C H A P T E R
N ow weighing more than 350 pounds,
17-year-old Monica sat on the edge
of her bed, staring at her reﬂection in the
C H A P T E R
8.1 Overview of Digestion
C O N C E P T S
mirror on the wall. Disgusted with what she The gastrointestinal tract (GI tract) contains a number of organs that carry out
saw, she mulled her options silently in her several processes as they digest food. The wall of the tract typically has four layers
head. She had tried dieting—Jenny Craig, and is modiﬁed in each of the organs.
Weight Watchers, the Atkins diet, and many
8.2 First Part of the Digestive Tract
others—and had lost a few pounds here and
The mouth receives food where it is chewed as chemical digestion begins before
there, but the weight loss never seemed to food enters the pharynx and the esophagus, which takes it to the stomach.
last. She had even consulted a nutritionist,
to no avail. Unable to control her weight by 8.3 The Stomach and Small Intestine
any other means, she decided to ask her doc- The stomach stores food and continues chemical digestion, which is completed in
tor about surgical remedies. the small intestine. The products of digestion are absorbed by the small intestine
into the blood or into the lymph.
Monica is considering bariatric surgery,
a term used to describe several procedures 8.4 Three Accessory Organs and Regulation of Secretions
that are performed on the digestive system The pancreas produces pancreatic juice, which is sent to the small intestine for the
to promote weight loss. One common pro- chemical digestion of food, and the hormone insulin, which ordinarily causes cells,
cedure is known as “stomach stapling.” In including liver cells, to take up glucose from the blood. The liver is a major metabolic
organ, which stores glucose as glycogen and produces bile. Bile is stored in the
this procedure, the stomach is reduced to
gallbladder before it is sent to the small intestine for emulsiﬁcation of fats.
the size of a golf ball, and the digestive tract
is rerouted so that food bypasses the ﬁrst 8.5 The Large Intestine and Defecation
two feet of the intestine. Bariatric surgery The large intestine absorbs water and vitamins produced by bacteria that normally
compels patients to eat less because they be- reside in the large intestine, and it carries out defecation.
come very ill if they overeat. After bariatric
8.6 Nutrition and Weight Control
surgery is performed, patients remain at a
Planning nutritious meals and snacks to control one’s weight involves making
lifelong risk of nutritional deﬁciencies. healthy and informed food choices.
With nearly two-thirds of Americans
overweight and one-third obese, we are
mired in an obesity epidemic. Bariatric sur-
gery is becoming more widely used, and an
increasing number of obese teens, like Mon-
ica, are asking for these procedures. As you
read this chapter, you will learn how the di-
gestive system is organized, how it ordinar-
ily works, and how its function is regulated
by other organs of the body.
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144 Part II Maintenance of the Human Body
longer, more active and productive life. Unfortunately,
8.1 Overview of Digestion poor diet and lack of physical activity now rivals smoking
The organs of the digestive system are located within a tube as a major cause of preventable death in the United States.
called the gastrointestinal tract (GI tract), which is depicted • Digestion can be mechanical or chemical. Mechanical
in Figure 8.1. Food, whether it is a Big Mac or a shrimp salad, digestion occurs when food is divided into pieces that
consists of the organic macromolecules you studied in Chap- can be acted on by the digestive enzymes. Actually,
ter 2, namely carbohydrates, fats, and proteins. These mole- we aid mechanical digestion when we cut up our
cules are too big to cross plasma membranes, and the purpose food prior to ingestion. Mechanical digestion occurs
of digestion is to hydrolyze these macromolecules to their primarily in the mouth and stomach.
unit molecules. The unit molecules, namely sugars, amino All parts of the tract, except the large intestine,
acids, fatty acids, and glycerol, can cross plasma membranes. contain digestive enzymes that hydrolyze particular
These are the nutrients that are carried by the blood to our foods to molecular nutrients. Chemical digestion begins
cells. Our food also contains water, salts, vitamins, and min- in the mouth and is not completed until food reaches
erals that help the body function normally. Speciﬁcally, these the small intestine. The thick semiﬂuid mass of partly
processes are necessary to the digestive process. digested food that is passed from the stomach to the
• Ingestion occurs when the mouth takes in food. Ingestion small intestine is termed chyme.
can be associated with our diet. The expression “we • Movement of GI tract contents along the digestive
dig our graves with our teeth” recognizes that tract is very important in order for the tract to fulﬁll its
our diet is very important to our health. other functions. For example, food must be passed
By learning about good nutrition, you along from one organ to the next, and indigestible
increase your likelihood of enjoying a remains must be expelled.
Accessory organs Digestive tract organs
Salivary glands teeth chew food; tongue tastes and
secrete saliva: contains digestive enzyme pushes food for chewing and swallowing
passageway where food is swallowed
passageway where peristalsis pushes
food to stomach
major metabolic organ: Stomach
processes and stores nutrients; secretes acid and digestive enzyme
produces bile for emulsification of fats for protein; churns mixing food with
secretions and sends chyme to
Gall bladder small intestine
stores bile from liver; sends it to
the small intestine
Pancreas mixes chyme with digestive enzymes
produces pancreatic juice: contains digestive for final breakdown; absorbs nutrient
enzymes, and sends it to the small intestine; molecules into body; secretes
produces insulin and secretes it into the digestive hormones into blood
blood after eating
absorbs water and salt to form feces
The human gastrointestinal (GI) tract. Rectum
Food is digested within the organs of the stores and regulates elimination
tract; the accessory organs assist this
process in the ways noted. Anus
mad86867_ch08_143-168.indd 144 12/1/06 2:36:27 PM
Chapter 8 Digestive System and Nutrition 145
Mucosa supply vessel
inner mucous membrane
layer that is modified
according to the
broad band of loose
connective tissue that
contains nerves, blood,
and lymphatic vessels
two layers of
Serosa Figure 8.2 Wall of the gastrointestinal tract.
thin outermost tissue The wall of the gastrointestinal tract contains the four
that is the visceral layers noted.
The second layer in the GI wall is called the submucosa.
The submucosal layer is a broad band of loose connective tis-
sue that contains blood vessels, lymphatic vessels, and nerves.
These are the vessels that will carry the nutrients absorbed by
the mucosa. Lymph nodules, called Peyer’s patches, are also
• Absorption occurs as unit molecules produced by in the submucosa. Like the tonsils, they help protect us from
digestion (i.e., nutrients) cross the wall of the GI tract disease. Because the submucosa contains blood vessels, it can
and enter the cells lining the tract. From there, the be the site of an inﬂammatory response (see page 128) that
nutrients enter the blood for delivery to the cells. leads to inﬂammatory bowel disease (IBD), characterized by
• Elimination: Molecules that cannot be digested chronic diarrhea, abdominal pain, fever, and weight loss.
need to be eliminated from the body. The removal of The third layer is termed the muscularis, and it contains
indigestible wastes through the anus, in the form of two layers of smooth muscle. The inner, circular layer encir-
feces, is defecation. cles the tract; the outer, longitudinal layer lies in the same
direction as the tract. The contraction of these muscles, which
Wall of the Digestive Tract are under nervous control, accounts for movement of digested
food from the esophagus to the anus. The muscularis can be
We can compare the GI tract to a garden hose that has a associated with irritable bowel syndrome (IBS), in which con-
beginning (mouth) and an end (anus). The so-called lumen tractions of the wall cause abdominal pain, constipation,
is the central space that contains water (food being digested). and/or diarrhea. The underlying cause of IBS is not known,
The wall of the GI tract has four layers (Fig. 8.2), and we will although some suggest stress as an underlying cause.
associate each layer with a particular disorder. The fourth layer of the tract is the serosa (serous mem-
The ﬁrst layer of the wall next to the lumen is called the brane layer), which secrete a serous ﬂuid. The serosa is a part
mucosa. The mucosa is more familiarly called a mucous of the peritoneum, the internal lining of the abdominal cavity.
membrane, and, of course, it produces mucus, which pro- The appendix is a worm-shaped blind tube projecting from
tects the wall from the digestive enzymes inside the lumen. the ﬁrst part of the large intestine on the right side of the abdo-
In the mouth, stomach, and small intestine, the mucosa men. An inﬂamed appendix (appendicitis) has to be removed
either contains glands that secrete and/or receive digestive because should the appendix burst, the result can be peritoni-
enzymes from glands that secrete digestive enzymes. tis, a life-threatening infection of the peritoneum.
Diverticulosis is a condition in which portions of the
mucosa literally have pushed through the other layers and
formed pouches, where food can collect. The pouches can be Check Your Progress 8.1
likened to an inner tube that pokes through weak places in a 1. Name and describe the processes that occur during the digestive
tire. When the pouches become infected or inﬂamed, the process.
condition is called diverticulitis. This happens in 10–25% of 2. What are the four layers of the GI tract? Associate an illness with
people with diverticulosis. each of the layers.
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146 Part II Maintenance of the Human Body
8.2 First Part of the Digestive Tract The Teeth and Tongue
Mechanical digestion occurs when our teeth chew food into
The mouth, the pharynx, and the esophagus are in the ﬁrst pieces convenient for swallowing. Monica, in the opening
part of the GI tract. story, will have to chew her food thoroughly after her opera-
tion because her stomach will no longer be able to continue
The Mouth the process of mechanical digestion. Monica might even have
The mouth receives food and begins the process of mechani- to put her food in a blender before eating it.
cal and chemical digestion. The mouth is bounded externally During the ﬁrst two years of life, the 20 smaller decidu-
by the lips and cheeks. The lips extend from the base of the ous, or baby, teeth appear. These are eventually replaced by
nose to the start of the chin. The red portion of the lips is 32 adult teeth (Fig. 8.3a). The third pair of molars, called the
poorly keratinized, and this allows blood to show through. wisdom teeth, sometimes fail to erupt. If they push on the
The roof of the mouth separates the nasal cavities from other teeth and/or cause pain, they can be removed by a den-
the oral cavity. The roof has two parts: an anterior (toward tist or oral surgeon. Each tooth has two main divisions: a
the front) hard palate and a posterior (toward the back) soft crown and a root (Fig. 8.3b). The crown has a layer of enamel,
palate (Fig. 8.3a). The hard palate contains several bones, an extremely hard outer covering of calcium compounds;
but the soft palate is composed entirely of muscle. The soft dentin, a thick layer of bonelike material; and an inner pulp,
palate ends in a ﬁnger-shaped projection called the uvula. which contains the nerves and the blood vessels. Dentin and
The tonsils, which are lymphatic tissue and help protect us pulp are also found in the root.
from disease, are also in the back of the mouth, on either side Tooth decay, called dental caries, or cavities, occurs
of the tongue, and in the nasopharynx (called adenoids). when bacteria within the mouth metabolize sugar and give
Three pairs of salivary glands send juices (saliva) by way off acids, which erode teeth. Tooth decay can be painful
of ducts to the mouth. One pair of salivary glands lies at the when it is severe enough to reach the nerves of the inner
sides of the face immediately below and in front of the ears. pulp. Two measures can prevent tooth decay: eating a lim-
These glands swell when a person has the mumps, a disease ited amount of sweets, and daily brushing and ﬂossing of
caused by a viral infection. Salivary glands have ducts that teeth. Fluoride treatments, particularly in children, can make
open on the inner surface of the cheek at the location of the the enamel stronger and more resistant to decay. Gum dis-
second upper molar. Another pair of salivary glands lies ease, which is now known to be linked to cardiovascular
beneath the tongue, and still another pair lies beneath the disease, is more apt to occur with aging. Inﬂammation of the
ﬂoor of the oral cavity. The ducts from these salivary glands gums (gingivitis) can spread to the periodontal membrane,
open under the tongue. You can locate the openings if you use which lines the tooth socket. A person then has periodonti-
your tongue to feel for small ﬂaps on the inside of your cheek tis, characterized by a loss of bone and loosening of the teeth
and under your tongue. Saliva is a solution of mucus and so that extensive dental work may be required. Stimulation
water, which also contains bicarbonate and an enzyme called of the gums in a manner advised by your dentist is helpful
salivary amylase that begins the process of digesting starch. in controlling this condition. Medications are also available.
Figure 8.3 Adult mouth
and teeth. enamel
a. The chisel-shaped incisors crown dentin
bite; the pointed canines tear; pulp
the fairly ﬂat premolars grind; hard palate gum
and the ﬂattened molars crush
food. b. Longitudinal section of a
tooth. The crown is the portion jawbone
that projects above the gum line
and can be replaced by a dentist uvula root canal
if damaged. When a “root canal” tonsil
is done, the nerves are removed.
When the periodontal membrane periodontal
is inﬂamed, the teeth can loosen. molars (3) membrane
mad86867_ch08_143-168.indd 146 12/1/06 2:36:49 PM
Chapter 8 Digestive System and Nutrition 147
Voluntary phase Involuntary phase
hard palate food bolus
glottis is closed
larynx rises and
trachea epiglottis covers esophagus lower esophageal
(airway)open glottis spincter relaxes and stomach
food enters stomach
a. Swallowing b. Peristalsis
Figure 8.4 Swallowing of food and peristalsis.
a. When food is swallowed, the tongue pushes a bolus of food up against the soft palate (left). Then, the soft palate closes off the nasal cavities, and the
epiglottis closes off the larynx so the bolus of food enters the esophagus (right). b. Peristalsis moves food through a sphincter into the stomach.
The tongue is covered by mucous membrane, which Unfortunately, we have all had the unpleasant experience
contains the sensory receptors called taste buds (see page of having food “go the wrong way.” The wrong way may be
249). When taste buds are activated by the presence of food, either into the nasal cavities or into the trachea. If it is the lat-
nerve impulses travel by way of nerves to the brain. The ter, coughing will most likely force the food up out of the tra-
tongue is composed of skeletal muscle, and it assists the chea and into the pharynx again.
teeth in carrying out mechanical digestion by moving food A rhythmic contraction called peristalsis pushes the food
around in the mouth. In preparation for swallowing, the along the esophagus and continues in all the organs of the
tongue forms chewed food into a mass called a bolus that it digestive tract. The esophagus plays no role in the chemical
pushes toward the pharynx. digestion of food. Its sole purpose is to move the food bolus
from the mouth to the stomach. Sphincters are muscles that
encircle tubes and act as valves; tubes close when sphincters
The Pharynx and Esophagus contract, and they open when sphincters relax. The entrance
Both the mouth and the nasal passages lead to a cavity called of the esophagus to the stomach is marked by a constriction,
the pharynx (Fig. 8.4). The food passage and air passage often called a lower gastroesophageal sphincter, although the
cross in the pharynx because the trachea (windpipe) is ante- muscle is not as developed as in a true sphincter. Relaxation
rior to (in front of) the esophagus, a long, narrow tube that of the sphincter allows food to pass into the stomach, while
takes food to the stomach. contraction prevents the acidic contents of the stomach from
backing up into the esophagus. When food or saliva is swal-
Swallowing lowed, the sphincter relaxes for a few seconds to allow the
Swallowing has a voluntary phase—from day-to-day liv- food or saliva to pass from the esophagus into the stomach,
ing, you know that you can swallow voluntarily. (Try it!) and then it closes again.
However, once food or drink is pushed back into the phar- Heartburn, discussed in the Health Focus on page 148,
ynx, swallowing becomes a reﬂex action performed auto- occurs due to acid reﬂux, when some of the stomach’s con-
matically (without you willing it). During swallowing, food tents escape into the esophagus. When vomiting occurs, the
normally enters the esophagus, a muscular tube that takes abdominal muscles and the diaphragm, a muscle that sepa-
food to the stomach, because other possible avenues are rates the thoracic and abdominal cavities, contract.
blocked. The soft palate moves back to close off the nasal
passages, and the trachea moves up under the epiglottis to
cover the glottis. The glottis is the opening to the larynx Check Your Progress 8.2
(voice box), and therefore, the air passage. We do not breathe 1. Describe the mechanical digestion and the chemical digestion
when we swallow. The up-and-down movement of the that occurs in the mouth.
Adam’s apple, the front part of the larynx, is easy to observe 2. What ordinarily prevents food from entering the nose or entering
when a person swallows. the trachea when you swallow?
mad86867_ch08_143-168.indd 147 12/1/06 2:36:54 PM
148 Part II Maintenance of the Human Body
In the absence of other symptoms, that burning sensation in
your chest may have nothing to do with your heart. Instead,
it is likely due to acid reﬂux. The stomach contents are more
acidic than those of the esophagus. When the stomach con-
tents pass upward into the esophagus, the acidity begins to
erode the lining of the esophagus, producing the burning sen-
sation associated with heartburn. Sometimes the acidic ﬂuid
passes upward and is tasted in the back of the mouth, and then
we say we have acid indigestion.
Almost everyone has had acid reﬂux and heartburn at some
time. The burning sensation occurs in the area of the esophagus
that lies behind the heart, and that is why it is termed heart-
burn. We might think it means we have a heart problem. Figure 8A A possible candidate for acid reﬂux.
When heartburn or acid reﬂux becomes a chronic condi- Obesity, a rich diet, and inactivity makes one prone to GERD
tion, the patient is diagnosed with gastroesophageal reﬂux (gastroesophageal reﬂux disease).
disease (GERD). The term signiﬁes that the stomach (gastric
refers to the stomach) and the esophagus are involved in the
• Avoid high-fat meals, such as those served by
disease. In GERD, patients’ reﬂux is more frequent, remains
fast-food chains. Fatty foods stay in the stomach
in the esophagus longer, and will often contain higher levels
longer and, therefore, cause the stomach to secrete
of acid than in a patient with typical heartburn or acid re-
ﬂux. People diagnosed with GERD may also experience pain
• Don’t overeat. In the movie Supersize Me, Morgan
in their chest, feel like they are choking, and have trouble
Spurlock doesn’t just have acid reﬂux after eating
swallowing. An anatomical disorder may be present. For
a meal consisting of a supersize hamburger, coke,
example, normally during swallowing, wavelike contrac-
and fries. He throws up! Vomiting involves the same
tions (peristalsis) of the esophagus will move food down
processes as does acid reﬂux.
the esophagus and into the stomach. Patients with weak or
• Eat several small meals instead of three large meals
abnormal esophageal contractions will have difﬁculty push-
a day. Avoid foods that lead to stomach acidity,
ing food into the stomach. These weak contractions can also
including alcohol, any form of fat, cakes and candy,
prevent reﬂux from being pushed back into the stomach after
tomato sauces, caffeinated beverages, and citrus
it has entered the esophagus, thus producing GERD. When
fruits. Eat complex carbohydrates such as multigrain
patients are lying down, the effects of abnormal esophageal
bread, brown rice, and pasta instead of foods high in
contractions become more severe. This is because gravity is
reﬁned sugar. Besides providing you with vitamins
not helping to return reﬂux to the stomach.
and minerals, complex carbohydrates can help you
Many people take over-the-counter medications for acid
control acid reﬂux!
reﬂux. These drugs, being basic, neutralize stomach acid.
• Light exercise such as riding a bike at a slow pace,
Some progress to taking prescribed medications, which shut
walking, yoga, and light weight lifting are usually
down and reduce acid production. However, persons with
extremely helpful in heartburn patients. Both diet
acid reﬂux could try modifying their eating habits ﬁrst.
and exercise should help control weight.
The pressure applied to the abdominal wall from
Diet and Exercise obesity places pressure on the stomach and causes
It has been found that diet can help control acid reﬂux. Here the normal acid contents of the stomach to reﬂux
are some tips: upward into the esophagus.
duodenum of the small intestine below. The stomach stores
8.3 The Stomach and Small Intestine food, initiates the digestion of protein, and controls the
The stomach and small intestine complete the digestion of movement of chyme into the small intestine. Notice that the
food, which began in the mouth. stomach does not absorb nutrients; however, it does absorb
alcohol because alcohol is fat soluble and can pass through
the membrane for that reason.
The Stomach The stomach wall has the usual four layers, but two of
The stomach (Fig. 8.5) is a thick-walled, J-shaped organ that them are modiﬁed for particular functions. The muscularis
lies on the left side of the body beneath the diaphragm. The contains three layers of smooth muscle (Fig. 8.5a). In addi-
stomach is continuous with the esophagus above and the tion to the circular and longitudinal layers, the stomach also
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Chapter 8 Digestive System and Nutrition 149
contains a layer of smooth muscle that runs obliquely to the sphincter, which closes and squeezes most of the chyme
other two. The oblique layer also allows the stomach to back, letting only a small amount to enter the small intestine
stretch and to mechanically break down food into smaller at one time (Fig. 8.5d).
fragments that are mixed with gastric juice. Since Monica’s
stomach will be the size of a golf ball after surgery, her stom- Historical Aspects
ach will not be able to carry out mechanical digestion; there- The stomach is well known for providing new scientiﬁc
fore, it will not be able to store food and she might be subject information. Not much was known, in general, about diges-
to GERD, as described in the Health Focus on page 148. tion in 1822 when an opportunity arose for a surgeon, named
The mucosa of the stomach has deep folds, the rugae, William Beaumont, to study how the stomach functions.
which disappear as the stomach ﬁlls to an approximate After a patient, Alexis St. Martin, was shot, leaving a hole in
capacity of 1 liter. The mucosa of the stomach has millions of his stomach, Beaumont began performing experiments on
gastric pits, which lead into gastric glands (Fig. 8.5b and c). digestion. Most of the experiments consisted of tying a piece
The gastric glands produce gastric juice. Gastric juice con- of food to a string and inserting it through the hole in St.
tains an enzyme called pepsin, which digests protein, plus Martin’s stomach. Every few hours, Beaumont would
hydrochloric acid (HCl) and mucus. HCl causes the stomach remove the food and observe how well it had been digested.
to have a high acidity with a pH of about 2, and this is ben- Through careful observations, he discovered most of what
eﬁcial because it kills most bacteria present in food. Although we know today about the stomach.
HCl does not digest food, it does break down the connective Also, about a 150 years later, in 1982, Dr. Marshall and
tissue of meat and activates pepsin. Dr. Warren began their work on the cause of ulcers. As
Normally, the stomach empties in about 2–6 hours. related in Chapter 1, they discovered that the bacterium
When food leaves the stomach, it is a thick, soupy liquid Heicobacter pyloris is able to overcome the protective function
called chyme. Chyme enters the small intestine in squirts. of mucus in the stomach and the result is a gastric ulcer.
The peristaltic waves move the chyme toward the pyloric (Gastric always refers to the stomach.)
lower gastroesophageal has three layers
sphincter of muscle
pyloric mucosa layer
sphincter has rugae
c. Gastric pits in mucosa gastric pit SEM 3,260�
cells that pyloric
b. Gastric glands d. How the stomach empties
Figure 8.5 Structure and function of the stomach.
a. Structure of the stomach showing that the muscularis has three muscle layers and the mucosa has folds called rugae. b. Gastric glands present in the mucosa
secrete mucus, HCl, and pepsin, an enzyme that digests protein. c. Micrograph of gastric pits. d. Peristalsis in the stomach controls the secretion of chyme into
the small intestine at the pyloric sphincter.
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150 Part II Maintenance of the Human Body
The Small Intestine jections called villi (sing., villus), which give the intestinal
wall a soft, velvety appearance (Fig. 8.6). A villus has an outer
The small intestine is named for its small diameter (com- layer of columnar epithelial cells, and each of these cells has
pared with that of the large intestine), but perhaps it should thousands of microscopic extensions called microvilli. Col-
be called the long intestine. The small intestine averages lectively, in electron micrographs, microvilli give the villi a
about 6 m (18 ft) in length, compared with the large intes- fuzzy border known as a “brush border.” Since the microvilli
tine, which is about 1.5 m (4½ ft) in length. bear the intestinal enzymes, these enzymes are called brush-
border enzymes. The microvilli greatly increase the surface
Digestion Is Completed in the Small Intestine area of the villus for the absorption of nutrients.
Notice in Table 8.1 that the small intestine contains enzymes Nutrients are absorbed into the vessels of a villus (Fig.
to digest all types of foods, primarily carbohydrates, pro- 8.7). A villus contains blood capillaries and a small lymphatic
teins, and fats. These enzymes are secreted by the pancreas capillary, called a lacteal. As you know, the lymphatic system
and enter via a duct at the duodenum, the name for the ﬁrst is an adjunct to the cardiovascular system; its vessels carry a
25 cm of the small intestine. Also, a duct brings bile from the ﬂuid called lymph to the cardiovascular veins. Sugars
liver and gallbladder into the duodenum (see Fig. 8.8). Bile (digested from carbohydrates) and amino acids (digested
emulsiﬁes fat—emulsiﬁcation causes fat droplets to disperse from proteins) enter the blood capillaries of a villus. Glycerol
in water. After fat is mechanically broken down to fat drop- and fatty acids (digested from fats) enter the epithelial cells
lets by bile, it is hydrolyzed to glycerol and fatty acids by of the villi, and within these cells are joined and packaged as
lipase present in pancreatic juice. Pancreatic amylase begins lipoprotein droplets, called chylomicrons, which enter a lac-
and an intestinal enzyme ﬁnishes the digestion of carbohy- teal. After nutrients are absorbed, they are eventually carried
drates to glucose. Similarly, pancreatic trypsin begins and an to all the cells of the body by the bloodstream.
intestinal enzyme ﬁnishes the digestion of proteins to amino
acids. The intestine has a slightly basic pH because pancre-
atic juice contains sodium bicarbonate (NaHCO3), which
neutralizes chyme. Monica’s surgery is associated with Lactose is the primary sugar in milk. People who do not have
nutritional deﬁciencies because her GI tract now bypasses the brush border enzyme called lactase can not digest lactose.
the duodenum. The result is a condition called lactose intolerance, character-
ized by diarrhea, gas, bloating, and abdominal cramps after
Nutrients Are Absorbed in the Small Intestine drinking milk and other dairy products. Diarrhea occurs
The wall of the small intestine absorbs the molecules, namely because the indigestible lactose causes ﬂuid retention in the
sugars, amino acids, fatty acids, and glycerol, which are the small intestine. Gas, bloating, and cramps occur when bacte-
products of the digestive process. The wall of the small intes- ria break down the lactose anaerobically, producing gas.
tine is modiﬁed for absorption. It has been suggested that the Persons with lactose intolerance can eat dairy products
surface area of the small intestine is approximately that of a such as cheese and yogurt in which the lactose has already
tennis court. What factors contribute to increasing its surface been broken down; lactose-free milk, and/or take a dietary
area? The wall of the small intestine contains ﬁngerlike pro- supplement that aids in the digestion of lactose.
Section of intestinal wall villus
blood villus microvilli
Figure 8.6 Anatomy of the small intestine.
The wall of the small intestine has folds that bear
ﬁngerlike projections called villi. The products of lymphatic
digestion are absorbed by microvilli into the blood vessel
capillaries and the lacteals of the villi. Villi arteriole
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Chapter 8 Digestive System and Nutrition 151
Table 8.1 Major Digestive Enzymes
Enzyme Produced By Site of Action Optimum pH Digestion
Salivary amylase Salivary glands Mouth Neutral Starch H2O → maltose
Pancreatic amylase Pancreas Small intestine Basic Starch H2O → maltose
Maltase Small intestine Small intestine Basic Maltose H2O → glucose glucose
Pepsin Gastric glands Stomach Acidic Protein H2O → peptides
Trypsin Pancreas Small intestine Basic Protein H2O → peptides
Peptidases Small intestine Small intestine Basic Peptide H2O → amino acids
NUCLEIC ACID DIGESTION:
Nuclease Pancreas Small intestine Basic RNA and DNA H2O → nucleotides
Nucleosidases Small intestine Small intestine Basic Nucleotide H2O → base sugar phosphate
Lipase Pancreas Small intestine Basic Fat droplet H2O → monoglycerides fatty acids
Figure 8.7 Digestion and carbohydrate protein
absorption of nutrients. +
a. Starch is digested to glucose, pancreatic amylase bile salts fat globules
which is actively transported into trypsin
the epithelial cells of intestinal villi. emulsification
From there, glucose moves into the maltase
epithelial peptidase lipase
bloodstream. b. Proteins are digested
cell of glucose
to amino acids, which are actively intestinal monoglycerides
transported into the epithelial cells of villus amino acids and free
intestinal villi. From there, amino acids fatty acids
move into the bloodstream. c. Fats
are emulsiﬁed by bile and digested to
pH = basic pH = basic chylomicron
monoglycerides and fatty acids. These pH = basic
diffuse into epithelial cells, where they
recombine and join with proteins to lymphatic
blood capillary blood capillary
form lipoproteins, called chylomicrons. capillary
Chylomicrons enter a lacteal. a. Carbohydrate digestion b. Protein digestion c. Fat digestion
Obesity: Diabetes Type 2 and Switching to a healthy diet, increasing physical activity,
and losing weight improves the ability of the hormone insulin
Cardiovascular Disease to function properly in type 2 diabetics. The high fatty acid
The nutrients absorbed at the small intestine have a profound levels that accompany diabetes type 2 can lead to an increased
effect on the body because these molecules can affect our health. risk of cardiovascular disease. It is well worthwhile to prevent
For example, the intake of too much sugar and fat can result in diabetes type 2 by switching to a healthy diet and engaging in
obesity, which is associated with diabetes type 2 and cardiovas- daily exercise because all diabetics are at risk for blindness,
cular disease. Nutritionists point out that consuming too much kidney disease, and cardiovascular disease.
energy from any source contributes to body fat, which increases
a person’s risk of obesity and associated illness. Still, foods such Check Your Progress 8.3
as doughnuts, cakes, pies, cookies, and white bread, which are 1. a. What are the functions of the stomach, and (b) how is the wall
high in reﬁned carbohydrates (starches and sugars), and fried of the stomach modiﬁed to perform these functions?
foods, which are high in fat, may very well be responsible for 2. a. What are the functions of the small intestine, and (b) how is the
the current epidemic of obesity among Americans. wall of the small intestine modiﬁed to perform these functions?
mad86867_ch08_143-168.indd 151 12/1/06 2:37:22 PM
152 Part II Maintenance of the Human Body
blood via the hepatic portal vein from the capillary bed of the
8.4 Three Accessory Organs and GI tract and ﬁlters blood in the capillaries of the lobules. In a
Regulation of Secretions sense, the liver acts like a sewage treatment plant when it
First, we will take a look at three accessory organs of diges- removes poisonous substances from the blood and detoxiﬁes
tion before considering how the secretions of these organs them, while blood courses through its lobules (Table 8.2).
and those of the GI tract are regulated. The liver is also a storage organ. It removes iron and the
vitamins A, D, E, K, and B12 from blood and stores them. After
we eat and in the presence of insulin, the liver stores glucose
Three Accessory Organs as glycogen, and between eating, it breaks down glycogen to
The pancreas is a ﬁsh-shaped, spongy, grayish pink organ keep the blood glucose level relatively constant. If need be,
that stretches across the back of the abdomen behind the the liver converts glycerol (from fats) and amino acids to glu-
stomach. Most pancreatic cells produce pancreatic juice, cose molecules. As amino acids are converted to glucose, the
which enters the duodenum via the pancreatic duct (Fig. 8.8). liver combines their amino groups with carbon dioxide to
Pancreatic juice contains sodium bicarbonate (NaHCO3) and form urea, the usual nitrogenous waste product in humans.
digestive enzymes for all types of food. Sodium bicarbonate On the other hand, the liver makes the plasma proteins
neutralizes acid chyme from the stomach. Pancreatic amylase and helps regulate the quantity of cholesterol in the blood. It
digests starch, trypsin digests protein, and lipase digests fat. does this by producing bile salts, which are derived from
The pancreas is also an endocrine gland that secretes the cholesterol. Bile has a yellowish green color because it also
hormone insulin into the blood. A hormone is a substance pro- contains bilirubin, derived from the breakdown of hemoglo-
duced by one set of cells that affects a different set of cells, the bin, another function of the liver. Bile is stored in the gall-
so-called target cells. When the blood glucose level rises rap- bladder, a pear-shaped organ just below the liver, until it is
idly, the pancreas produces an overload of insulin to bring the sent via the bile ducts to the duodenum. Gallstones form
level under control. Over the years, the body’s cells can become when liquid stored in the gallbladder hardens into pieces of
insulin resistant with the result that diabetes type 2 can occur. stonelike material. Monica may have a tendency toward
The liver, which is the largest and a major metabolic gallstones after her surgery because her gallbladder is no
gland in the body, lies mainly in the upper-right section of the longer ridding itself of bile. In the small intestine, bile salts
abdominal cavity, under the diaphragm (see Fig. 8.1). The emulsify fat. When fat is emulsiﬁed, it breaks up into drop-
liver contains approximately 100,000 lobules that serve as its lets, providing a much larger surface area, which can be
structural and functional units (Fig. 8.8b). The liver receives acted upon by a digestive enzyme from the pancreas.
common hepatic duct
b. bile duct portal vein
common bile duct
Figure 8.8 The liver.
a. The location of the liver, gallbladder, and pancreas in relation to the small
intestine. b. The liver contains over 100,000 lobules. Each lobule contains
a. many cells that perform the various functions of the liver (see Table 8.2).
mad86867_ch08_143-168.indd 152 12/1/06 2:37:29 PM
Chapter 8 Digestive System and Nutrition 153
Table 8.2 Functions of the Liver
1. Destroys old red blood cells; excretes bilirubin, a breakdown product of
hemoglobin in bile, a liver product
2. Detoxiﬁes blood by removing and metabolizing poisonous substances
3. Stores iron (Fe2 +) and the fat-soluble vitamins A, D, E, and K
4. Makes plasma proteins, such as albumins and ﬁbrinogen, from amino acids
5. Stores glucose as glycogen after a meal, and breaks down glycogen
to glucose to maintain the glucose concentration of blood between pancreas
6. Produces urea after breaking down amino acids
7. Helps regulate blood cholesterol level, converting some to bile salts
Liver Disorders duodenum secretin
Hepatitis and cirrhosis are two serious diseases that affect gastrin
the entire liver and hinder its ability to repair itself. There- blood vessel
fore, they are life-threatening diseases. When a person has
a liver ailment, jaundice may occur in which bile pigments Figure 8.9 Hormonal control of digestive gland secretions.
leak into the blood. Jaundice is a yellowish tint to the whites Gastrin (blue), from the lower stomach, feeds back to stimulate the upper
of the eyes and also to the skin of light-pigmented persons. part of the stomach to produce digestive juice. Secretin (green) and CCK
Jaundice can result from hepatitis, inﬂammation of the (purple) from the duodenal wall stimulate the pancreas to secrete digestive
liver. Viral hepatitis occurs in several forms. Hepatitis A is juice and the gallbladder to release bile.
usually acquired from sewage-contaminated drinking
water. Hepatitis B, which is usually spread by sexual con-
tact, can also be spread by blood transfusions or contami-
nated needles. The hepatitis B virus is more contagious food, the parasympathetic nervous system automatically
than the AIDS virus, which is spread in the same way. Vac- stimulates gastric secretion. Also, when a person has eaten a
cines are now available for hepatitis A and hepatitis B. meal particularly rich in protein, the stomach produces the
Hepatitis C, for which there is no vaccine, is usually hormone gastrin. Gastrin enters the bloodstream, and soon
acquired by contact with infected blood and can lead to the secretory activity of gastric glands is increasing.
chronic hepatitis, liver cancer, and death. Cells of the duodenal wall produce two other hormones
Cirrhosis is another chronic disease of the liver. First, the that are of particular interest—secretin and CCK (cholecys-
organ becomes fatty, and then liver tissue is replaced by inac- tokinin). Acid, especially hydrochloric acid (HCl) present in
tive ﬁbrous scar tissue. Cirrhosis of the liver is often seen in chyme, stimulates the release of secretin, while partially
alcoholics, due to malnutrition and to the excessive amounts digested protein and fat stimulate the release of CCK. Soon
of alcohol (a toxin) the liver is forced to break down. Physi- after these hormones enter the bloodstream, the pancreas
cians are now beginning to see cirrhosis of the liver in obese increases its output of pancreatic juice. Pancreatic juice buf-
people who are overweight due to a diet high in fatty foods. fers the acidic chyme entering the intestine from the stom-
The liver has amazing regenerative powers and can ach and helps digest food. CCK also causes the liver to
recover if the rate of regeneration exceeds the rate of dam- increase its production of bile, and the gallbladder to con-
age. During liver failure, however, there may not be enough tract and release stored bile. Figure 8.9 summarizes the
time to let the liver heal itself. Liver transplantation is usu- actions of gastrin, secretin, and CCK.
ally the preferred treatment for liver failure, but artiﬁcial
livers have been developed and tried in a few cases. The
liver is a vital organ, and its failure leads to death. Check Your Progress 8.4
1. What are the three main accessory organs that assist with
the digestive process?
Regulation of Digestive Secretions 2. How does each accessory organ contribute to the digestion
The secretions of digestive juices are controlled by the ner- of food?
vous system and by hormones. When you look at or smell 3. How are digestive secretions regulated in the body?
mad86867_ch08_143-168.indd 153 12/1/06 2:37:32 PM
154 Part II Maintenance of the Human Body
Escherichia coli were the major inhabitants of the colon, but
8.5 The Large Intestine and new culture methods show that over 99% of the colon bacte-
Defecation ria are other types of bacteria. The bacteria in the large intes-
tine break down indigestible material, and also produce B
The large intestine includes the cecum, the colon, the rectum,
complex vitamins and most of the vitamin K needed by our
and the anal canal (Fig. 8.10). The large intestine is larger in
bodies. In this way, they perform a service for us.
diameter than the small intestine (6.5 cm compared with 2.5
The large intestine forms feces. Feces are three-quarters
cm), but it is shorter in length (see Fig. 8.1).
water and one-quarter solids. Bacteria, dietary ﬁber (indigest-
The cecum, which lies below the junction with the small
ible remains), and other indigestible materials are in the solid
intestine, is the blind end of the large intestine. The cecum
portion. Bacterial action on indigestible materials causes the
usually has a small projection called the vermiform appen-
odor of feces and also accounts for the presence of gas. A
dix (vermiform means wormlike) (Fig. 8.10). In humans, the
breakdown product of bilirubin (see page 152) and the pres-
appendix also may play a role in ﬁghting infections. As men-
ence of oxidized iron cause the brown color of feces.
tioned previously, the appendix can become infected, a con-
Defecation, which is ridding the body of feces, is also a
dition called appendicitis.
function of the large intestine. Peristalsis occurs infrequently
The colon includes the ascending colon, which goes up
in the large intestine, but, when it does, feces are forced into
the right side of the body to the level of the liver; the trans-
the rectum. Feces collect in the rectum until it is appropriate
verse colon, which crosses the abdominal cavity just below
to defecate. At that time, stretching of the rectal wall initiates
the liver and the stomach; the descending colon, which
nerve impulses to the spinal cord, and shortly thereafter, the
passes down the left side of the body; and the sigmoid colon,
rectal muscles contract and the anal sphincters relax, allowing
which enters the rectum, the last 20 cm of the large intestine.
the feces to exit the body through the anus (Fig. 8.10). A per-
The rectum opens at the anus, where defecation, the expul-
son can inhibit defecation by contracting the external anal
sion of feces, occurs.
sphincter. Ridding the body of indigestible remains is another
way the digestive system helps maintain homeostasis.
Functions of the Large Intestine Water is considered unsafe for swimming when the coli-
form (nonpathogenic intestinal) bacterial count reaches a
The large intestine absorbs water, which is an important func-
certain number. A high count indicates that a signiﬁcant
tion that prevents dehydration of the body. Note that the large
amount of feces has entered the water. The more feces pres-
intestine does not produce any digestive enzymes and it does
ent, the greater the possibility that disease-causing bacteria
not absorb any nutrients.
are also present.
The large intestine absorbs vitamins produced by bacte-
ria called the intestinal ﬂora. For many years, it was believed
Disorders of the Colon and Rectum
The large intestine is subject to a number of disorders. Many
transverse colon of these can be prevented or minimized by a good diet and
good bowel habits.
Diarrhea The major causes of diarrhea are infection of the
lower intestinal tract and nervous stimulation. In the case of
infection, such as food poisoning caused by eating contami-
ascending nated food, the intestinal wall becomes irritated, and peri-
colon small descending
stalsis increases. Water is not absorbed, and the diarrhea that
results rids the body of the infectious organisms. In nervous
diarrhea, the nervous system stimulates the intestinal wall,
and diarrhea results. Prolonged diarrhea can lead to dehydra-
cecum tion because of water loss and to disturbances in the heart’s
contraction due to an imbalance of salts in the blood.
Constipation When a person is constipated, the feces are
vermiform dry and hard. One reason for this condition is that socialized
persons have learned to ignore the urge to defecate. An in-
take of water and ﬁber can help regularity of defecation. The
internal anal sphincter external anal sphincter frequent use of laxatives is discouraged. If, however, it is nec-
anal canal anus essary to take a laxative, a bulk laxative is the most natural
because, like ﬁber, it produces a soft mass of cellulose in the
Figure 8.10 Large intestine. colon. Lubricants, such as mineral oil, make the colon slip-
The regions of the large intestine are the cecum, colon, rectum, and anal canal. pery; saline laxatives, such as milk of magnesia, act osmoti-
mad86867_ch08_143-168.indd 154 12/1/06 2:37:37 PM
Chapter 8 Digestive System and Nutrition 155
Swallowing a Camera create discomfort in the patient, the considerable risks in-
volved with using anesthetics and painkillers are elimi-
PillCam has become a viable alternative to traditional nated. And ﬁnally, the doctor does not need to be present
endoscopy procedure, during which a doctor uses a re- for the entire procedure, saving valuable time and money
tractable tubelike instrument with an embedded camera, for both doctor and patient.
called an endoscope, to visualize the patient’s GI tract. Colonoscopy is a routine procedure used to exam-
With a gulp of water, PillCam is swallowed, and its travels ine and diagnose colon cancer. It employs a colonoscope,
through the digestive system begin. Instead of spending an instrument similar to an endoscope, that is inserted
an uncomfortable half day or more at the doctor’s ofﬁce, a through the anus into the large intestine and is capable of
patient simply visits the doctor in the morning, swallows removing precancerous tissue before it becomes invasive.
the camera, dons the recording device, and goes about his Since PillCam cannot be used to remove tissue samples for
or her daily routine. analysis, its use as a replacement for the colonoscopy is
Propelled by the normal muscular movement of the di- somewhat limited.
gestive system, PillCam embarks on a four- to eight-hour
journey through the digestive system. As it travels through
the stomach, the twists and turns of the small intestine, and
the large intestine, PillCam continuously captures high- Figure 8B Patient
quality, wide-angle ﬁlm footage of its journey and continu- swallowing PillCam.
ously beams this information to the recording device worn PillCam traverses through the
by the patient. entire GI tract, taking pictures
At the end of the day, PillCam reaches the end of its after it is swallowed.
journey, and it is simply defecated normally. Later, the re-
cording device is returned to the doctor’s ofﬁce so that the
data may be retrieved. The doctor can view PillCam’s jour-
ney as a 90-minute movie.
Like the food that we eat, PillCam traverses the nu-
merous twists and turns of the intestine with ease. This
provides a more accurate diagnosis, since a larger portion
of the GI tract can be examined. Since PillCam does not
cally—they prevent water from being absorbed. Some laxa- layer, where nerves and blood vessels are. Colitis causes diar-
tives are irritants that increase peristalsis. rhea, rectal bleeding, abdominal cramps, and urgency.
Chronic constipation is associated with the development
Polyps and Cancer The colon is subject to the development
of hemorrhoids, enlarged and inﬂamed blood vessels at the
of polyps, small growths arising from the epithelial lining.
anus. Other contributing factors include pregnancy, aging,
Polyps, whether benign or cancerous, can be removed surgi-
and anal intercourse.
cally. If colon cancer is detected while still conﬁned to a polyp,
Diverticulosis As mentioned previously (see page 145), the expected outcome is a complete cure. Some investigators
diverticulosis is the occurrence of little pouches of mucosa believe that dietary fat increases the likelihood of colon cancer
that have pushed out through weak spots in the muscularis. because dietary fat causes an increase in bile secretion. It could
A frequent site is the last part of the descending colon. be that intestinal bacteria convert bile salts to substances that
promote the development of cancer. On the other hand, ﬁber
Irritable Bowel Syndrome (IBS) (spastic colon) Also men-
in the diet seems to inhibit the development of colon cancer.
tioned previously (see page 145), IBS is a condition in which
Regular elimination reduces the time that the colon wall is
the muscularis contracts powerfully but without its normal
exposed to any cancer-promoting agents in feces.
coordination. The symptoms are abdominal cramps, gas,
As discussed in the Health Focus above, endoscopy by
constipation, and urgent, explosive stools (feces discharge).
means of a ﬂexible tube (and camera) inserted into the tract,
Inﬂammatory Bowel Disease (IBD) (colitis) IBD is a col- usually from the anus, is gradually being replaced by the Pill-
lective term for a number of inﬂammatory disorders. Ulcer- Cam, a camera you swallow by mouth!
ative colitis and Crohn’s disease are the most common of
these. Ulcerative colitis affects the large intestine and rectum,
but in Crohn’s disease, the inﬂammation can be more wide-
Check Your Progress 8.5
spread and ulcers can penetrate more deeply. Ulcers are pain- 1. What are the different parts of the large intestine?
ful and cause bleeding because they erode the submucosal 2. What are the functions of the large intestine?
mad86867_ch08_143-168.indd 155 12/1/06 2:37:42 PM
156 Part II Maintenance of the Human Body
8.6 Nutrition and Weight Control How Obesity Is Deﬁned
Today, obesity is often deﬁned as a body mass index (BMI)
Obesity—that is, being grossly overweight, has doubled in
of 35 or greater. The table in Figure 8.11 shows your BMI, as
the United States in only 20 years. Nearly one-third (33%) of
long as you already know your weight and height. The BMI
adults are now obese. Furthermore, being overweight is now
can also be calculated by taking the weight of the individual
prevalent among children and adolescents. These statistics
in kilograms and then dividing the weight by the square of
are of great concern because excess body fat is associated
the height in meters. Most people ﬁnd that using the table is
with a higher risk for premature death, diabetes type 2,
a lot easier. As a general rule,
hypertension, cardiovascular disease, stroke, gallbladder
disease, respiratory disfunction, osteoarthritis, and certain healthy BMIs=25.0 to 30.9;
kinds of cancers. overweight BMIs=31.0 to 35.2;
Obesity is also on the rise throughout the world. The obese BMIs=35.3 to 39.9; and
term globesity has been coined in recognition of the world-
morbidly obese BMIs=40 or more.
wide problem. In all regions, obesity appears to escalate as
income increases. In Brazil and Colombia, the ﬁgure of over- Your BMI is suppose to give you an idea of how much of
weight people is about 40%—comparable with a number of your weight is due to adipose tissue—that is, fat. In general,
European countries. the taller you are, the more you could weigh without it being
The Health Focus on page 157 tells us about the various due to fat. Using BMI in this way works for most people,
unhealthy ways people have tried to keep their weight especially if they tend to be sedentary. But your BMI number
under control. The conclusion is that, while dieting, “Eat a should be used as a general guide only because it does not
variety of foods, watch your weight, and exercise.” To reverse a take into account ﬁtness, bone structure, or gender. For exam-
trend toward obesity, eat fewer calories, be more active, and ple, a weight lifter might have an obese BMI, not because he
make wiser food choices. has body fat, but because he is very muscular.
Body Mass Index (BMI)
18 20 22 24 26 28 30 32 34 36
50 75 100 125 150 175 200 225 250 275
Source: U.S. Department of Agriculture: Dietary Guidelines for Americans 2005
Figure 8.11 Determining your BMI.
Match your weight with your height, then determine your body mass index (BMI). Healthy BMIs = 25.0 to 30.9; overweight BMIs = 31.0 to 35.2; obese BMIs =
35.3 to 39.9; morbidly obese BMIs = 40 or more.
mad86867_ch08_143-168.indd 156 12/1/06 2:37:44 PM
Chapter 8 Digestive System and Nutrition 157
Searching for the Magic Weight-Loss Bullet
While dieting, “Eat a variety of foods, watch your weight, and ex-
ercise” doesn’t sound very glamorous, and not only that, you
can’t sell it to the public and make a lot of money. No wonder
the public, which is always looking for the magic weight-loss
bullet, is offered so many solutions to being overweight, most
of which are not healthy. The solutions involve trendy diet
programs, new prescription medications, and even surgery.
The latter two options are for people who, like Monica, have
tried a low-calorie diet and, hopefully, regular physical activ-
ity but have been unsuccessful in losing weight. Prescription
medications should only be taken under a physician’s supervi-
sion, and, of course, surgeries are only done by physicians.
TRENDY DIET PROGRAMS: Various diets for the over-
weight have been around for many years, and here are some
recently touted: suits against the manufacturers of the prescription drug fen-
phen for causing heart problems and the belief that a drug
The Pritikin Diet This diet encourages the consumption
called Meridia elevates blood pressure, new drugs continue to
of large amounts of carbohydrates and ﬁber in the
enter clinical trials. A drug called L-Marc is now being tested
form of whole grains and vegetables. The diet is so
around the nation, and there is no lack of volunteers. Another
low-fat that the dieter may not be able to consume a
one called Acomplia, which is suppose to block pleasure
sufﬁcient amount of “healthy” fats.
receptors in the brain, is in late-stage clinical trials. Another
The Atkin's Diet This diet is just the opposite of the
idea is a weight-loss nasal spray. The availability of so many
Pritikin Diet because it is a low-carbohydrate (carb)
possible prescription drugs make it seem likely that the magic
diet. It is based on the assumption that if we eat
bullet may not be synonymous with the magic pill.
more protein and fat, our bodies lose weight by
burning stored body fat. The Atkin’s diet is thought SURGICAL PROCEDURES: For people who are morbidly
by many to be a serious threat to homeostasis obese, surgery may be recommended. There are three common
because it puts a strain on the body to maintain the types of weight-loss surgery; gastroplasty, gastric bypass,
blood glucose level, the breakdown of fat lowers and gastric banding. Gastroplasty involves partitioning or
blood pH, and the excretion of nitrogen from stapling a small section of the stomach to reduce total food
protein breakdown stresses the kidneys. intake. Gastric bypass surgery involves attaching the lower
The Zone Diet and the South Beach Diet As a reaction to part of the small intestine to the stomach, so that most of
the Atkin's diet, these diets recommend only “healthy” the food bypasses both the stomach and small intestine,
fats and permit low-sugar carbs. In other words, these decreasing the surface area for absorption. Gastric banding
diets are bringing us back, once again, to “Eat a variety is a new procedure in which stomach size is reduced using
of foods, maintain your weight, and exercise.” a constricting band. About one-third to one-half of people
You may also have heard of the caveman diet. A profes- who received obesity surgery lose signiﬁcant amounts
sor at Colorado State University goes back in time, claiming to of weight and keep this weight off for at least ﬁve years.
have found the lifestyle change necessary for weight manage- The downside of these operations is that people face the
ment. If you would rather just ﬂush your system of toxins and possibility of a lifetime of post-surgery problems, including
keep to your own lifestyle, some nutritionists claim that by chronic diarrhea, vomiting, intolerance to dairy products,
consuming certain foods, such as cayenne pepper, mustard, dehydration, and nutritional deﬁciencies, due to alterations
cinnamon, green vegetables, and omega-3-rich ﬁsh, you can in nutrient digestion and absorption. Careful consideration
boost your metabolic rate and ﬂush fat away. of the potential beneﬁts and risks must be weighed.
Then again, according to a professor at Brigham Young Many people also try liposuction to remove fat cells from
University, the cure for endless dieting and the key to reach- localized areas in the body, but this is not recommended to
ing a healthy weight is to listen to your body. Using a hunger treat obesity or morbid obesity. This procedure is clearly not
scale of 1 to 10, with 1 being starving and 10 being very over- the solution to long-term weight loss, as the millions of fat
full, keep around 3 to 5 and you will eat less. Unfortunately, cells that remain in the body after liposuction enlarge if the
such sensible advice doesn’t seem to have caught on yet. person continues to overeat.
We are facing an obesity epidemic and must do something
PRESCRIPTION DRUGS: Ever since Health and Human about it. Are these weight-loss options the answer? There are
Services Secretary Tommy Thompson said that Medicare and signs that the public is no longer searching for the magic bullet
Medicaid will consider obesity a disease, drugmakers have and, instead, is focusing on “To achieve or maintain a healthy
revved up the search for a magic weight-loss pill. Despite law- weight, eat a variety of healthy foods and exercise.”
mad86867_ch08_143-168.indd 157 12/1/06 2:38:01 PM
158 Part II Maintenance of the Human Body
Classes of Nutrients Table 8.3 Reducing High-Glycemic Index Carbohydrates
A nutrient can be deﬁned as a component of food that per-
forms a physiological function in the body. Nutrients pro- To reduce dietary sugar:
vide us with energy, promote growth and development, and
regulate cellular metabolism. 1. Eat fewer sweets, such as candy, soft drinks, ice cream, and pastry.
2. Eat fresh fruits or fruits canned without heavy syrup.
3. Use less sugar—white, brown, or raw—and less honey and syrups.
Carbohydrates are either simple or complex. Glucose is a
4. Avoid sweetened breakfast cereals.
simple sugar preferred by the body as an energy source.
Complex carbohydrates with several sugar units are digested 5. Eat less jelly, jam, and preserves.
to glucose. While body cells can utilize fatty acids as an 6. Eat fresh fruit; especially avoid artiﬁcial fruit juices.
energy source, brain cells require glucose. For this reason 7. When cooking, use spices, such as cinnamon, instead of sugar to ﬂavor
alone, it is necessary to include carbohydrates in the diet foods.
because the body is unable to convert fatty acids to glucose. 8. Do not put sugar in tea or coffee.
Any product made from reﬁned grains, such as white 9. Avoid potatoes and processed foods made from reﬁned carbohydrates,
bread, cake, and cookies, should be minimized in the diet. such as white bread, rice, and pasta.
During reﬁnement, ﬁber and also vitamins and minerals
are removed from grains, so primarily starch remains. In
contrast, sources of complex carbohydrates, such as beans,
peas, nuts, fruits, and whole-grain products, are recom- gators tell us that a chronically high insulin level may lead
mended as a good source of vitamins, minerals, and also to insulin resistance, diabetes type 2, and increased fat de-
ﬁber (Fig. 8.12). Insoluble ﬁber adds bulk to fecal material position. Deposition of fat is associated with coronary heart
and stimulates movements of the large intestine, prevent- disease, liver ailments, and several types of cancer.
ing constipation. Soluble ﬁber combines with bile salts and Table 8.3 gives suggestions on how to reduce your intake
cholesterol in the small intestine and prevents them from of dietary sugars.
Can Carbohydrates Be Harmful? Some nutritionists hy- Carbohydrates are the preferred energy source for the
pothesize that the high intake of reﬁned carbohydrates and body, but select complex carbohydrates in whole grains,
fructose sweeteners processed from cornstarch may be re- beans, nuts, and fruits, contain ﬁber, vitamins, and minerals
sponsible for obesity in the United States. In addition, these in addition to carbohydrate.
foods are said to have a high glycemic index (GI), because
the blood glucose response to these foods is high. When the
blood glucose level rises rapidly, the pancreas produces an
overload of insulin to bring the level under control. Investi-
Dietary proteins are digested to amino acids, which cells use
to synthesize hundreds of cellular proteins. Of the 20 differ-
ent amino acids, eight are essential amino acids that must
be present in the diet. Children will not grow if their diets
lack the essential amino acids. Eggs, milk products, meat,
poultry, and most other foods derived from animals contain
all nine essential amino acids and are “complete” or “high-
quality” protein sources.
Legumes (beans and peas) (Fig. 8.13), other types of veg-
etables, seeds and nuts, and also grains supply us with
amino acids, but each of these alone is an incomplete protein
source, because of a deﬁciency in at least one of the essential
amino acids. Absence of one essential amino acid prevents
utilization of the other 19 amino acids. Therefore, vegetari-
ans are counseled to combine two or more incomplete types
of plant products to acquire all the essential amino acids.
Also, tofu, soymilk, and other foods made from processed
soybeans are complete protein sources. A balanced vegetar-
Figure 8.12 Fiber-rich foods. ian diet is quite possible with a little planning.
Plants provide a good source of carbohydrates. They also provide a good Amino acids are not stored in the body, and a daily
source of vitamins, minerals , and ﬁber when they are not processed (reﬁned). supply is needed. However, it does not take very much
mad86867_ch08_143-168.indd 158 12/1/06 2:38:04 PM
Chapter 8 Digestive System and Nutrition 159
protein to meet the daily requirement. Two servings of Lipids
meat a day (one serving is equal in size to a deck of cards) Fats, oils, and cholesterol are lipids. Saturated fats, which
is usually plenty. are solids at room temperature, usually have an animal ori-
Can Proteins Be Harmful? While the body is harmed if gin. Two well-known exceptions are palm oil and coconut
the amount of protein in the diet is severely limited, it is oil, which contain mostly saturated fats and come from the
also likely harmed if the diet contains an overabundance plants mentioned (Fig. 8.14). Butter and meats, such as mar-
of protein. The average American eats about twice as much bled red meats and bacon, contain saturated fats.
protein as needed, and some people may be on a diet that Oils contain unsaturated fatty acids, which do not pro-
encourages the intake of proteins instead of carbohydrates mote cardiovascular disease. Corn oil and safﬂower oil are
as an energy source. Also, bodybuilders, weight lifters, and high in polyunsaturated fatty acids. Polyunsaturated oils
other athletes may include amino acid or protein supple- are nutritionally essential because they are the only type of
ments in the diet because they think these supplements fat that contains linoleic acid and linolenic acid, two fatty
will increase muscle mass. However, excess amino acids acids the body cannot make. Since these fatty acids must be
are not always converted into muscle tissue. The liver re- supplied by diet, they are called essential fatty acids.
moves the nitrogen portion during deamination and uses it Olive oil and canola oil are well known to contain a larger
to form urea, which is excreted in urine. The water needed percentage of monounsaturated fatty acids than other types of
for excretion of urea can cause dehydration when a person cooking oils. Omega-3 fatty acids—with a double bond in the
is exercising and losing water by sweating. High-protein third position—are believed to be especially protective against
diets, especially those rich in animal proteins, can also in- heart disease. Some cold-water ﬁshes, such as salmon, sardines,
crease calcium loss in urine. Excretion of calcium can lead and trout, are rich sources of omega-3, but they contain only
to kidney stones. about half that of ﬂaxseed oil, the best source from plants.
Certain types of meat, especially red meat, are known Can Lipids Be Harmful? Cardiovascular disease is often
to be high in saturated fats, while other sources of protein, due to arteries blocked by plaque, which contains saturated
such as chicken, ﬁsh, and eggs, are more likely to be low in fats and cholesterol. Cholesterol is carried in the blood by
saturated fats. As discussed next, an intake of saturated two types of lipoproteins: low-density lipoprotein (LDL)
fats leads to cardiovascular disease. and high-density lipoprotein (HDL). LDL is thought of as
“bad” because it carries cholesterol from the liver to the
Sufﬁcient proteins are needed to supply the essential cells, while HDL is thought of as “good” because it carries
amino acids. Meat and dairy sources of protein may supply cholesterol to the liver, which takes it up and converts it to
unwanted saturated fat, but vegetable sources do not. bile salts. Saturated fats tend to raise LDL cholesterol levels,
while unsaturated fats lower LDL cholesterol levels.
canola oil 6 62 32
safflower oil 9 12 79
olive oil 14 77 9
beef fat 51 44 5 14
butter 62 33 5 33
coconut oil 82 11 7
Numbers in Percent
% saturated fat % polyunsaturated fat
Figure 8.13 Beans as food. % monounsaturated fat % cholesterol
Beans are a good source of complex carbohydrates and protein. But beans
don’t supply all nine of the essential amino acids. To ensure a complete Figure 8.14 Fat and oils.
source of protein in the diet, beans should be eaten in combination with a This illustration gives the percentages of saturated and unsaturated fatty acids
grain, such as rice. in fats and oils. Which one do you judge to be the most healthy to use?
mad86867_ch08_143-168.indd 159 12/1/06 2:38:10 PM
160 Part II Maintenance of the Human Body
Trans-fatty acids (trans fats) arise when unsaturated
fatty acids are hydrogenated to produce a solid fat. Found Table 8.4 Reducing Certain Lipids
largely in commercially produced products, trans fats may
reduce the function of the cell-membrane receptors that To reduce saturated fats and trans fats in the diet:
clear cholesterol from the bloodstream. Trans fats are found
in commercially packaged goods, such as cookies and 1. Choose poultry, ﬁsh, or dry beans and peas as a protein source.
crackers; commercially fried foods, such as french fries 2. Remove skin from poultry, and trim fat from red meats before cooking;
from some fast-food chains; packaged snacks, such as place on a rack so that fat drains off.
microwaved popcorn; as well as in vegetable shortening 3. Broil, boil, or bake rather than fry.
and some margarines. Any packaged goods that contain 4. Limit your intake of butter, cream, trans fats, shortenings, and tropical oils
partially hydrogenated vegetable oils or “shortening” most (coconut and palm oils).*
likely contain trans fats. 5. Use herbs and spices to season vegetables instead of butter, margarine, or
Table 8.4 gives suggestions on how to reduce dietary sauces. Use lemon juice instead of salad dressing.
saturated fat and cholesterol. It is not a good idea to rely on 6. Drink skim milk instead of whole milk, and use skim milk in cooking and
commercially produced low-fat foods. In some products, baking.
sugars have replaced the fat, and in others, protein is used. To reduce dietary cholesterol:
Unsaturated fats such as those in oils do not lead 1. Avoid cheese, egg yolks, liver, and certain shellﬁsh (shrimp and lobster).
Preferably, eat white ﬁsh and poultry.
to cardiovascular disease and are preferred. Fats and
oils contain many more calories per gram than do 2. Substitute egg whites for egg yolks in both cooking and eating.
carbohydrates and protein. 3. Include soluble ﬁber in the diet. Oat bran, oatmeal, beans, corn, and fruits,
such as apples, citrus fruits, and cranberries, are high in soluble ﬁber.
Minerals are divided into major minerals and trace miner- Calcium
als. The body contains more than 5 grams of each major min- Calcium (Ca2) is a major mineral needed for the con-
eral and less than 5 grams of each trace mineral. Table 8.5 struction of bones and teeth and for nerve conduction and
lists the selected minerals and gives their functions and food muscle contraction. Many people take calcium supple-
sources. The major minerals are constituents of cells and ments to counteract osteoporosis, a degenerative bone
body ﬂuids and are structural components of tissues. disease that afﬂicts an estimated one-fourth of older men
The trace minerals are parts of larger molecules. For and one-half of older women in the United States. Osteo-
example, iron (Fe2) is present in hemoglobin, and iodine porosis develops because bone-eating cells called osteo-
(I) is a part of thyroxine and triiodothyronine, hormones clasts are more active than bone-forming cells called
produced by the thyroid gland. Zinc (Zn2), copper (Cu2), osteoblasts. Therefore, the bones are porous, and they
and manganese (Mn2) are present in enzymes that cata- break easily because they lack sufﬁcient calcium. Due to
lyze a variety of reactions. As research continues, more and recent studies that show consuming more calcium does
more elements are added to the list of trace minerals con- slow bone loss in elderly people, the guidelines have been
sidered essential. During the past three decades, for exam- revised. A calcium intake of 1,000 mg a day is recom-
ple, very small amounts of selenium, molybdenum, mended for men and for women who are premenopausal,
chromium, nickel, vanadium, silicon, and even arsenic and 1,300 mg a day is recommended for postmenopausal
have been found to be essential to good health. Table 8.5 women. To achieve this amount, supplemental calcium is
lists the functions of various minerals and gives their food most likely necessary.
sources and signs of deﬁciency and toxicity. Vitamin D is an essential companion to calcium in pre-
Occasionally, individuals do not receive enough iron venting osteoporosis. Other vitamins may also be helpful;
(especially women), calcium, magnesium, or zinc in their for example, magnesium has been found to suppress the
diets. Adult females need more iron in their diet than males cycle that leads to bone loss. In addition to adequate cal-
(18 mg compared with 10 mg) because they lose hemoglo- cium and vitamin intake, exercise helps prevent osteoporo-
bin each month during menstruation. Stress can bring on a sis. Risk factors for osteoporosis include drinking more
magnesium deﬁciency, and due to its high-ﬁber content, a than nine cups of caffeinated coffee per day and smoking.
vegetarian diet may make zinc less available to the body. Medications are also available that slow bone loss while
However, a varied and complete diet usually supplies increasing skeletal mass. These are still being studied for
enough of each type of mineral. their effectiveness and possible side effects.
mad86867_ch08_143-168.indd 160 12/1/06 2:38:10 PM
Chapter 8 Digestive System and Nutrition 161
Table 8.5 Minerals
Mineral Functions Food Sources Conditions with
Too Little Too Much
MAJOR (MORE THAN 100 MG/DAY NEEDED)
Calcium Strong bones and teeth, nerve Dairy products, Stunted growth in children, Kidney stones; interferes
(Ca2) conduction, muscle contraction leafy green vegetables low bone density in adults with iron and zinc absorption
Phosphorus Bone and soft tissue growth; Meat, dairy products, Weakness, confusion, Low blood and bone
(PO43) part of phospholipids, sunﬂower seeds, food pain in bones and joints calcium levels
ATP, and nucleic acids additives
Potassium Nerve conduction, Many fruits and Paralysis, irregular Vomiting, heart attack,
(K) muscle contraction vegetables, bran heartbeat, eventual death death
Sulfur (S2) Stabilizes protein shape, Meat, dairy products, Not likely In animals, depresses
neutralizes toxic substances legumes growth
Sodium (Na) Nerve conduction, Table salt Lethargy, muscle cramps, Edema, high blood
pH and water balance loss of appetite pressure
Chloride (Cl) Water balance Table salt Not likely Vomiting, dehydration
Magnesium Part of various enzymes Whole grains, Muscle spasm, irregular Diarrhea
(Mg2) for nerve and muscle leafy green vegetables heartbeat, convulsions,
contraction, protein synthesis confusion, personality changes
TRACE (LESS THAN 20 MG/DAY NEEDED)
Zinc (Zn2) Protein synthesis, wound healing, Meats, legumes, Delayed wound healing, Anemia, diarrhea,
fetal development and growth, whole grains night blindness, diarrhea, vomiting, renal failure,
immune function mental lethargy abnormal cholesterol levels
Iron (Fe2) Hemoglobin synthesis Whole grains, meats, Anemia, physical Iron toxicity disease, organ
prune juice and mental sluggishness failure, eventual death
Copper (Cu2) Hemoglobin synthesis Meat, nuts, legumes Anemia, stunted growth Damage to internal
in children organs if not excreted
Iodine (I) Thyroid hormone synthesis Iodized table salt, seafood Thyroid deﬁciency Depressed thyroid function, anxiety
Selenium Part of antioxidant enzyme Seafood, meats, eggs Vascular collapse, possible Hair and ﬁngernail
(SeO42) cancer development loss, discolored skin
Manganese Part of enzymes Nuts, legumes, green Weakness and confusion Confusion, coma, death
Sodium Table 8.6 Reducing Dietary Sodium
Sodium plays a major role in regulating the body’s water
balance, as does chloride (Cl). The recommended amount To reduce dietary sodium:
of sodium intake per day is 500 mg, although the average
American takes in 4,000–4,700 mg every day. In recent 1. Use spices instead of salt to ﬂavor foods.
years, this imbalance has caused concern because sodium 2. Add little or no salt to foods at the table, and add only small amounts of
in the form of salt intensiﬁes hypertension (high blood salt when you cook.
pressure) if you already have it. About one-third of the 3. Eat unsalted crackers, pretzels, potato chips, nuts, and popcorn.
sodium we consume occurs naturally in foods; another 4. Avoid hot dogs, ham, bacon, luncheon meats, smoked salmon,
third is added during commercial processing; and we add sardines, and anchovies.
the last third either during home cooking or at the table in 5. Avoid processed cheese and canned or dehydrated soups.
the form of table salt.
6. Avoid brine-soaked foods, such as pickles or olives.
Clearly, it is possible to cut down on the amount of sodium
in the diet. Table 8.6 gives recommendations for doing so. 7. Read nutrition labels to avoid high-salt products.
mad86867_ch08_143-168.indd 161 12/1/06 2:38:11 PM
162 Part II Maintenance of the Human Body
Vitamins Vitamins C, E, and A are believed to defend the body
against free radicals, and therefore, they are termed antioxi-
Vitamins are organic compounds (other than carbohydrate, dants. These vitamins are especially abundant in fruits and
fat, and protein) that the body uses for metabolic purposes vegetables. The food pyramid in Figure 8.16 suggests we eat
but is unable to produce in adequate quantity. Many vita- 2½ cups of vegetables and 2 cups of fruit a day. To achieve
mins are portions of coenzymes, which are enzyme helpers. this goal, think in terms of salad greens, raw or cooked veg-
For example, niacin is part of the coenzyme NAD, and ribo- etables, dried fruit, and fruit juice, in addition to apples and
ﬂavin is part of another dehydrogenase, FAD, discussed in oranges and other fresh fruits.
Chapter 3. Coenzymes are needed in only small amounts Dietary supplements may provide a potential safeguard
because each can be used over and over again. Not all vita- against cancer and cardiovascular disease, but nutritionists
mins are coenzymes; vitamin A, for example, is a precursor do not think people should take supplements instead of
for the visual pigment that prevents night blindness. If vita- improving their intake of fruits and vegetables. There are
mins are lacking in the diet, various symptoms develop (Fig. many beneﬁcial compounds in fruits that cannot be obtained
8.15). Altogether, there are 13 vitamins, which are divided from a vitamin pill. These compounds enhance one anoth-
into those that are fat soluble (Table 8.7) and those that are er’s absorption or action and also perform independent bio-
water soluble (Table 8.8). logical functions.
Antioxidants Vitamin D
Over the past 20 years, numerous statistical studies have been Skin cells contain a precursor cholesterol molecule that is
done to determine whether a diet rich in fruits and vegetables converted to vitamin D after UV exposure. Vitamin D
can protect against cancer. Cellular metabolism generates free leaves the skin and is modiﬁed ﬁrst in the kidneys and then
radicals, unstable molecules that carry an extra electron. The in the liver until ﬁnally it becomes calcitriol. Calcitriol pro-
most common free radicals in cells are superoxide (O2) and motes the absorption of calcium by the intestines. The lack
hydroxide (OH). In order to stabilize themselves, free radi- of vitamin D leads to rickets in children (Fig. 8.15a). Rick-
cals donate an electron to DNA, to proteins (including ets, characterized by bowing of the legs, is caused by defec-
enzymes), or to lipids, which are found in plasma membranes. tive mineralization of the skeleton. Most milk today is
Such donations most likely damage these cellular molecules fortiﬁed with vitamin D, which helps prevent the occur-
and thereby may lead to disorders, perhaps even cancer. rence of rickets.
a. b. c.
Figure 8.15 Illnesses due to vitamin deﬁciency.
a. Bowing of bones (rickets) due to vitamin D deﬁciency. b. Dermatitis (pellagra) of areas exposed to light due to niacin (vitamin B3) deﬁciency. c. Bleeding gums
are another early sign of vitamin C deﬁciency.
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Chapter 8 Digestive System and Nutrition 163
Table 8.7 Fat-Soluble Vitamins
Vitamin Functions Food Sources Conditions with
Too Little Too Much
Vitamin A Antioxidant synthesized from beta- Deep yellow/orange and leafy, Night blindness, Headache, dizziness,
carotene; needed for healthy eyes, dark green vegetables, fruits, impaired growth nausea, hair loss,
skin, hair, and mucous membranes, cheese, whole milk, butter, of bones and teeth abnormal development
and for proper bone growth eggs of fetus
Vitamin D A group of steroids needed for Milk fortiﬁed with vitamin D, Rickets, Calciﬁcation of soft tissues,
development and maintenance of ﬁsh liver oil; also made in the decalciﬁcation and diarrhea, possible
bones and teeth skin when exposed to sunlight weakening of bones renal damage
Vitamin E Antioxidant that prevents oxidation Leafy green vegetables, fruits, Unknown Diarrhea, nausea,
of vitamin A and polyunsaturated vegetable oils, nuts, whole- headaches, fatigue,
fatty acids grain breads and cereals muscle weakness
Vitamin K Needed for synthesis of substances Leafy green vegetables, Easy bruising Can interfere with
active in clotting of blood cabbage, cauliﬂower and bleeding anticoagulant medication
Table 8.8 Water-Soluble Vitamins
Vitamin Functions Food Sources Conditions with
Too Little Too Much
Vitamin C Antioxidant; needed for forming Citrus fruits, leafy green Scurvy, delayed wound Gout, kidney stones,
collagen; helps maintain capillaries, vegetables, tomatoes, healing, infections diarrhea, decreased
bones, and teeth potatoes, cabbage copper
Thiamine Part of coenzyme needed for cellular Whole-grain cereals, Beriberi, muscular Can interfere
(vitamin B1) respiration; also promotes activity of dried beans and peas, weakness, enlarged with absorption
the nervous system sunﬂower seeds, nuts heart of other vitamins
Riboﬂavin Part of coenzymes, such as FAD;* Nuts, dairy products, whole- Dermatitis, blurred Unknown
(vitamin B2) aids cellular respiration, including grain cereals, poultry, vision, growth failure
oxidation of protein and fat leafy green vegetables
Niacin Part of coenzymes NAD;+ needed Peanuts, poultry, whole- Pellagra, diarrhea, High blood sugar
(nicotinic for cellular respiration, including grain cereals, leafy green mental disorders and uric acid,
acid) oxidation of protein and fat vegetables, beans vasodilation, etc.
Folacin Coenzyme needed for production of Dark leafy green Megaloblastic May mask B12
(folic acid) hemoglobin and formation of DNA vegetables, nuts, beans, anemia, spina biﬁda deﬁciency
Vitamin B6 Coenzyme needed for synthesis Whole-grain cereals, bananas, Rarely, convulsions, Insomnia,
of hormones and hemoglobin; CNS beans, poultry, nuts, leafy vomiting, seborrhea, neuropathy
control green vegetables muscular weakness
Pantothenic Part of coenzyme A needed for Nuts, beans, dark green Rarely, loss of appetite, Unknown
acid oxidation of carbohydrates and fats; vegetables, poultry, mental depression,
aids in the formation of hormones fruits, milk numbness
and certain neurotransmitters
Vitamin B12 Complex, cobalt-containing compound; Dairy products, ﬁsh, Pernicious anemia Unknown
part of the coenzyme needed for poultry, eggs,
synthesis of nucleic acids and myelin fortiﬁed cereals
Biotin Coenzyme needed for metabolism of Generally in foods, especially Skin rash, nausea, Unknown
amino acids and fatty acids eggs fatigue
*FAD ﬂavin adenine dinucleotide
NAD nicotinamide adenine dinucleotide
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164 Part II Maintenance of the Human Body
How to Plan Nutritious Meals
Many serious disorders in Americans are linked to a diet that
results in excess body fat. While genetics is a factor in being
overweight, a person cannot become fat without taking in
more food energy (Calories, Kcal) than is needed. A person
needs calories (energy) for their basal metabolism, which is
the number calories your body burns at rest to maintain nor-
mal body functions, and a person needs calories for exercise.
The less exercise, the less calories that are needed beyond the
basal metabolic rate. So, the ﬁrst step in planning a diet is to
limit the number of calories to an amount that you will use up
each day. Let’s say you do all the necessary calculations
(which are beyond the scope of this book) and discover that,
as a woman, the maximum number of calories you can take in
each day is 2,000, and as a man, you can afford 2,500 calories GRAINS VEGETABLES FRUITS MILK MEAT & BEANS
without the anxiety of gaining weight. Eat 6 oz. Eat 21/2 cups Eat 2 cups Consume 3 Eat 51/2 oz.
Now, the new food pyramid developed by the U.S. Depart- every day every day every day cups every every day
ment of Agriculture (USDA) can be used to help you decide day; ages
2–8, 2 cups
how those calories should be distributed among the foods to
Source: U.S. Dept. of Agriculture
eat (Fig. 8.16). The new pyramid emphasizes food that should
be eaten often and omits foods that should not be eaten on a Figure 8.16 The 2005 USDA food guide pyramid.
regular basis. Additionally, the USDA provides recommenda- This food guide pyramid can be helpful when planning nutritious meals
tions concerning the minimum quantity of foods in each group and snacks.
that should be eaten daily. In general ,we should:
• eat a variety of foods. Foods from all food groups
should be included in the diet. dry beans and peas can be used to meet protein needs.
• eat more of these foods: fruits, vegetables, whole Choose lean meats such as poultry, ﬁsh high in omega-
grains, and fat-free or low-fat milk products. Choose 3 fatty acids, such as salmon, trout, and herring, in
dark-green vegetables, orange vegetables, and dry moderate sized portions. Include oils that are rich in
beans and peas over potatoes, corn, or green peas more monounsaturated and polyunsaturated fatty acids in
often. When eating grains, choose whole grains such as the diet. Choose and prepare foods and beverages with
brown rice, oatmeal, whole wheat bread. Choose fruit little added sugars and salt.
as a snack or a toping for foods, instead of sugar. • be physically active every day. If weight loss is needed,
• eat less of foods high in saturated or trans fats, added decrease calorie intake slowly, while maintaining
sugars, cholesterol, salt, and alcohol. Remember that adequate nutrient intake and increasing physical activity.
Table 8.9 Preparing Meals and Snacks Using the Food Guide Pyramid
Breakfast Snack Lunch Snack Dinner Dessert
½ cup oat ﬂakes cereal Whole-wheat mufﬁn Toasted American Apple 3 oz. hamburger ½ cup frozen vanilla
cheese sandwich patty yogurt topped with
(whole-grain bread) sliced strawberries
8 oz. nonfat milk 2 Tbsp peanut butter 1 cup vegetable salad 4 graham Whole-wheat bun
mixed with 1 tsp crackers
½ cup blueberries 8 oz. nonfat milk 1 Tbsp olive oil and ½ cup baked
vinegar salad dressing beans
1 slice whole-grain toast 4 oz. tomato juice 1 cup steamed
1 tsp soft maragarine 1 tsp soft maragarine
4 oz. orange juice
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Chapter 8 Digestive System and Nutrition 165
a. Anorexia nervosa b. Bulimia nervosa c. Muscle dysmorphia
Figure 8.17 Eating disorders.
a. People with anorexia nervosa have a mistaken body image and think they are fat, even though they are thin. b. Those with bulimia nervosa overeat and then
purge their bodies of the food they have eaten. c. People with muscle dysmorphia think their muscles are underdeveloped. They spend hours at the gym and are
preoccupied with diet as a way to gain muscle mass.
Table 8.9 gives you a sample menu, developed by a nutrition- weights are often normal and they tend to conceal their bing-
ist, so you can see what a good diet for one day would look ing and purging practices. Women are more likely than men
like. Table 8.9 also gives you an idea of what a nutritionist to develop bulimia; an estimated 4% of young women suffer
means by a “serving size.” from this condition.
Other abnormal eating practices include binge-eating dis-
order and muscle dysmorphia. Many obese people suffer
Eating Disorders from binge-eating disorder, a condition characterized by epi-
People with eating disorders are dissatisﬁed with their body sodes of overeating that are not followed by purging. Stress,
image. Social, cultural, emotional, and biological factors all anxiety, anger, and depression can trigger food binges. A per-
contribute to the development of an eating disorder. Serious son suffering from muscle dysmorphia (Fig. 8.17c) thinks his
conditions such as obesity, anorexia nervosa, and bulimia or her body is underdeveloped. Body-building activities and
nervosa can lead to malnutrition, disability, and death. a preoccupation with diet and body form accompany this
Regardless of the eating disorder, early recognition and condition. Each day, the person may spend hours in the gym
treatment are crucial. working out on muscle-strengthening equipment. Unlike
Anorexia nervosa is a severe psychological disorder anorexia nervosa and bulimia, muscle dysmorphia affects
characterized by an irrational fear of getting fat that results more men than women.
in the refusal to eat enough food to maintain a healthy body
weight (Fig. 8.17a). A self-imposed starvation diet is often
accompanied by occasional binge eating that is followed by Check Your Progress 8.6
purging and extreme physical activity to avoid weight gain. 1. a. What is the rationale behind using BMI to judge obesity?
Binges usually include large amounts of high-calorie foods, b. What disorders are associated with obesity?
and purging episodes involve self-induced vomiting and 2. Why might carbohydrates and why might fats be the cause of the
laxative abuse. About 90% of people suffering from anorexia obesity epidemic today?
nervosa are young women; an estimated 1 in 200 teenage 3. a. What type fats are dangerous to our health, and (b) what type
girls is affected. is protective for cardiovascular disease?
A person with bulimia nervosa binge-eats, and then 4. Proteins provide the essential amino acids; why shouldn’t they be
purges to avoid gaining weight (Fig. 8.17b). The binge-purge eaten as an energy source?
cycle behavior can occur several times a day. People with 5. In general, what does the new USDA food pyramid tell you?
bulimia nervosa can be difﬁcult to identify because their body 6. Name and describe four eating disorders.
mad86867_ch08_143-168.indd 165 12/1/06 2:38:30 PM
166 Part II Maintenance of the Human Body
Summarizing the Concepts 8.5 The Large Intestine and Defecation
• The large intestine consists of the cecum, the colon
(including the ascending, transverse, and descending
8.1 Overview of Digestion colon), and the rectum, which ends at the anus.
The organs of the digestive system are located within • The large intestine absorbs water, salts, and some
the GI tract. The processes of digestion require ingestion, vitamins; forms the feces; and carries out defecation.
digestion, movement, absorption, and elimination. All • Disorders of the large intestine include diverticulosis,
parts of the tract have four layers, called the mucosa, irritable bowel syndrome, inﬂammatory bowel disease,
submucosa, muscularis, and serosa. polyps, and cancer.
8.2 First Part of the Digestive Tract 8.6 Nutrition and Weight Control
• In the mouth, teeth chew the The nutrients released by the
food, saliva contains salivary digestive process should
amylase for digesting starch, provide us with adequate
and the tongue forms a bolus for energy, essential amino
swallowing. acids and fatty acids, and
• The air passage and food all necessary vitamins
passages cross in the pharynx. and minerals. Today,
During swallowing, the air
obesity is on the
passage is blocked off by the soft
palate and epiglottis; food enters increase, possibly GRAINS VEGETABLES FRUITS MILK MEAT & BEANS
the esophagus, and peristalsis because people
begins. The esophagus moves eat too much food and make improper choices of food.
food to the stomach. Obesity is associated with many illnesses, including
diabetes type 2 and cardiovascular disease. The food guide
8.3 The Stomach and Small pyramid shows foods to emphasize and foods to minimize
Intestine for good health.
• The stomach expands and • Carbohydrates are necessary in the diet, but simple sugars
stores food and also churns, mixing food with the acidic and reﬁned starches cause a rapid release of insulin that
gastric juices. This juice contains pepsin, an enzyme that can lead to diabetes type 2.
• Proteins supply essential amino acids.
• The duodenum of the small intestine receives bile from
• Unsaturated fatty acids, particularly the omega-3 fatty
the liver and pancreatic juice from the pancreas. Bile
acids, are protective against cardiovascular disease.
emulsiﬁes fat and readies it for digestion by lipase.
• Saturated fatty acids lead to plaque, which occludes
• The pancreas produces enzymes that digest starch
(pancreatic amylase), protein (trypsin), and fat (lipase). The
intestinal enzymes ﬁnish the process of chemical digestion. • Vitamins and minerals are also required by the body in
• Small nutrient molecules are absorbed at the villi in the
walls of the small intestine.
8.4 Three Accessory Organs and Regulation Understanding Key Terms
Three accessory organs of digestion send secretions to the absorption 145 epiglottis 147
duodenum via ducts. These organs are the pancreas, liver, anorexia nervosa 165 esophagus 147
and gallbladder. anus 154 essential amino acids 158
• The pancreas produces pancreatic juice, which contains appendix 145 essential fatty acids 159
digestive enzymes for carbohydrate, protein, and fat. bile 150, 152 ﬁber 154
• The liver produces bile, destroys old blood cells, detoxiﬁes binge-eating disorder 165 gallbladder 152
blood, stores iron, makes plasma proteins, stores glucose bolus 147 gallstone 152
as glycogen, breaks down glycogen to glucose, produces body mass index (BMI)-156 gastric 149
urea, and helps regulate blood cholesterol levels. bulimia nervosa 165 gastric gland 149
• The gallbladder stores bile, which is produced by the liver. cecum 154 glottis 147
The secretions of digestive juices chyme 144 glycemic index (GI) 158
are controlled by the nervous cirrhosis 153 hard palate 146
system and by hormones. colon 154 heartburn 147
• Gastrin produced by the lower part constipation 154 hemorrhoid 155
of the stomach stimulates via the defecation 154 hepatitis 153
bloodstream the upper part of the dental caries 146 hormone 152
stomach to secrete pepsin. diaphragm 147 ingestion 144
• Secretin and CCK produced by diarrhea 154 jaundice 153
the duodenal wall stimulate the digestion 144 lacteal 150
pancreas to secrete its juices and duodenum 150 lactose intolerance 150
the gallbladder to release bile. elimination 145 large intestine 154
mad86867_ch08_143-168.indd 166 12/1/06 2:38:41 PM
Chapter 8 Digestive System and Nutrition 167
lipase 150 pharynx 147 9. Tracing the path of food in the following list (a–f), which step
liver 150 plaque 159 is out of order ﬁrst?
lumen 145 polyp 155 a. mouth d. small intestine
mineral 160 rectum 154 b. pharynx e. stomach
movement 144 rugae 149 c. esophagus f. large intestine
mucosa 145 salivary amylase 146
muscle dysmorphia 165 salivary gland 146 10. The appendix connects to the
muscularis 145 serosa 145 a. cecum. d. liver.
nutrient 158 small intestine 150 b. small intestine. e. All of these are correct.
obesity 156 soft palate 146 c. esophagus.
osteoporosis 160 sphincter 147 11. Which association is incorrect?
pancreas 152 stomach 148 a. mouth—starch digestion
pancreatic amylase 152 submuscosa 145 b. esophagus—protein digestion
pepsin 149 trypsin 152 c. small intestine—starch, lipid, protein digestion
periodontitis 146 vermiform appendix 154 d. stomach—food storage
peristalsis 147 villus 150 e. liver—production of bile
peritonitis 145 vitamin 162
12. Why can a person not swallow food and talk at the same time?
Match the key terms to these deﬁnitions. a. In order to swallow, the epiglottis must close off the trachea.
b. The brain cannot control two activities at once.
a. Essential requirement in the diet, needed in small c. In order to speak, air must come through the larynx to
amounts. Often a part of a coenzyme. form sounds.
b. Fat-digesting enzyme secreted by the pancreas. d. A swallowing reﬂex is only initiated when the
mouth is closed.
c. Lymphatic vessel in an intestinal villus; it aids in e. Both a and c are correct.
the absorption of fats.
13. Which association is incorrect?
d. Muscular tube for moving swallowed food from the a. pancreas—produces alkaline secretions and enzymes
pharynx to the stomach. b. salivary glands—produce saliva and amylase
e. Organ attached to the liver that serves to store and c. gallbladder—produces digestive enzymes
concentrate bile. d. liver—produces bile
14. Which of the following could be absorbed directly
Testing Your Knowledge of the Concepts without need of digestion?
a. glucose d. protein
b. fat e. nucleic acid
1. Argue that absorption is the most important of the ﬁve c. polysaccharides
processes of digestion over the other four processes.
(pages 144–45) 15. Peristalsis occurs
a. from the mouth to the small intestine.
2. List the main organs of the digestive tract, and state b. from the beginning of the esophagus to the anus.
the contribution of each to the digestive process. c. only in the stomach.
(pages 146–47) d. only in the small and large intestine.
3. Discuss the absorption of the products of digestion into the e. only in the esophagus and stomach.
lymphatic and cardiovascular systems. (page 150)
16. An organ is a structure made of two or more tissues
4. Name the enzymes involved in the digestion of starch, performing a common function. Which of the four tissue
protein, and fat, and tell where these enzymes are active and types are present in the wall of the digestive tract?
what they do. (page 151) a. epithelium d. muscle tissue
b. connective tissue e. All of these are correct.
5. Why are the pancreas, liver, and gallbladder considered
c. nervous tissue
accessory organs of digestion and not an organ of
digestion? (page 152) 17. Which association is incorrect?
6. Name and state the functions of the hormones that assist a. protein—trypsin d. maltose—pepsin
the nervous system in regulating digestive secretions. b. fat—bile e. starch—amylase
(page 153) c. fat—lipase
7. What is the chief contribution of each of these in the body— 18. Most of the products of digestion are absorbed across the
carbohydrates, proteins, fats, fruits, and vegetables? (pages a. squamous epithelium of the esophagus.
158–59, 162) b. striated walls of the trachea.
c. convoluted walls of the stomach.
8. Which three eating disorders involve binge eating? How are d. ﬁngerlike villi of the small intestine.
these three disorders different from one another? (page 165) e. smooth wall of the large intestine.
mad86867_ch08_143-168.indd 167 12/1/06 2:38:41 PM
168 Part II Maintenance of the Human Body
19. Bile 34. Two layers of smooth muscle
a. is an important enzyme for the digestion of fats.
In questions 35–40, match each statement to an answer in the key.
b. cannot be stored.
Answers are used more than once. Some may have more than one
c. is made by the gallbladder.
d. emulsiﬁes fat.
e. All of these are correct. Key:
a. gastrin d. All of these are correct.
20. Which of the following is not a function of the liver in adults? b. secretin e. None of these is correct.
a. produces bile d. produces urea c. CCK
b. detoxiﬁes alcohol e. makes red blood cells
c. stores glucose 35. Stimulates gallbladder to release bile
21. The large intestine 36. Hormone carried in bloodstream
a. digests all types of food. 37. Stimulates the stomach to digest protein
b. is the longest part of the intestinal tract.
c. absorbs water. 38. Enzyme that digests food
d. is connected to the stomach. 39. Secreted by duodenum
e. is subject to hepatitis.
40. Secreted by the stomach
In questions 22–26, match each function to an organ in the key.
In questions 41–45, match each statement to a vitamin or mineral
Key: in the key.
a. mouth b. esophagus
c. stomach d. small intestine Key:
e. large intestine a. calcium d. iodine
b. vitamin K e. vitamin A
22. Removes nondigestible remains c. sodium
23. Serves as a passageway 41. Needed to make thyroid hormone
24. Stores food 42. Needed for night vision
25. Absorbs nutrients 43. Needed for bones, teeth, and muscle contraction
26. Receives food 44. Needed for nerve conduction, pH, and water balance
27. The amino acids that must be consumed in the diet are called 45. Needed for making clotting proteins
essential. Nonessential amino acids
a. can be produced by the body.
b. are only needed occasionally.
Thinking Critically About the Concepts
c. are stored in the body until needed.
The opening story discusses bariatric surgery, which reduces
d. can be taken in by supplements.
the size of the stomach and enables food to bypass a section of
28. Which of the following are often organic portions of the small intestine. The surgery is generally done when obese
important coenzymes? individuals have unsuccessfully tried numerous ways to lose
a. minerals c. protein weight and their health is compromised by their weight. There
b. vitamins d. carbohydrates are many risks associated with the surgery, but it helps a number
29. The products of digestion are of people lose a considerable amount of weight. After people
a. large macromolecules needed by the body. undergo the surgery, there are several lifestyle changes they must
b. enzymes needed to digest food. make to avoid nutritional deﬁciencies and to compensate for the
c. small nutrient molecules that can be absorbed. small size of their stomach.
d. regulatory hormones of various kinds. 1a. Why do some people who had bariatric surgery process
e. the food we eat. their food in a blender (or have to chew thoroughly) before
In questions 30–34, match each statement to a layer of the wall of swallowing their food?
the esophagus in the key. Answers are used more than once. b. Why should people who had bariatric surgery drink liquids
between meals rather than with meals?
a. mucosa c. muscularis 2. What risk is there to the esophagus after bariatric surgery?
b. submucosa d. serosa
3. The text on page 149 describes how William Beaumont was
30. Loose connective tissue that contains lymph nodules able to observe the workings of the stomach through a ﬁstula
in St. Martin’s stomach.
31. Contains a layer of epithelium that lines the lumen
a. What do you think Beaumont observed after he inserted a
32. Very thin layer of squamous epithelium that secretes a ﬂuid,
piece of meat into St. Martin’s stomach?
keeping the organ moist
b. What do you think Beaumont observed after he inserted a
33. Contains digestive glands and mucus-secreting goblet cells piece of bread into St. Martin’s stomach?
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