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					   Digestion, Nutrition,
      and Excretion

Lifetime Nutrition and Wellness
What does the digestive system do?

• Stores and digests food

• Transfers nutrients to the body

• Eliminates waste

• Absorbs water into the body
The Digestion Process

• Digestion- the process of breaking down
  food into usable nutrients

• Digestive System- a long, hollow tube that
  extends from the mouth through the entire
  body
Human Digestive System
5 Tasks of Digestive Systems

  1. Mechanical processing
  2. Secretion
  3. Digestion
  4. Absorption of water into the body
  5. Elimination of waste
    The Mouth

•    Mechanical and chemical digestions both begin in
     the mouth.
•    Chewing is the first step in mechanical digestion.
•    During chewing, salivary glands produce saliva,
     which mixes with the chewed food. Enzymes in
     the saliva kill bacteria and begin the process of
     chemical digestion by breaking down starches to
     sugars.
The Mouth
•       Human teeth are well adapted for chewing many
        kinds of food. The 32 teeth of the normal adult
        have three basic shapes, each with a different
        function:
    •     Incisors - sharp front teeth used for biting into and
          tearing pieces of food.
    •     Canines - pointed teeth (vampire) next to incisors,
          used to tear or shred food.
    •     Molars - teeth at the back of the mouth, have large
          flat surfaces that crush and grind food.
Mechanical Processing




                        Digestive - 8
The Mouth

• Once the teeth and salivary glands have completed the initial
  processing, the food is ready to be swallowed.
• Gathering the food together in a ball called a bolus(soft,
  flexible mass;) the tongue pushes it toward the back of the
  mouth and the pharynx.
• The pharynx is an area at the back of the throat that connects
  the nose and mouth to the GI track and respiratory tracts.
• In the pharynx, the GI track and the respiratory system cross
  each other.
• As the tongue moves food into the pharynx, it presses down
  on a small flap of cartilage called the epiglottis. When the
  epiglottis is depressed, it closes the entrance to the respiratory
  track and guides the food down the GI track.
The Esophagus
• Once the bolus enters the esophagus, muscles in
  the esophagus wall move food toward the
  stomach.
   • Waves of muscular contractions called peristalsis move
     food through the digestive track.
   • Contractions of the muscles move the bolus to a valve
     called the sphincter where the esophagus joins the
     stomach. The sphincter allows food to pass into the
     stomach but usually not letting it move back up into the
     esophagus.
The Esophagus

Collapsible, muscular tube
Conducts food through the thoracic cavity and
 diaphragm into the stomach through
 peristalsis.
No chemical digestion occurs
The Stomach
• The partially digested food is now in the stomach.
• The stomach is a muscular sac with thick,
  expandable walls.
• The stomach walls are made of layers of muscles
  that contract in opposite directions.
• Mechanical digestion occurs when the stomach
  walls contract strongly, mixing and churning the
  food. These contractions are responsible for the
  "growling" noises our stomach makes, they are the
  loudest when we have an empty stomach.
The Stomach
• Chemical digestion in the stomach begins with the
  actions of hydrochloric acid and an enzyme called
  pepsin. Glands in the stomach secrete both
  substances.
• Pepsin breaks down protein, and works best in an
  acidic environment, which is provided by the
  hydrochloric acid.
• Another fluid secreted by glands in the stomach is
  mucus. Mucus lubricates food so that it can travel
  through the digestive tract more easily.
• Mucus also coats the walls of the stomach, protecting
  the muscle tissue from being broken down by other
  digestive fluids.
The Stomach

• After about three hours (2-3 hours) of mechanical
  and chemical treatment in the stomach, food is
  reduced to a soft pulp called chyme.
   • Chyme is a thick liquid made up of partially digested
     proteins, starches, and acids, and undigested sugars and
     fats.
• At this point, the pyloric valve between the
  stomach and small intestine opens, allowing small
  amounts of chyme to pass into the small intestine.
The Stomach

• By the time chyme has left the stomach,
  most proteins have been broken down into
  smaller polypeptides.
  • Sugars and fats have not yet been chemically
    altered.
  • Some starch molecules have been broken down
    into disaccharides.
The Stomach

• Expands to store food
• Mixes with gastric juices:
   • Hydrochloric acid
   • Mucus (protective)
   • Pepsin
• In 2-6 hours soupy chyme leaves the
  stomach
The Small Intestine
• As chyme is pushed through the pyloric valve, it
  enters the duodenum, the first part of the small
  intestine.
• The small intestine performs three major functions
  on chyme that enters from the stomach.
   • The small intestine digests carbohydrates and fats
   • completes the digestion of proteins
   • absorbs digested nutrients.
• The small intestine is long (7m—23 ft), but its
  diameter (2.5cm) is smaller than the large
  intestine.
  The Small Intestine
• Some of the digestive fluids that contain enzyme
  activators and enzymes that digest food in the small
  intestine come from glands located in the small intestine.
• These glands produce enzymes that digest proteins and
  carbohydrates.
• The pancreas, an organ located behind the stomach,
  secretes pancreatic fluid into the small intestine.
• Pancreatic fluid contains enzymes that digest proteins,
  fats, and carbohydrates.
• Pancreatic fluid also contains sodium bicarbonate, which
  neutralizes the hydrochloric acid in chyme, protecting the
  small intestine.
The Small Intestine
• The liver is a large brownish organ that lies above
  the stomach in the abdomen. One of the functions
  of the liver is to secrete a yellow-brown liquid
  called bile.
• Bile is stored in a small sac called the gallbladder.
  The entrance of food into the small intestine
  stimulates the release of bile to the small intestine
  through a duct.
• Bile is produced by the liver and stored in the
  gallbladder until needed.
• Fats in the small intestine are broken down into
  smaller droplets by bile.
The Small Intestine

• Longest segment of digestive tract
• Receives chyme from stomach
• First section (duodenum) is where most
  digestion takes place
• Receives secretions from liver, gallbladder,
  and pancreas
Secretions into Small Intestine
from Accessory Structures
• Liver – produces bile
• Gallbladder – stores and secretes bile to
  emulsify fat
• Pancreas – pancreatic juice, contains:
   • Trypsin – digest proteins
   • Lipase – digest fats (lipids)
   • NaHCO2 – basic, neutralizes the acidic
     chyme
Additional Functions of the Liver

•   Detox blood
•   Store irons and minerals
•   Make plasma protein
•   Store glucose as glycogen
•   Produce urea from amino acids
•   Remove bilirubin
•   Regulate blood cholesterol level when
    producing bile salts
Hormonal control of digestive gland
secretions




                               Digestive - 27
    The Small Intestine
•    One of the main functions of bile is to dissolve
     cholesterol. Bile is a salt containing detergent and if
     the amount of salt in the bile is insufficient, sharp,
     painful crystals can form, known as gallstones.
•    Most nutrients are absorbed into the bloodstream
     through the cells that line the small intestine.
•    The internal surface of the intestine is lined with
     fingerlike projections called villi.
•    Villi increase the surface area of the lining of the
     small intestine, making absorption more efficient.
    The Small Intestine
•       Nutrients are absorbed through blood vessels and lymph
        vessels in the villi.
    •      Blood vessels absorb carbohydrates (sugars) and proteins (amino
           acids).
    •      Lymph vessels called lacteals absorb fats and fatty acids. Most of
           the nutrients used by the body are absorbed through the lining of
           the small intestine.
•       Undigested material leaves the small intestine through a
        valve and enters the large intestine or colon.
•       An organ called the appendix is located near the junction of
        the small and large intestine. The appendix is a finger-
        shaped pouch, which does not serve any known function. If
        the appendix becomes infected with bacteria, resulting in
        appendicitis, the appendix must be removed.
Walls of Small Intestine
• Projections increase
  surface area
  available for
  absorption




                           one villus
                                      INTESTINAL LUMEN


Nutrient
Absorption                                          monosaccharides
                           carbohydrates

• Occurs mainly in        proteins               amino acids

  small intestine
• Various methods of
  absorption:
• Osmosis, transport     EPITHELIAL
                            CELL
  proteins, diffusion



                          INTERNAL
                        ENVIRONMENT
     Fat Absorption
     bile salts

                                      bile salts
                                            +
                                                          micelles
 fat globules     emulsification    fatty acids,
(triglycerides)     droplets       monoglycerides




                                                    triglycerides + proteins
 EPITHELIAL
    CELL
                                                              chylomicrons



                     INTERNAL ENVIRONMENT
The Large Intestine or Colon
•   The large intestine, also called the colon, is
    about 6 cm wide and 1.5 m long.
•   The large intestine absorbs water from the
    material remaining in the digestive tract.
•   Water-soluble vitamins are absorbed along with
    the water.
•   When most of the water has been removed from
    the undigested material, a solid waste matter
    called feces remains.
The Large Intestine or Colon

•   Peristalsis propels the feces through the large
    intestine and into the rectum, the last few inches
    of the large intestine. Feces collected in the
    rectum are eliminated through the anus.
•   Sometimes a disease or disorder prevents the
    large intestine from absorbing enough water -
    the result is diarrhea, or watery feces. Severe
    diarrhea can result in a loss of water, or
    dehydration, that can be fatal
Large Intestine
• Includes: cecum, colon, rectum, anus
• Concentrates and stores feces
• Does not produce digestive enzymes
• Absorbs water, salts, and some vitamins
  Actively transports sodium ions out of
  lumen; water follows
• Resident bacteria produce vitamins
6. Large Intestine (colon) - large 5-
    foot tube framing the small intestines.
Note:
 cecum
 appendix
 rectum

Colon contains
 large number
 of vitamin-
 producing
 bacteria [B1, B2, B6, B12, folic acid & biotin]
Cecum and Appendix

                   ascending
                   portion of large
                   intestine




                        last
                        portion of
                        small
cecum   appendix        intestine



                                      Fig. 24-8, p.409
Defecation reflex




                    Digestive - 38
Fats (3-6 hours)
Proteins (3 hrs)
Carbs (1-2 hrs)

				
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posted:3/29/2013
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