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Digestive system-Sum091

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					                                      Digestive System Notes



Learning Objectives:
1. List the organs of the alimentary canal and the accessory digestive organs.
2. Name and define the six digestive processes.
3. Explain the relationship of the digestive organs to the peritoneum.
4. Identify the four layers of the alimentary canal organs.
5. Describe the anatomy of the mouth, lips, cheeks, palate, tongue, salivary glands, teeth, pharynx,
    and esophagus.
6. Explain the processes of mastication and deglutition.
7. Discuss the gross and microscopic anatomies of the stomach.
8. List and explain the phases of the regulation of gastric secretion.
9. Examine gastric motility and emptying.
10. Describe the gross and microscopic anatomies of the small intestine.
11. Define the roles of the liver, gall bladder, and pancreas in digestion.
12. Discuss the motility of the small intestine and its requirements for optimal activity.
13. Explain the gross and microscopic anatomies of the large intestine.
14. Describe defecation and the motility of the large intestine.
15. Define chemical digestion and explain the process as it relates to the breakdown of
    carbohydrates, proteins, lipids, and nucleic acids.
16. Describe the absorption of carbohydrates, proteins, lipids, nucleic acids, vitamins, electrolytes,
    and water.



I. Digestive System: Overview
       A. The alimentary canal or gastrointestinal (GI) tract digests and absorbs food
       B. Alimentary canal – mouth, pharynx, esophagus, stomach, small intestine, and large
           intestine
       C. Accessory digestive organs – teeth, tongue, gallbladder, salivary glands, liver, and pancreas


II. Digestive Process
       A. The GI tract is a “disassembly” line
               1. Nutrients become more available to the body in each step
       B. There are six essential activities:
               1. Ingestion, propulsion, and mechanical digestion
               2. Chemical digestion, absorption, and defecation
       C. Gastrointestinal Tract Activities
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              1. Ingestion – taking food into the digestive tract
              2. Propulsion – swallowing and peristalsis
              3. Peristalsis – waves of contraction and relaxation of muscles in the organ walls
              4. Mechanical digestion – chewing, mixing, and churning food
              5. Chemical digestion – catabolic breakdown of food
                      a. hydrolysis – catalyzed by digestive enzymes in digestive juices
              6. Absorption – movement of nutrients from the GI tract to the blood or lymph
              7. Defecation – elimination of indigestible solid wastes


III. Peritoneum and Peritoneal Cavity
       A. Peritoneum – serous membrane of the abdominal cavity
       B. Visceral – covers external surface of most digestive organs
       C. Parietal – lines the body wall
       D. Peritoneal cavity
              1. Lubricates digestive organs
              2. Allows them to slide across one another
       E. Mesentery – double layer of peritoneum that provides:
              1. Vascular and nerve supplies to the viscera
              2. Hold digestive organs in place and store fat
       F. Retroperitoneal organs – organs outside the peritoneum
       G. Peritoneal organs (intraperitoneal) – organs surrounded by peritoneum


IV. Histology of the Alimentary Canal
       A. From esophagus to the anal canal the walls of the GI tract have the same four tunics
       B. From the lumen outward they are the mucosa, submucosa, muscularis externa, and serosa
       C. Each tunic has a predominant tissue type and a specific digestive function
       D. Mucosa
              1. Moist epithelial layer that lines the lumen of the alimentary canal
              2. Three major functions:
                      a. Secretion of mucus
                      b. Absorption of end products of digestion
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                    c. Protection against infectious disease
             3. Consists of three layers: a lining epithelium, lamina propria, and muscularis
                mucosae
                    a. Mucosa: Epithelial Lining
                           1. Simple columnar epithelium and mucus-secreting goblet cells
                           2. Mucus secretions:
                                   a. Protect digestive organs from digesting themselves
                                   b. Ease food along the tract
                           3. Stomach and small intestine mucosa contain:
                                   a. Enzyme-secreting cells
                                   b. Hormone-secreting cells (making them endocrine and
                                       digestive organs)
                    b. Mucosa: Lamina Propria and Muscularis Mucosae
                           4. Lamina Propria
                                   a. Loose areolar and reticular connective tissue
                                   b. Nourishes the epithelium and absorbs nutrients
                                   c. Contains lymph nodes (part of MALT) important in defense
                                       against bacteria
                           5. Muscularis mucosae – smooth muscle cells that produce local
                                movements of mucosa


      E. Mucosa: Other Sublayers
             1. Submucosa – dense connective tissue containing elastic fibers, blood and lymphatic
                vessels, lymph nodes, and nerves
             2. Muscularis externa – responsible for segmentation and peristalsis
                    a. involuntary contractions
                           i.   physically breaks down food
                           ii. mixes food with digestive enzymes
                           iii. propels along GI tract
             3. Serosa – the protective visceral peritoneum
                    a. Replaced by the fibrous adventitia in the esophagus
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                    b. Retroperitoneal organs have both an adventitia and serosa


V. Mouth
      A. Oral or buccal cavity:
             1. Is bounded by lips, cheeks, palate, and tongue
             2. Has the oral orifice as its anterior opening
             3. Is continuous with the oropharynx posteriorly
      B. To withstand abrasions:
             1. The mouth is lined with stratified squamous epithelium
             2. The gums, hard palate, and dorsum of the tongue are slightly keratinized
      C. Lips and Cheeks
             1. lips – fleshy folds around opening of mouth – covered by skin on outside and
                    mucous membrane on inside

             2. cheeks – keep food between teeth and assist in speech


      D. Palate
             1. Hard palate – underlain by palatine bones and palatine processes of the maxillae
                    a. Assists the tongue in chewing
             2. Soft palate – mobile fold formed mostly of skeletal muscle
                    a. Closes off the nasopharynx during swallowing
                    b. Uvula projects downward from its free edge and prevents entry of food into
                        nasal cavity
      E. Tongue
             1. Occupies the floor of the mouth and fills the oral cavity when mouth is closed
             2. Skeletal muscle covered with mucous membrane
             3. Functions include:
                    a. Gripping and repositioning food during chewing
                    b. Mixing food with saliva and forming the bolus
                    c. Initiation of swallowing by forcing food into back of mouth, and speech
             4. Lingual frenulum secures the tongue to the floor of the mouth
             5. Superior surface bears three types of papillae
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             6. Filiform – give the tongue roughness and provide friction
             7. Fungiform – scattered widely over the tongue and give it a reddish hue
             8. Circumvallate – V-shaped row in back of tongue
             9. Sulcus terminalis – groove that separates the tongue into two areas:
                    a. Anterior 2/3 residing in the oral cavity
                    b. Posterior third residing in the oropharynx


      F. Salivary Glands
             1. Produce and secrete saliva that:
                    a. Cleanses the mouth
                    b. Moistens and dissolves food chemicals
                    c. Aids in bolus formation
                    d. Contains enzymes that break down starch
             2. Three pairs of extrinsic glands – parotid, submandibular, and sublingual
                    a. Parotid – lies anterior to the ear between the masseter muscle and skin
                            1. Parotid duct opens into the vestibule next to second upper molar
                    b. Submandibular – lies along the medial aspect of the mandibular body
                            1. Its ducts open at the base of the lingual frenulum
                    c. Sublingual – lies anterior to the submandibular gland under the tongue
                            1. It opens via 10-12 ducts into the floor of the mouth
             3. Intrinsic salivary glands (buccal glands) – scattered throughout the oral mucosa
             4. Saliva: Source and Composition
                    a. Secreted from serous and mucous cells of salivary glands
                    b. 97-99.5% water, hypo-osmotic, slightly acidic solution containing
                            1. Electrolytes – Na+, K+, Cl–, PO42–, HCO3–
                            2. Digestive enzyme – salivary amylase
                            3. Proteins – mucin, lysozyme, defensins, and IgA
                            4. Metabolic wastes – urea and uric acid
             5. Control of Salivation
                      a. Intrinsic glands keep the mouth moist
                      b. Extrinsic salivary glands secrete serous, enzyme-rich
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                          saliva in response to:
                            1. Ingested food which stimulates chemoreceptors and pressoreceptors
                            2. The thought of food
                      c. Strong sympathetic stimulation inhibits salivation and results in dry mouth


      G. Teeth
             1. Primary and permanent dentitions have formed by age 21
             2. Primary – 20 deciduous teeth that erupt at intervals between 6 and 24 months
             3. Permanent – enlarge and develop causing the root of deciduous teeth to be
                 resorbed and fall out between the ages of 6 and 12 years
             4. All but the third molars have erupted by the end of adolescence
             5. Usually 32 permanent teeth
             6. Classification of Teeth
                    a. Teeth are classified according to their shape and function
                            i.   Incisors – chisel-shaped teeth for cutting or nipping
                            ii. Canines – fanglike teeth that tear or pierce
                            iii. Premolars (bicuspids) and molars – have broad crowns with
                                 rounded tips; best suited for grinding or crushing
             7. During chewing, upper and lower molars lock together generating crushing force
             8. Tooth Structure
                    a. Two main regions – crown and the root
                    b. Crown – exposed part of the tooth above the gingiva
                    c. Enamel – acellular, brittle material composed of calcium salts and
                        hydroxyapatite crystals; the hardest substance in the body
                            i.   Encapsules the crown of the tooth
                    d. Root – portion of the tooth embedded in the jawbone
                    e. Neck – constriction where the crown and root come together
                     f. Cementum – calcified connective tissue
                            i.   Covers the root
                            ii. Attaches it to the periodontal ligament
                    g. Periodontal ligament
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                              i.   Anchors the tooth in the alveolus of the jaw
                              ii. Forms the fibrous joint called a gomaphosis
                       h. Gingival sulcus – depression where the gingiva borders the tooth
                       i. Dentin – bonelike material deep to the enamel cap that forms the bulk of
                           the tooth
                        j. Pulp cavity – cavity surrounded by dentin that contains pulp
                       k. Pulp – connective tissue, blood vessels, and nerves
                       l. Root canal – portion of the pulp cavity that extends into the root
               9. Digestion – results from mastication – mechanical
                       a. mechanical – mixing of food with saliva and shaping food into bolus
                       b. chemical – salivary amylase – carbohydrate breakdown


VI. Pharynx
      A. From the mouth, the oro- and laryngopharynx allow passage of:
               1. Food and fluids to the esophagus
               2. Air to the trachea
      B. Lined with stratified squamous epithelium and mucus glands
      C. Has two skeletal muscle layers
      D. swallowed food from mouth to oropharynx to laryngopharynx to esophagus


VII. Esophagus
      A.     Muscular tube going from the laryngopharynx to the stomach by going through the
             mediatinum and diaphragm
      B. Lies posterior to trachea and anterior to vertebral column
      C.     Joins the stomach at the cardiac orifice
      D. lined with stratified squamous
      E. Functions: transport food to stomach and secretes mucous
      F. swallowing – movement of food from mouth to stomach (involves the mouth, pharynx
             and esophagus)
               1. Coordinated activity of the tongue, soft palate, pharynx, esophagus, and 22
                   separate muscle groups
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                2. Buccal phase – bolus is forced into the oropharynx
                3. Pharyngeal-esophageal phase – controlled by the medulla and lower pons
                          a. Pharyngeal - all routes except into the digestive tract are sealed off
                          b. Esophageal - peristalsis moves food through the pharynx to the esophagus
                                   i. peristalsis - regular contractions of the muscularis that push food
                                      through entire GI
                                         1. circular muscles above bolus contract – constrict wall of
                                             esophagus and squeeze bolus downward
                                         2. longitudinal muscles, bottom of bolus contract – shorten
                                             the section of esophagus below bolus and pushing walls
                                             outward
                                         3. bolus moves and repeat 1 and 2 above


VIII. Stomach
      A. Chemical breakdown of proteins begins and food is converted to chyme
      B.     Structure:
                1. Cardiac region – surrounds the cardiac orifice
                2. Fundus – dome-shaped region beneath the diaphragm
                3. Body – midportion of the stomach
                4. Pyloric region – made up of the antrum and canal which terminates at the pylorus
                          i.   The pylorus is continuous with the duodenum through the pyloric sphincter


      C.     Microscopic Anatomy of the Stomach
                1. same 4 basic layers as GI tract
                2. rugae – large folds
                3. surface mucous cells – nonciliated simple columnar secretes mucous
                4. Muscularis – 3 layers of smooth muscle
                          a. Outer – longitudinal
                          b. Middle – circular
                          c. Inner – oblique
                          d. The extra layer of smooth muscles

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                                 i.    Allows the stomach to churn, mix, and pummel food physically
                                 ii.       Breaks down food into smaller fragments
               5. Submucosa – areolar CT connects muscularis to mucosa (epithelial lining)
               6. Epithelial lining is composed of:
                       a. Goblet cells that produce a coat of alkaline mucus
                      b. The mucous surface layer traps a bicarbonate-rich fluid beneath it
                       c. Gastric pits contain gastric glands that secrete gastric juice, mucus, and
                          gastrin
                      d. Glands of the Stomach
                                 i.    Gastric glands of the fundus and body have a variety of secretory
                                       cells
                                           1. Mucous neck cells – secrete acid mucus
                                           2. Parietal cells – secrete HCl and intrinsic factor
                 Secretes                  3. Chief cells – produce pepsinogen
                 gastric juice
                                                  1. Pepsinogen is activated to pepsin by HCl in the
                                                      stomach
                                           4. Enteroendocrine cells (or G cells) – secrete gastrin,
                                               histamine, endorphins, serotonin, cholecystokinin (CCK), and
                                               somatostatin into the lamina propria


      D.     Stomach Lining (Why doesn’t the stomach digest itself?)
               1. The stomach is exposed to the harshest conditions in the digestive tract
               2. To keep from digesting itself, the stomach has a mucosal barrier with:
                       a. A thick coat of bicarbonate-rich mucus on the stomach wall
                      b. Epithelial cells that are joined by tight junctions
                       c. Gastric glands that have cells impermeable to HCl
                      d. Damaged epithelial cells are quickly replaced
                       e. Pepsinogen is not active until HCl is present


      E.     Digestion in the Stomach
               1. The stomach:
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                    a. Holds ingested food
                    b. Degrades this food both physically and chemically
                           i.   Mechanical and chemical digestion
                    c. Delivers chyme to the small intestine
                    d. Enzymatically digests proteins with pepsin
                    e. Secretes intrinsic factor required for absorption of vitamin B12
             2. Before food enters the stomach, the sight, smell, taste, or thought of food
                initiates: (Cephalic phase)
                    a. reflexes stimulate parasympathetic of the ANS and enteric neurons
                    b. stimulate gastric glands → gastric juices – pepsinogen, HCI, mucous
                           i.   gastrin
                                   1. stimulates secretion of large amounts of gastric juice
                                   2. increases stomach mobility
                                   3. relaxes pyloric sphincter
             3. Once the food enters (Gastric phase and Intestinal phase):
                    a. Stomach stretches
                    b. Activation of stretch receptors (neural activation)
                    c. There are proteins that buffer some of the stomach acid
                    d. initiate mixing waves gentle rippling peristaltic movements to form
                       chime
                           i.   chyme is produced when waves macerate food and mix with
                                secretions of gastric glands
                    e. gastric emptying
                           i.   each mixing wave forces a small amount of chyme through
                                pyloric sphincter, after stomach has emptied some contents it
                                slows down so it doesn’t overload duodenum
                           ii. stimulated by:
                                   1. distention of stomach
                                   2. gastrin
                                   3. partially digested proteins
                                   4. vagus nerve

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                     f. If fat are present, it stimulates cholecystokinin – CCK – slows stomach
                        emptying
                     g. secretin
                            i.    hormone – released by endothelial cells in mucosa which
                                  decreases secretion of gastric juice
                            ii. major stimulus is entry of acid chyme of SI
             4. Main event: protein digestion which begins with pepsin
                     a. Pepsin breaks proteins to peptides
                     b. There is some lipid digestion by gastric lipases


             5. Absorption – epithelial cells are impermeable to most materials
                     a. However, it can absorb little of: water, ions, short chain fatty acids and
                        some drugs such as aspirin and alcohol.


IX. Anatomy of Small Intestine:
      A. Where most of digestion and absorption occur
      B. Enters 2-4 hours after meal
      C. Structure
             1. Runs from pyloric sphincter to the ileocecal valve
             2. Has three subdivisions: duodenum, jejunum, and ileum
             3. The bile duct and main pancreatic duct are located in the duodenum:
                     a. Join the duodenum at the hepatopancreatic ampulla
                     b. Are controlled by the sphincter of Oddi
             4. The jejunum extends from the duodenum to the ileum
             5. The ileum joins the large intestine at the ileocecal valve


      D. Microscopic Anatomy
             1. Structural modifications of the small intestine wall increase surface area
             2. Plicae circulares: deep circular folds of the mucosa and submucosa
             3. Villi – fingerlike extensions of the mucosa
             4. Microvilli – tiny projections of absorptive mucosal cells’ plasma
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                   membranes
       E. Histology of the Wall
               1. The epithelium of the mucosa is made up of:
                       a. Absorptive cells and goblet cells
                      b. Enteroendocrine cells
                       c. Interspersed T cells called intraepithelial lymphocytes (IELs)
                              1. IELs immediately release cytokines upon encountering Ag
               2. Cells of intestinal crypts secrete intestinal juice
               3. Peyer’s patches are found in the submucosa
               4. Brunner’s glands in the duodenum secrete alkaline mucus


       F. Intestinal Juice
               1. Secreted by intestinal glands in response to distension or irritation of the mucosa
               2. Slightly alkaline and isotonic with blood plasma
               3. Largely water, enzyme-poor, but contains mucus


X. Accessory Digestive Organs
       A.    Digestion in the small intestine would not be possible without the secretion from the
             accessory digestive organs


       B. Liver
               1. The largest gland in the body
               2. Superficially has four lobes – right, left, caudate, and quadrate
               3. The falciform ligament:
                       c. Separates the right and left lobes anteriorly
                      d. Suspends the liver from the diaphragm and anterior abdominal wall
               4. The gallbladder rests in a recess on the inferior surface of the right lobe
               5. Microscopic Anatomy
                       a. Hexagonal-shaped liver lobules are the structural and functional units of
                          the liver
                      b. Composed of hepatocyte (liver cell) plates radiating
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                       outward from a central vein
                    c. Liver sinusoids – enlarged, leaky capillaries located between hepatic plates
                   d. Kupffer cells – hepatic macrophages found in liver sinusoids
                    e. Hepatocytes’ functions include:
                           i.       Production of bile
                           ii.      Processing bloodborne nutrients
                           iii.     Storage of fat-soluble vitamins
                           iv.      Detoxification
             6. Composition of Bile
                    a. A yellow-green, alkaline solution containing bile salts, bile pigments,
                       cholesterol, neutral fats, phospholipids, and electrolytes
                   b. Bile salts are cholesterol derivatives that:
                           i.     Emulsify fat – converts large globules of triglycerides into droplets
                                  (makes more soluble in water of intestinal fluid)
                           ii.    Facilitate fat and cholesterol absorption
                           iii. Help solubilize cholesterol
                           iv. The chief bile pigment is bilirubin, a waste product of heme
                                    1. after serving as emulsifying agent reabsorption by active
                                        transport in the ileum → portal blood to liver
                   c. Bile Secretion (after a meal…)
                           i.     Vagus (Parasympathetic impulse) nerve stimulates liver to
                                  increase bile production
                           ii.    fatty acids and amino acids in chime stimulate CCK secretion
                                  and acidic chime stimulates secretin secretion into blood
                           iii. CCK causes contraction of gallbladder wall and bile come out
                                  into the cystic duct and common bile duct. It also relaxes the
                                  hepatopancreatic ampulla, which allows bile to flow into the
                                  duodenum.
                           iv.    Secretin stimulates secretion of pancreatic juice (rich in
                                  bicarbonate) and stimulates secretion of bicarbonate by
                                  hepatocyte into the bile.
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             6. Functions:
                   a. Carbohydrate metabolism – very important in maintaining normal
                      blood glucose level
                             i.   glycogen, triglycerides – storage
                             ii. converts into glucose
                                     1. amino acids
                                     2. lactic acid
                                     3. fructose
                                     4. galactose
                                     5. glycogen and triglycerides
                   b. Lipid metabolism
                             i.   hepatocytes store triglycerides
                             ii. breakdown fatty acids → ATP
                             iii. synthesizes lipoprotein – transports fatty acids, triglycerides, and
                                  cholesterol to and from body cells
                             iv. synthesizes cholesterol
                             v. cholesterol → bile salts, gallstones are usually crystallized
                                  cholesterol
                   c. Protein metabolism
                             i.   hepatocytes remove amino group from amino acids
                                     1. amino acids → ATP
                                     2. amino acids converted to carbs. or fats
                             ii. convert NH3 → urea
                                     1. to detoxify ammonia by protein deamination
                                     2. without functioning hepatocytes, protein digestion is toxic
                                         due to production of ammonia
                             iii. synthesizes most plasma proteins – albumins, fibrinogen
                   d. Process drugs and hormones
                             i.   detoxify alcohol
                             ii. secretes drugs into bile – penicillin, erythromycin, sulfonamides
                             iii. inactivates
                                     1. thyroid hormones
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                                    2. steroids – estrogen, aldosterone
                    e. Excretes bilirubin
                           i.   heme of RBC
                           ii. excreted in bile
                           iii. metabolized in SI and eliminated in feces
                    f. Stores vitamins and minerals
                           i.   A, B12, D, E, K
                           ii. iron, copper
                    g. Synthesizes active form of vitamin D

      C. Pancreas
             1. Location
                    a. Lies deep to the greater curvature of the stomach
                    b. The head is encircled by the duodenum and the tail abuts the spleen
             2. Exocrine function
                    a. Secretes pancreatic juice which breaks down all categories of foodstuff
                    b. Acini (clusters of secretory cells) contain zymogen granules with digestive
                       enzymes
                    c. 99 % of the small cluaters in pancreas
             3. The pancreas also has an endocrine function
                    a. Islets of Langerhans
                           i.    release of insulin and glucagon
             4. Composition and Function of Pancreatic Juice
                    a. Water solution of enzymes and electrolytes (primarily HCO3–)
                    b. Neutralizes acid chyme
                    c. Provides optimal environment for pancreatic enzymes
                    d. Enzymes are released in inactive form and activated in the duodenum
                           i.   Examples include
                                    a. Trypsinogen is activated to trypsin
                                    b. Procarboxypeptidase is activated to carboxypeptidase
                    e. Active enzymes secreted

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                              i.     Amylase, lipases, and nucleases
                              ii. These enzymes require ions or bile for optimal activity
                      f. Functions of enzymes:
                              i.        pancreatic amylase – digest carbs
                              ii.       trypsin, chymotrypsin, carboxypeptidase – protein digesting
                              iii.      pancreatic lipase – triglyceride digesting
                              iv.       ribonuclease, deoxyribonuclease – nucleic acid digesting
                              v.        enterokinase – protein digesting ones are inactive so don’t
                                        digest pancreas, this activates trypsin which then activates
                                        others
              5. Regulation of Pancreatic Secretion
                      a. Secretin and CCK are released when fatty or acidic chyme enters the
                          duodenum
                      b. CCK and secretin enter the bloodstream
                      c. Upon reaching the pancreas:
                              i.        CCK induces the secretion of enzyme-rich pancreatic juice
                              ii.       Secretin causes secretion of bicarbonate-rich pancreatic juice
                      d. Vagal stimulation also causes release of pancreatic juice



XI. Digestion in the Small Intestine
       A. As chyme enters the duodenum:
              1. Carbohydrates and proteins are only partially digested
              2. No fat digestion has taken place
       B. Digestion continues in the small intestine
       D. Virtually all nutrient absorption takes place in the small intestine


       E. Mechanical digestions (Motility in the Small Intestine)
              1. The most common motion of the small intestine is segmentation
                      a. It is initiated by intrinsic pacemaker cells (Cajal cells)
                      b. Localized contractions that slosh chime back and forth mixing with digestive
                          juices and bringing it into contact with mucose
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             2. After nutrients have been absorbed:
                     a. Segmentation stops and peristalsis begins with each wave starting distal to
                        the previous
                    b. Meal remnants, bacteria, mucosal cells, and debris are moved into the large
                        intestine


      F. Chemical Digestion
             1. Absorption: via cotransport with Na+, and facilitated diffusion
             2. Enter the capillary bed in the villi
             3. Transported to the liver via the hepatic portal vein
             4. Enzymes used: salivary amylase, pancreatic amylase, and brush border enzymes
             5. Proteins
                    a. pancreatic juice enzymes - trypsin, chymotrypsin, elastase,
                        carboxypeptidase – actions different because split bonds between
                        different amino acids
                    b. completed by peptidases – epithelial cells – final → tripeptides,
                        dipeptides, amino acids
             6. Carbohydrates
                    a. starches and dextrins not reduced to maltose by the time chyme leaves
                        stomach → pancreatic amylase
                    b. 3 enzymes in intestinal juice → disaccharides → monosaccharides
                    c. surface of epithelial cells
                    d. maltase → maltose → 2 glucose
                    e. sucrase → sucrose → glucose + fructose
                    f. lactase → lactose → glucose + galactose

             7. Fats
                    a. bile salts – emulsification – small drops – easy access for enzymes
                    b. pancreatic lipase
                            a. removes 2 of 3 fatty acids from glycerol forming fatty acids +
                                monoglycerides

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             8. Nucleotides
                   a. ribonuclease - RNA
                   b. deoxyribonuclease - DNA


      G. Absorption
            1. Electrolyte Absorption
                   a. Most ions are actively absorbed along the length of small intestine
                   b. Na+ is coupled with absorption of glucose and amino acids
                   c. Ionic iron is transported into mucosal cells where it binds to ferritin
                   d. Anions passively follow the electrical potential established by Na+
                   e. K+ diffuses across the intestinal mucosa in response to osmotic gradients
                    f. Ca2+ absorption:
                          i.   Is related to blood levels of ionic calcium
                          ii. Is regulated by vitamin D and parathyroid hormone (PTH)
             2. Water Absorption
                   a. 95% of water is absorbed in the small intestines by osmosis
                   b. Water moves in both directions across intestinal mucosa
                   c. Net osmosis occurs whenever a concentration gradient is established by
                       active transport of solutes into the mucosal cells
                   d. Water uptake is coupled with solute uptake, and as water moves into
                       mucosal cells, substances follow along their concentration gradients
             3. Monosaccharides
                   a. Glucose and galactose through active transport
                   b. Fructose by facilitated diffusion
                   c. It comes out of epithelial cells to blood capillaries by facilitated diffusion
                       and goes into the general circulation
             4. Amino acids
                   a. Mainly absorbed in duodenum and jejunum
                   b. Half come food and half from dead cells and digestive juices
                   c. Move into epithelial cells through active transport
                   d. Peptides inside of cell will be digested into amino acids

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                       e. Move to capillaries by diffusion and then general circulation
               5. Lipids
                       a. All dietary lipds are absorbed via simple diffusion
                       b. About 95% of lipids are absorbed in the SI
                       c. Lipids reach the epithelial cells of the villi by being dissolved in micelles
                              i.   Micelles – spherical aggregate of bile salts that dissolved fatty
                                   acids and monoglycerides
                       d. Inside the epithelial cells, lipase breaks down monoglycerides to fatty
                           acids and glycerol which are then recombine to from triglycerides.
                              i.   Triglycerides combine with cholesterol and proteins to form
                                   chylomicrons which are release by exocytosis and move to
                                   lacteals
               6. Vitamins
                       a. Fat-soluble vitamins (A, D, E, and K) are included with lipid in micelles
                           and are absorbed by simple diffusion
                       b. Water-soluble vitamins such as vitamin c and most vitamin B are
                           absorbed also by simple diffusion
                              i.   Vitamin B12 combines with intrinsic factor produced by the
                                   stomach and then it is absorbed by active transport in the ileum
XII. Large Intestine
       A. Functions:
               1. Other than digestion of enteric bacteria, no further digestion takes place
               2. Vitamins, water, and electrolytes are reclaimed
               3. Its major function is propulsion of fecal material toward the anus
               4. Though essential for comfort, the colon is not essential for life


       B. Structure
               1. Is subdivided into the cecum, appendix, colon, rectum, and anal canal
               2. Colon
                       a. Has distinct regions: ascending colon, transverse colon, descending colon,
                           and sigmoid colon
                       b. The transverse and sigmoid portions are anchored via
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                           mesenteries called mesocolons
                     c. The sigmoid colon joins the rectum
                     d. The anal canal, the last segment of the large intestine, opens to the exterior
                           at the anus
             3. Valves and Sphincters of the Rectum and Anus
                     a. The anus has two sphincters:
                               i. Internal anal sphincter composed of smooth muscle
                              ii. External anal sphincter composed of skeletal muscle
                              iii. These sphincters are closed except during defecation
             4. LI wall has no villi or circular folds


      C. Microscopic Anatomy
             1. Colon mucosa is simple columnar epithelium except in the anal canal
             2. Has numerous deep crypts lined with goblet cells
             3. Anal canal mucosa is stratified squamous epithelium
             4. Anal sinuses exude mucus and compress feces
             5. Superficial venous plexuses are associated with the anal canal
                     a. Inflammation of these veins results in itchy varicosities called hemorrhoids


      D. Bacterial Flora
             1. The bacterial flora of the large intestine consists of:
                     a. Bacteria surviving the small intestine that enter the cecum and
                     b. Those entering via the anus
             2. These bacteria:
                     a. Colonize the colon
                     b. Ferment indigestible carbohydrates
                     c. Can break down proteins and amino acids
                     d. Decompose bilirubin into stercobilin which gives feces its brown color
                     e. Release irritating acids and gases (flatus)


      E. Motility of the Large Intestine
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             1. Haustral contractions
                       a. Slow segmenting movements that move the contents of the colon
             2. Presence of food in the stomach:
                       a. Activates the gastrocolic reflex
                       b. Initiates peristalsis that forces contents toward the rectum


      F. Defecation
             1. Feces – water, inorganic salts, epithelial cells, bacteria, unabsorbed food and
                 indigestible food
             2. Distension of rectal walls caused by feces:
                       a. Stimulates contraction of the rectal walls
                       b. Relaxes the internal anal sphincter
                       c. Voluntary signals stimulate relaxation of the external anal sphincter and
                           defecation occurs


XIII. Homeostatic Imbalances
      1. Mumps
      2. Halitosis
      3. Dental caries (or cavities)
      4. Gingivitis
      5. Heartburn
      6. Hiatal hernia
      7. Gastric or peptic ulcer
      8. Vomiting or emesis
      9. Hepatitis
      10. Cirrhosis
      11. Gallstones
      12. Appendicitis and peritonitis
      13. Diarrhea
      14. Diverticulosis
      15. Lactose intolerance

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             Digestive - 22
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