Digestive System

Digestive System Anatomy Divisions:

1. Alimentary canal (GI tract)
   1. oral cavity to anus
2. Accessory organs:
   1. salivary
   2. liver
   3. pancreas
   4. gall bladder teeth

Digestive System Functions:

1. Ingestion of food
2. Propulsion
   1. Deglutition (swallowing in oropharynx)
   2. Peristalsis (esophagus to the large intestine)
3. Mechanical Digestion
   1. Mastication (mouth)
   2. segmentation (SI)
   3. Churning (stomach)
4. Chemical digestion (catabolism)
5. Absorption (nutrients enter blood)
6. defecation (elimination)
7. Location of Functions
   1. Mouth:
      1. Ingestion
      2. Propulsion
      3. Digestion (Mechanical & Chemical) (starch)
   2. Pharynx & Esophagus:
      1. Propulsion
   3. Stomach:
      1. Digestion Mechanical & Chemical (protein)
   4. Small Intestine:
      1. Digestion Mechanical & Chemical (fat & others)
      2. Propulsion
      3. Absorption- CHO, fat, protein vitamins/electrolytes, water
   5. Large Intestine:
      1. Propulsion
      2. Absorption- vitamins/NaCl, water
      3. Chem Digestion (vit B,K)
      4. Defecation

Anatomy

1. 4 layers of GI tract:
   1. Mucosa - secretory epithelium
   2. Submucosa - contains nerves, blood vessels
   3. Muscularis - peristalsis, 2 or > layers
   4. Serosa - outer layer, anchors to abdominal cavity, holds organs in place (greater omentum)
2. Salivary glands: (Parotid, sublingual, submandibular)
   1. provide taste, cleanse teeth
   2. Initiate starch breakdown - amylase
   3. forms 'bolus'
   4. Parasympathetic - (inc) saliva
      Sympathetic - (dec) saliva
   5. ' mumps' inflamed parotid
3. Esophagus:
   1. 10 '' collapsable food tube
   2. pierces diaphragm at esophageal hiatus
      (hiatal hernia- 'heartburn')
   3. stratified squamous (mucus)
   4. cardiac sphincter --> stomach
      'gastroesophageal'
4. Anatomy of the Stomach (gastric)
   1. Fundus
      1. cardiac region
      2. body
   2. Pylorus
   3. Rugae (interior folds)
   4. Muscularis layer (3)
      1. circular
      2. longitudinal
      3. oblique
   5. bolus becomes chyme
   6. Secretory cells:
      1. mucus secreting
      2. parietal cells --> HCl acid (pH1.5-3.5)
         intrinsic factor (Vit B12)
      3. Chief cells --> Pepsinogen
         1. activates pepsin (proteinase)
      4. enteroendocrine --> Gastrin
5. Stomach Physiology
   1. Gastric emptying (~ 4 hrs)
      1. depends on meal composition
      2. (increase) stretch --> (increase) emptying
      3. pacemaker cells ( peristaltic rhythm)
   2. Gastric secretion- neural/ hormonal control: Gastritis (ulcerative)
      1. Stimulatory (parasymp)
         1. Medulla--> Vagus (sight, smell or stomach distension)
         2. Peptides, caffeine --> Gastrin
         3. food in Duodenum --> enteric Gastrin
      2. Gastric inhibition
         1. Sympathetic innervation:
         2. excess acidity in stomach inhibits Gastrin
         3. Intestinal contents (acid)--> Secretin CCK, GIP
            decrease secretion
6. Small Intestine:
   1. Duodenum - site of most digestion Jejunum, ileum
   2. Large surface area:
      1. Villi- fingers of mucosa (contain capillary & lacteal)
      2. Microvilli or brush border (digestive enzymes for protein, disaccharides)
   3. pH 6.5- 7.8
   4. Brunners' glands --> mucus
   5. secrete enterogastrones:
      1. Secretin, CCK, GIP:
         1. inhibit gastric motility & secretion
   6. most digestive 'juice' provided by liver & pancreas
7. Large intestine:
   1. Cecum
   2. Veriform appendix (no digestive role)
   3. Colon:
      1. ascending
      2. transverse
      3. descending
      4. sigmoid
   4. Anus:
      1. internal sphincter (involuntary)
      2. external sphincter (voluntary)
   5. Key features:
      1. no brush border for nutrient absorption (only water, NaCl, Vit)
      2. stores & concentrates residues
      3. muscularis layer --> tenae coli
         1. ribbons causing walls to form sacs or haustra
         2. contract--> slow segmentation
      4. Bacterial flora
         1. responsible for flatus
         2. synthesis of Vit B, K
   6. Disorders of L.I.
      1. Diarrhea- abnormal rapid motility
      2. Constipation- abnormal slow motility
      3. Diverticulitis - abnormal outpouching
      4. colorectal cancers (lo fiber, hi fat diet may contribute)
   7. Defecation reflex
      1. Stretch of Rectum --> stimulates spinal reflex (parasympathetic)
      2. conscious control of external anal sphincter
   8. Gastrocolic reflex (following meal)
      1. Mass movement ( 2-4 times/day)
      2. Hemorrhoids- enlarged veins irritated if constipated
8. Liver and gall bladder
   1. Accessories to small intestine
   2. Digestive role - to produce Bile (~ 1 liter / day)
   3. delivers bile to duodenum
   4. Liver produces bile, gall bladder stores and concentrates it
      1. 'chole' - bile
      2. 'cyst' - bladder
      3. 'hepat' - liber
9. Liver
   1. Largest Gland
   2. 4 lobes located under diaphragm
   3. falciform ligament
   4. gall bladder - located rt lobe
   5. liver other functions:
      1. detoxify blood
      2. pick up nutrients from blood (hepatic portal vein)
   6. Hepatic abnormalities
      1. Hepatitis-
         1. virus A- contaminated food,
         2. B virus- blood
         3. Jaundice (yellow skin) -accumulation of bile pigments in blood (deposit in skin) & occur with gallstones (block cystic duct)
      2. Cirrhosis -
         1. chronic inflamation
         2. scarring of liver
         3. reduced detox ability
   7. Regulation of Bile secretion
      1. Acid, fatty chyme enters duodenum
      2. CCK, Secretin released in blood
      3. Increased bile salt from liver
      4. Vagus
      5. CCK causes the gall bladder contract
         1. also relaxes Sphincter of Oddi
            (common bile Duct & pancreatic duct)
10. Pancreas-
    1. located inferior to stomach
    2. Acinar cells secrete digestive juices
       1. released to duodenum via duct Wirsung
    3. Pancreatic juice components:
       1. alkaline (bicarbonate)
       2. Enzymes (released in inactive form)
          1. proteases: trypsinogen (trypsin)
             chymotrypsinogen
             procarboxypeptidase
          2. lipase (lipids)
          3. amylase
          4. Nuclease
       3. release under control of CCK, Secretin
       4. combine with Intestinal juice (pH 6.5-7.5)
       5. produced by glands (crypts of Lieberkuhn)
          (also digestive enzymes - brush border: complete CHO & protein digestion)
11. Chemical Digestion/Absorption
    1. Carbohydrates: (starch --> glucose)
       1. amylase (saliva, pancreatic): starch broken down into maltose
       2. disaccharide broken down into glucose, fructose, galactose via brush border enzymes: maltase, sucrase, lactase, a-dextrinase
       3. absorption most in duodeum (before ileum)
          1. glucose (carrier-mediated active transport)
          2. fructose (facilitated diffusion)
       4. path of CHO absorption: (Same as protein)
          1. villus--> capillary blood --> liver (hepatic vein)--> blood or stored
    2. Protein Chemical Digestion
       1. stomach (pepsin) protein broken down into polypeptide
       2. duodenum (pancreatic) peptides broken down into amino acids (brush border)
    3. Lipid chemical digestion
       1. Triglyceride composed of fatty acids(FA) & glycerol
          1. location: small intestine
             1. bile salts (liver) emulsify
             2. pancreatic lipase broken down to FA
       2. Absorption:
          1. Micelles (FA, bile salts, monogly cerides) small droplets that permit fat absorption in intestinal mucosa (by ileum)
          2. Chylomicrons formed in epithelium (fat globules combined with protein)
          3. enter lacteal (lymph capillary) in villas
          4. lymph enters venous blood
          5. lipoprotein lipase breaks down triglyceride (chylomicron) into FFA + glycerol
          6. following a meal, serum appears milky due to fat