1. Digestive Physiology

2. Overview
Inside gastrointestinal (GI) tract, food is broken down by hydrolysis reactions into molecular monomers
Most digestion of nutrients and absorption of monomers occurs in small intestine (90%)
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3. Functions of the Digestive System
Ingestion--taking food into mouth
Mastication--chewing food and mixing it with saliva
Deglutition--swallowing food
Peristalsis--rhythmic wave-like contractions that move food through GI tract
Digestion– mechanical and chemical breakdown of food
Absorption--Is passage of digested end products into blood or lymph
Storage and Elimination--Includes temporary storage and subsequent elimination of indigestible components of food
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4. Functions of Digestive System
Secretion:
Includes release of exocrine and endocrine products into GI tract
Exocrine secretions include: HCl, H2O, HCO3-, bile, lipase, pepsin, amylase, trypsin, elastase, and histamine
Endocrine includes hormones secreted into stomach and small intestine to help regulate GI system
e.g. gastrin, secretin, cholescytokinin, gastric inhibitory peptide, and somatostatin
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5. Digestive System
Is composed of GI tract (alimentary canal) and accessory digestive organs
Organs include oral cavity, pharynx, esophagus, stomach, small and large intestine
Accessory organs include teeth, tongue, salivary glands, liver, gallbladder, and pancreas

6. Regulation of GI Tract
Parasympathetic effects, arising from vagus and spinal nerves, stimulate motility and secretions of GI tract
Sympathetic activity reduces peristalsis and secretory activity
GI tract contains an intrinsic system that controls its peristaltic movements--the enteric nervous system
GI motility is also influenced by paracrine and hormonal signals
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7. From Mouth to Stomach continued
Peristalsis propels food thru esophagus and GI tract
= wave-like muscular contractions
After food passes into stomach, the lower esophageal sphincter constricts, preventing reflux
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8. Stomach Is most distensible part of GI tract Empties into the duodenum
Functions in:
storage of food;
initial chemical digestion of proteins
some mechanical digestion of all nutrients
kills bacteria with high acidity (HCl)
moves soupy food mixture (chyme) into intestine
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9. Stomach Gastric mucosa has gastric pits in its folds
Cells that line folds deeper in the mucosa, are exocrine gastric glands
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10. Stomach continued
Gastric glands contain cells that secrete different products that form gastric juice
1. Goblet cells secrete mucus
2. Parietal cells secrete HCl and intrinsic factor (necessary for B12 absorption in intestine)
3. Chief cells secrete pepsinogen (precursor to pepsin)
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11. Stomach continued
4. Enterochromaffin-like cells secrete histamine and serotonin
5. G cells secrete gastrin
6. D cells secrete somatostatin
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12. HCl in Stomach
Is secreted into stomach lumen by proton pumps of epithelial parietal cells in response to the histamine secreted by ECL cells ; and ACh from vagus (parasympathetic stimulation)
These are indirect effects since release of histamine is due to gastrin release from G cells
Proton pump inhibitors (medicines) are common and work to reduce stomach acids (treat ulcers)
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13. HCl in Stomach continued
Makes gastric juice very acidic which denatures proteins to make them more digestible
Also converts pepsinogen into pepsin
Pepsin is more active at low pHs
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14. Digestion and Absorption in Stomach
Proteins are partially digested by pepsin
Carbohydrate digestion by salivary amylase is soon inactivated by acidity
Water, alcohol and aspirin are the only commonly ingested substances that are absorbed here
18-35

15. Small Intestine (SI) continued
Surface area increased by foldings and projections
Large folds are plicae circulares
Microscopic finger-like projections are villi
Cell apical hair-like membrane projections are microvilli
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16. Intestinal Enzymes Attached to microvilli are brush border enzymes
Enzyme active sites are exposed to chyme
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18. Large Intestine or Colon
Has no digestive function of its own but absorbs H2O, electrolytes, B and K vitamins, and folic acid
Internal surface has no villi (relatively smooth)
Intestinal Flora: Contains large population of microflora
400 different species of commensal bacteria
Which produce folic acid and vitamin K and ferment indigestible foods to produce fatty acids
And reduce ability of pathogenic bacteria to infect colon
Antibiotics can kill commensals
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20. Fluid and Electrolyte Absorption in Colon
SI absorbs most water but colon absorbs 90% of water it receives
Begins with osmotic gradient set up by Na+/K+ pumps
Water follows by osmosis
Salt and water reabsorption stimulated by aldosterone
Colon can also excrete H2O via active transport of NaCl into intestinal lumen (an ANP influence)
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21. Accessory Organs:The Liver
Day 28
complete

22. Functions of the Liver Overall it over 500 functions!
Lipid metabolism: lipolysis, lipogenesis, synthesis of cholesterol
Protein metabolism: synthesizes the plasma proteins
(albumin, fibrinogen, alpha and beta globulins, and prothrombin); breaks down proteins and converts the to carbohydrates or lipid for storage.
Carbohydrate metabolism: helps to maintain normal blood glucose levels by;
breaking down glycogen into glucose and then secreting it into the blood
converting serum glucose into glycogen and triglycerides for storage

23. Functions of the Liver
Detoxification: processes drugs and hormones; detoxifies substances such as alcohol or excretes drugs such as the antibiotics into bile.
Synthesis of bile salts: bile salts are used in the small intestine for the emulsification and absorption of lipids, cholesterol, phospholipids, and lipoproteins.
Storage: stores glycogen, vitamins and minerals.
Phagocytosis: Kupffer cells phagocytize RBCs, WBCs, bacteria, and toxins.

24. Detoxification of Blood
Liver can remove hormones, drugs, and other biologically active molecules from blood by:
Excretion in bile
Phagocytosis by Kupffer cells
Chemical alteration of molecules
e.g. ammonia is produced by deamination of amino acids in liver
Liver then converts it to urea which is excreted in urine
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25. The Gallbladder
Stores and concentrates bile continuously produced by liver
When SI is empty, sphincter of Oddi in common bile duct closes and bile is forced up into gallbladder
Expands as it fills with bile
When food is in SI, sphincter of Oddi opens, gall bladder contracts, and bile is ejected thru ducts into duodenum
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26. The Pancreas Has both endocrine and exocrine functions
Endocrine function performed by Islets of Langerhans cells
Secrete insulin and glucagon
Exocrine secretions include bicarbonate (HCO3-) solution and digestive enzymes
These pass in pancreatic duct to small intestine
Exocrine secretory units are acini
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27. The Physiology of Digestion Regulation of Gastric Function
Extrinsic control of gastric function is divided into cephalic, gastric, and intestinal phases
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28. Cephalic Phase Refers to control by brain via vagus nerve
Stimulated by sight, smell, thought, and taste of food
Activation of vagus nerve stimulates:
Salivary glands to secrete saliva
Chief cells to secrete pepsinogen
G cells to secrete gastrin
ECL cells to secrete histamine
Parietal cells to secrete HCl
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29. Gastric Phase
The presence of proteins, polypeptides and amino acids in the stomach raises the pH. This change in chemical nature, along with stomach distension, activates the gastric (stomach) phase.
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30. Gastric Phase
Proteins, polypeptides and amino acids present in the stomach stimulate G cells to secrete gastrin and chief cells to secrete pepsinogen
Gastrin then stimulates ECL cells to secrete histamine which stimulates parietal cell secretion of HCl
This is a positive feedback mechanism: as more HCl and pepsinogen are secreted, more polypeptides and amino acids are liberated, and more digestive processes are stimulated.
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31. Gastric Phase
As polypeptides leave the stomach and move into the duodenum, the pH begins to drop again and the gastric phase slows.
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32. Summary of the Interactions Among Gastric Gland Cells and Secretions
1. Presence of proteins in stomach, an increased pH, and stomach distension (ie: stretch) all stimulate G cells to secrete gastrin and chief cells to secrete pepsinogen.
2. Gastrin stimulates ECL cells to secrete histamine.
3. Histamine stimulates parietal cells to secrete HCL.
4. HCl denatures proteins and activates pepsin from pepsinogen.
5. Pepsin digests proteins into polypeptides.
6. Polypeptides also stimulate G cells to secrete gastrin (a positive feedback effect).

33. Intestinal Phase Begins when chyme enters the small intestine
Arrival of chyme in duodenum causes a neural reflex that inhibits gastric motility and secretion
Fat in chyme stimulates SI to secrete enterogasterones--hormones that inhibit gastric motility and secretion
Include Somatostatin, Cholecystokinin, Secretin, and Gastric Inhibitory Peptide
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34. Intestinal Phase
Enterogasterones secreted by intestines whem chyme arrives:
Somatostatin: inhibits stomach acid secretion.
Cholecystokinin: stimulates secretion of pancreatic enzymes; stimulates contraction of gall bladder; brings about a feeling of fullness after eating; slows gastric motility and acid secretion.
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35. Intestinal Phase
Enterogasterones secreted by intestines whem chyme arrives:
Secretin: stimulates secretion of pancreatic HCO3- and bile from the liver; inhibits acid production and gastric motility.
Gastric Inhibitory Peptide: Inhibits gastric motility and secretion; stimulates secretion of insulin from pancreas.
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36. Digestion and Absorption of Carbohydrates
Most carbohydrates are ingested as starch-- structured of glucose
Salivary amylase begins starch digestion in the mouth and continues for awhile in the stomach (until it is denatured by the low pH)
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37. Digestion and Absorption of Carbohydrates
Pancreatic amylase secreted into duodenum converts starch to oligosaccharides
Oligosaccharides are hydrolyzed by small intestine brush border enzymes into monosaccharides
Monosaccharides are absorbed directly into the bloodstream
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38. Digestion and Absorption of Proteins
Chemical digestion begins in stomach when pepsin digests proteins to form polypeptides
In small intestine, endopeptidases (trypsin, chymotrypsin, elastase) from pancreas cleave peptide bonds from interior of polypeptides
Also in small intestine exopeptidases (carboxypeptidase, aminopeptidase) cleave peptide bonds from ends of polypeptides. Carboxypeptidase is a pancreatic enzyme while aminopeptidase is a brush border enzyme.
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39. Digestion and Absorption of Protein
Protein digestion in small intestine results in free amino acids, dipeptides, and tripeptides
Which are absorbed into small intestine cells where they are broken down into amino acids
Which are then secreted directly into the bloodstream
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40. Digestion and Absorption of Lipids
Small amount of lipid chemical digestion begins in mouth via lingual lipase
No chemical digestion in stomach (except in infants who can digest milk fats)
Arrival of lipids in duodenum causes secretion of bile from gall bladder
Fat is emulsified by bile salt micelles
Form tiny droplets of fat
Greatly increases surface area for digestion by pancreatic lipase
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41. Digestion and Absorption of Lipids continued
Pancreatic lipase then hydrolyzes exposed triglycerides to free fatty acids and monoglycerides which are then absorbed into epithelial cells
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42. Digestion and Absorption of Lipids continued
Products of fat digestion are dissolved in micelles which move to the brush border
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43. Digestion and Absorption of Lipids continued
Free fatty acids and monoglycerides leave micelles and are absorbed into epithelial cells
Inside epithelial cells, they are resynthesized into triglycerides and phospholipids, and then packaged into protein transport structures termed chylomicrons.
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