Unit Twelve: Gastrointestinal Physiology

1. Unit Twelve: Gastrointestinal Physiology Chapter 65: Digestion and Absorption In the Gastrointestinal Tract Guyton and Hall, Textbook of Medical Physiology, 12th edition

2. Digestion of the Various Foods by Hydrolysis • Hydrolysis of Carbohydrates • When cbhs are digested, the water molecule that • was removed during the synthesis reaction, is • added back to split the polysaccharide

3. Digestion of the Various Foods by Hydrolysis • Hydrolysis of Fats • The fat digesting enzymes return 3 water molecules • to the triglyceride and break the fatty acids away • from the glycerol • Hydrolysis of Proteins • Reverse of the condensation synthesis reaction also; • the hydrogen from water goes to one of amino acid • in a peptide bond and the hydroxyl ion goes to the • other amino acid

4. Digestion of the Various Foods by Hydrolysis • Digestion of Carbohydrates • Cbh foods of the diet- consists of sucrose, lactose, • and starches • Digestion in the mouth and stomach-ptyalin (alpha- • amylase)-eventually blocked by the acid of the • stomach • Small intestine-pancreatic amylase; usually • converted to maltose or other small glucose polymers

5. Digestion of the Various Foods by Hydrolysis • Digestion of Carbohydrates • Hydrolysis by intestinal epithelial enzymes- • enterocytes containing lactase, sucrase, maltase, • and alpha-dextrinase Fig. 65.1 Digestion of carbohydrates

6. Digestion of the Various Foods by Hydrolysis • Digestion of Proteins • In the stomach- pepsin is the most important peptic • enzyme of the stomach; active at pH 2-3 but • inactive at pH5 Fig. 65.2 Digestion of proteins

7. Digestion of the Various Foods by Hydrolysis • Digestion of Proteins • Most protein digestion results from actions of • pancreatic proteolytic enzymes: trypsin, • chymotrypsin, carboxypeptidase and proelastase • Digestion of peptides by peptidases in the entero- • cytes that line the small intestinal villi; cells have a • brush border containing hundreds of microvilli • d. More than 99% of the absorbed products are • individual amino acids

8. Digestion of the Various Foods by Hydrolysis • Digestion of Fats- the most abundant fats in the diet • are triglycerides Fig. 65.3 Hydrolysis of neutral fat catalyzed by lipase

9. Digestion of the Various Foods by Hydrolysis • Digestion of Fats • Triglycerides or neutral fats are more commonly • found in foods of animal origin • Digestion of fats in the stomach- small amount • (<10%) by a lingual lipase (lingual glands of the • tongue) • First step of fat digestion is emulsification by • bile acids and lecithin

10. Digestion of the Various Foods by Hydrolysis • Digestion of Fats • Triglycerides are digested by pancreatic lipase • End products of fat digestion are free fatty acids Fig. 65.4 Digestion of fats

11. Digestion of the Various Foods by Hydrolysis • Digestion of Fats • Bile salts from micelles that accelerate fat digestion • Digestion of cholesterol esters and phospholipids- • two other lipases in the pancreatic juice; • cholesterol ester hydrolase and phospholipase A

12. Basic Principles of GI Absorption • Anatomical Basis of Absorption- total quantity of • fluid absorbed each day by the intestines is • equal to the ingested fluid plus that secreted • by the various gastrointestinal secretions (8-9 • liters); about 7 is reabsorbed in the small • intestine. • Only about 1.5 passes through the ileocecal • valve into the colon each day; stomach is a • poor absorptive area (except alcohol and • aspirin for example)

13. Basic Principles of GI Absorption • Anatomical Basis of Absorption • Folds of Kerckring, villi, and microvilli increase the • intestinal mucosal absorptive area by 1000 fold Fig. 65.5 Longitudinal section of the small intestine, showing the valvulaeconniventes covered by villi

14. Basic Principles of GI Absorption • Anatomical Basis of Absorption • The folds are called valvulaeconniventes and • extend most of the way around the intestine • Also, there are millions of villi all the way down to • the ileocecal valve

15. Basic Principles of GI Absorption • Anatomical Basis of Absorption Fig. 65.6 Functional organization of the villus; longitudinal (A) and cross-sections (B)

16. Basic Principles of GI Absorption • Absorption in the Small Intestine • Absorption of water by osmosis • Absorption of ions- active transport of sodium; • sodium is also co-transported by several specific • carrier proteins: • Sodium-glucose co-transporter • Sodium-amino acid co-transporters • Sodium-hydrogen exchanger

17. Basic Principles of GI Absorption Fig. 65.8 Absorption of sodium, chloride, glucose, and amino acids through the intestinal epithelium; note the osmotic absorption of water

18. Basic Principles of GI Absorption Aldosterone greatly enhances the absorption of sodium. Absorption of choride ions in the small intestine- mainly by diffusion following an electrochemical gradient caused by sodium flow Absorption of bicarbonate ions in the duodenum and jejunum-when sodium is absorbed, small amounts of hydrogen are secreted into the lumen of the gut in exchange for some of the sodium.

19. Basic Principles of GI Absorption Hydrogens combine with bicarbonate to form carbonic acid which then dissociates to form water and carbon dioxide. Water remains as part of the chyme, but the carbon dioxide is absorbed into the blood, and expired through the lungs—this is called the “active absorption of bicarbonate ions” and is the same mechanism that occurs in the tubules of the kidney. Secretion of bicarbonate ions in the ileum and large intestine—simultaneous absorption of chloride ions

20. Basic Principles of GI Absorption • Active absorption of calcium, iron, potassium, • magnesium, and phosphate • Absorption of Nutrients • Cbh are mainly absorbed as monosaccharides • Proteins are absorbed as dipeptides, tripeptides, or • amino acids • Fats are absorbed as micelles into lacteals; small • amounts are absorbed as fatty acids into the portal • blood

21. Absorption in the Large Intestine-Formation of Feces • Absorption and Secretion of Electrolytes and Water • High capacity for the active absorption of sodium • and chloride (due to electrochemical gradient of • sodium) • Creates an osmotic gradient which in turn causes • absorption of water • Mucosa secretes bicarbonate to counter the acidity • of bacterial end products

22. Absorption in the Large Intestine-Formation of Feces • Maximum Absorption Capacity of the Large Intestine • 5-8 liters of fluids and electrolytes each day • Bacterial action in the colon-vitamin K, vitamin B-12, • thiamine, riboflavin, and gases (carbon dioxide, • hydrogen gas, and methane) • Composition of the feces- 75% water and 25% solid • matter; solid matter is 30% dead bacteria, 19-20% fat, • 10-20% inorganic matter, 2-3% protein, 30% undigested • roughage; color is due to bilirubin by-products; odor • due to bacterial actions