Gastrointestinal Physiology

1. Gastrointestinal Physiology Dr. Meg-angela Christi Amores

3. Functional anatomy • The gastrointestinal wall (from outer to inner) • Serosa • Longitudinal muscle • Circular muscle • Submucosa • Mucosa • Mucosal muscle

4. Functional anatomy • motor functions of the gut are performed by the different layers of smooth muscle • In the longitudinal muscle layer, the bundles extend longitudinally down the intestinal tract; • in the circular muscle layer, they extend around the gut • each muscle layer functions as a syncytium

5. Neural control – ENTERIC nervous system • a nervous system all its own • lies entirely in the wall of the gut • especially important in controlling gastrointestinal movements and secretion • two plexuses: • myenteric plexus or Auerbach's plexus • controls mainly the gastrointestinal movements • submucosal plexus or Meissner's plexus • controls mainly gastrointestinal secretion and local blood flow

6. Assignment • Describe the differences between the Myenteric plexus and the submucosal plexus

7. Hormonal control • Gastrin • secreted by the "G" cells of the antrum of the stomach • stimulation of gastric acid secretion and stimulation of growth of the gastric mucosa • Cholecystokinin • secreted by "I" cells in the mucosa of the duodenum and jejunum in response to digestive products of fat, fatty acids, and monoglycerides • strongly contracts the gallbladder, expelling bile into the small intestine

8. Hormonal Control • Secretin • secreted by the "S" cells in the mucosa of the duodenum in response to acidic gastric juice emptying into the duodenum • acts to promote pancreatic secretion of bicarbonate • to neutralize the acid in the small intestine • Gastric inhibitory peptide • secreted by the mucosa of the upper small intestine • decreasing motor activity of the stomach and therefore slows emptying of gastric contents

9. Secretory functions • 2 types of secretion • Digestive enzymes • Mucus • are formed only in response to the presence of food in the alimentary tract

10. SECRETIONS • Saliva • Esophageal secretions • Gastric secretions • Pancreatic secretions • Bile (liver) • Secretions of the small intestine • Secretions of the large intestine

11. Average daily volume of secretions

12. Salivary secretions • principal glands of salivation are the parotid, submandibular, and sublingual glands • two major types of protein secretion: • (1) a serous secretion that contains ptyalin (an α-amylase), which is an enzyme for digesting starches • (2) mucus secretion that contains mucin for lubricating and for surface protective purposes

13. Saliva • 0.5 milliliter of saliva, is secreted each minute • Becomes very little during SLEEP • secretion plays an exceedingly important role for maintaining healthy oral tissues • flow of saliva itself helps wash away pathogenic bacteria • contains several factors that destroy bacteria • contains significant amounts of protein antibodies that can destroy oral bacteria

14. Gastric secretions • oxyntic glands (also called gastric glands) • hydrochloric acid, pepsinogen, intrinsic factor, and mucus • body and fundus (proximal 80 per cent of stomach) • pyloric glands • mucus for protection of the pyloric mucosa from the stomach acid • Gastrin • antral portion of the stomach (distal 20%)

16. Gastric secretion • parietal cell (also called oxyntic cell), demonstrating that it contains large branching intracellular canaliculi • where HCl is produced

17. Gastric secretion • Pepsinogen • has no digestive activity when first secreted • as soon as it comes in contact with hydrochloric acid, it is activated to form active pepsin • for protein digestion in the stomach • Intrinsic factor • essential for absorption of vitamin B12 in the ileum

18. Gastric Secretion • Pyloric glands • almost no parietal cells • contain mostly mucous cells • Secrete mucus • Secrete gastrin in G cells • Act on histamine to stimulate gastric hydrochloric acid secretion

19. Pancreatic Secretions • Secretes DIGESTIVE enzymes, bicarbonates • secreted most abundantly in response to the presence of chyme in the upper portions of the small intestine • digesting proteins : trypsin, chymotrypsin, and carboxypolypeptidase • digesting carbohydrates: pancreatic amylase • fat digestion: pancreatic lipase, cholesterol esterase, phospholipase

20. Liver secretions - BILE • Functions: • fat digestion and absorption • 1) they help to emulsify the large fat particles of the food into many minute particles, the surface of which can then be attacked by lipase enzymes secreted in pancreatic juice • (2) they aid in absorption of the digested fat end products through the intestinal mucosal membrane • serves as a means for excretion of several important waste products from the blood

21. Liver secretions - BILE • Bile is secreted continually by the liver cells, but most of it is normally stored in the gallbladder • maximum volume that the gallbladder can hold is only 30 to 60 milliliters • When food begins to be digested in the upper gastrointestinal tract, the gallbladder begins to empty • the most potent stimulus for causing the gallbladder contractions is the hormone cholecystokinin

23. Small Intestinal Secretions • Duodenum: Brunner’s Glands – secrete mucus • Small intestine – crypts of Leiberkuhn – enzymes: • Peptidase (small peptides to amino acids) • Sucrase, maltase, isomaltase, lactase (splitting disaccharides to monosaccharides) • Intestinal lipase – neutral fats to glycerol and fatty acids

24. Large Intestinal Secretions • Crypts of Leiberkuhn – mucus • protects the intestinal wall against excoriation • provides an adherent medium for holding fecal matter together • protects the intestinal wall from the great amount of bacterial activity • mucus plus the alkalinity of the secretion

25. SO HOW is FOOD DIGESTED? • Bread: starch • Mouth: • Esophagus: • Stomach: • Pancreas/Liver: • Small intestine: • Large Intestine

26. Digestion and Absorption • To be continued......