Secretory function of the digestive system

1. Secretory function of the digestive system

2. Objectives • Describe the location of the salivary glands, composition and function of saliva. • Outline the composition of the pancreatic juice. • Understand regulation of pancreatic juice secretion. • Explain function of bile. • Describe the role of CCK in causing release of bile from the gall bladder, including the effects on the sphincter of Oddi. • Describe the mechanisms by which chyme from the stomach is neutralized in the duodenum. • Explain functions of the liver • Define enterohepatic circulation • List the functions of gastric acid

3. Major Salivary Glands

4. Composition of the saliva • 99.5% water • 0.5% solutes • ions = Na+, K+, Cl-, HCo3, Po4- • Organic substances = urea, uric acids, mucus, immunoglobulin A • Enzymes : lysozyme salivary amylase • Saliva is slightly acidic (pH 5.8 -7.4)

5. Secretion of the saliva • Daily secretion: 1-2L, basal flow rate 0.5 mL/min → 5mL/min (max.). • Parotid gland secretes watery fluid (serous) contains salivary amylase • Submandibular glands secrete mixture of watery and mucus secretion • Sublingual glands secrete mainly mucus • Two main secretions: • Serous: contains enzyme α-amylase • Mucous: contains mucin for lubrication

6. Secretion of the saliva • Saliva stimulated by • the presence of food in the mouth • Smell, sight, sound, or thought of food • It is under control of the autonomic nervous system. • Parasympathetic stimulation causes vasodilatation and profuse secretion of watery saliva • Sympathetic stimulation causes vasoconstriction and secretion of small amount of saliva ( mouth dryness) e.g. stress

7. Secretion of the saliva • Dehydration, • Salivary glands stop secreting saliva , resulting in mouth dryness and contribute to the sensation of thirst.

8. Functions of Saliva • Lubricating effect thanks to water and mucin which: • Moistens food • Facilitates swallowing • Facilitates speech • Protective effect • keeps the mouth and teeth clean • IgA and lysozymes (defense against bacteria) • HCO3- prevents dental carries • Diluting noxious substances and the corrosive gastric acid and pepsin → esophagus and mouth during vomitting

9. Functions of Saliva cont… • Aids digestion of: • Carbohydrates: α-amylase Amylase breaks down starch into smaller molecules (pH 6.8) Amylase stops functioning at pH 1.5 (stomach) • Dietary lipid: lingual lipase • Good messenger for dehydration

10. Gastric secretion H+ H+ H+

11. Gastric Juice • About 2-3 liters/day • Contents: • Water & mineral salts secreted by gastric glands • Mucus secreted by goblet cells • HCl & intrinsic factor secreted by parietal cells • Enzyme precursors ( pepsinogens) secreted by chief cells.

12. Functions of the gastric juice • waterchange into a liquid the food swallowed • HCl • acidifies the food and stop the action of salivary amylase • kills ingested microbes • provides acidic environment for the activation of pepsinogens.

13. Acid secretion H+ ions are pumped into the lumen in exchange with K+ by H+/K+ pump (ATPase) K+ taken up by the cells is recycled by the K+ channels. HCO-3is added to blood in exchange with Chloride ions (Cl--HCO3-) exchanger). ALKALINE TIDE (↑ pH after a heavy meal) Cl-diffuses into the lumen through chloride channels

14. Acid Secretion Parietal cells have receptors for Histamine, Ach and Gastrin 1.HistamineIt works in a paracrine fashion. 2.Acetylcholine: released from the enteric and vagus nerve endings. 3. Gastrin Produced by G cells in the pyloric antrum and duodenum Gastrin secretion is suppressed when pH in the pyloric antrum falls to 1.5 Gastrin and Ach stimulate Histamine release.

15. Secretion of gastric juice

16. Functions of the gastric juice cont… • Pepsinogens (inactive pro-enzyme) • Secretd by chief cells • activated to pepsin by HCl • breaks protein to smaller molecules • most effective at pH 1.5-3.5 What keeps pepsin from digesting the protein in stomach cells along with the food?

17. Functions of the gastric juice cont… • Mucus Prevents • Mechanical injury to the stomach by lubricating the contents • Chemical injury by acting as barrier between the stomach wall and the corrosive gastric juice

18. Functions of the gastric juice cont… • Gastric Lipase -Splits the short chain triglycerides in fat molecules into fatty acids and monoglycerides. -Limited role in the adult stomach

19. Functions of the gastric juice cont… • Other functions of the stomach • Temporary storage allowing time for digestive enzymes • Mechanical breakdown of the food • Absorbs water, some lipids and certain drugs • Production of intrinsic factor (Vitamin B12 absorption)

20. Pancreas

21. Pancreas Endocrine pancreas (1% of its mass), Islets of Langherans:Glucagon, Insulin hormones Exocrine pancreas (99%), acini: enzymes and HCO3-

22. Exocrine pancreas • Exocrine pancreas secretes pancreatic juice consisting of enzymes and aqueous alkaline (NaHCO3)secretions. The duct & centroacinar cells actively secrete the aqueous NaHCO3. The acinar cells secrete 3 types of pancreatic enzymes: • Proteolytic enzymes • Pancreatic amylase • Pancreatic lipase

23. Exocrine pancreas secretes pancreatic juice consisting of: • Aqueous alkaline secretions • Enzymes

24. Pancreatic aqueous alkaline-secretion • Large volume of juice with high concentration of NaHCO3. • The largest component of pancreatic secretion: 1-2 L/day • The bicarbonate component is important in: • Neutralizing the acidic chyme emptied into the duodenum. • Providing optimal pH for digestive enzyme activity

25. Exocrine pancreas secretes pancreatic juice consisting of: • Aqueous alkaline secretions • Enzymes

26. Pancreatic enzymes

27. Pancreatic proteolytic enzymes • The protein digesting enzymes are secreted as inactive proenzymes: zymogens. • In the duodenum:zymogens are converted to active enzymes when enteropeptidases converts trypsinogen to trypsin • Trypsin, in turn, activates the other pancreatic zymogens Trypsin: central enzyme catalysing the activation of all other pancreatic enzymes • Chemotrypsinogen Procarboxypeptidase • Proelastase Why proteolytic enzymes secreted in inactive form?

28. Regulation of pancreatic secretion • Secretion of aqueous alkaline component is determined by the amount of acid in the duodenum. • Secretion of enzymes depends on the amount of fat & proteins in the duodenum

31. Bile -Bile is a yellow, brownish or olive –green liquid secreted by hepatocyte (0.5L/day) -It has a pH of 7.6-8.6 -It is stored and concentrated in the gall bladder

32. Composition of Bile 1. Bile acids Are synthesized from cholesterol Exist mainly as conjugated (bile salts) 2. Phospholipids Major ones are the lecithins 3. Cholesterol 4. Bile pigments (bilirubin, breakdown product of RBCs) 5. Inorganic ions (mainly Cl- and HCO3-) 6. Water

33. Function of bile Fat emulsification and excretion of cholesterol (Emulsification is the process by which fat globules are broken into smaller pieces by the detergent action of bile salts and especially lethicin

34. Bile secretion and enterohepatic circulation • Total amount of bile salts is smaller than what is needed daily to promote lipid digestion and absorption. • The enterohepatic circulation allows the recycling of bile salts • Body can recycle the same bile salts

35. Regulation of bile secretion Relaxation of sphincter of Oddi

36. Functions of the Liver • Carbohydrate Metabolism • Important in maintaining normal blood glucose • Blood glucose low --- liver breakdown of glycogen to glucose and release to blood • Blood glucose high--- liver converts blood glucose into glycogen and triglycerides for storage • Lipid Metabolism • Stores some triglycerides • Synthesize cholesterol • Use cholesterol to form bile salts • Breakdown fatty acids to generate ATP • Synthesize lipoproteins

37. Functions of the Liver cont.. • Protein Metabolism • Deamination of amino acids • Synthesis of most plasma proteins such as albumin, globulin, fibrinogen and prothrombin • Processing of drugs and hormones • Detoxification and excretion of some drugs and hormones • Excretion of bilirubin

38. Functions of the Liver cont.. • Synthesis of bile acids • Storage • Glycogen • Vitamins (A, B12, D, E, K) • Minerals (copper and iron) • Phagocytosis • Reticuloendothelial (kupffer) cells phagocytosed aged RBCs, WBCs and some bacteria • Activation of Vitamin D