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

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
water change 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-3 is 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.Histamine It 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
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 23

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 25

26. Pancreatic enzymes

27. Why proteolytic enzymes secreted in inactive form?
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
-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 Lipid 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