Physiologic Anatomy of Biliary Secretion

Secretory Functions of the Alimentary Tract Bile Secretion By The Liver •   600 and 1000 ml/day Bile plays an important role in fat digestion and absorption, not because of any enzymes in the bile that cause fat digestion, but because bile acids in the bile perform two functions: • Emulsify the large fat particles into many minute particles, the surface of which can then be attacked by lipase. • They aid in absorption of the digested fat end products. Bile serves as a means for excretion of several important waste products from the blood (bilirubin and excesses of cholesterol) Secretory Functions of the Alimentary Tract Physiologic Anatomy of Biliary Secretion • • Storing and Concentrating Bile in the Gallbladder Maximum volume gallbladder can hold is 30-60 ml. as much as 12 hours of bile secretion (usually about 450ml) can be stored in the gallbladder because water, sodium, chloride, and most other small electrolytes absorbed, concentrating the remaining bile constituents that contain the bile salts, cholesterol, lecithin, and bilirubin. Secretory Functions of the Alimentary Tract Gallbladder Emptying • • Cholecystokinin Gallbladder is also stimulated less strongly by acetylcholine-secreting nerve fibers from both the vagi and ENS. Secretory Functions of the Alimentary Tract Function Of Bile Salts In Fat Digestion And Absorption • • • • Detergent action on the fat particles in the food Bile salts help in the absorption of :  Fatty acids  Monoglycerides  Cholesterol  Other lipids forming small physical complexes with these lipids; the complexes are called micelles. Without the presence of bile salts in the intestinal tract, up to 40 percent of the ingested fats are lost into the feces. Secretory Functions of the Alimentary Tract Enterohepatic Circulation of Bile Salts • •  • About 94 percent of the bile salts are reabsorbed into the blood from the small intestine, one half by diffusion through the mucosa in the early portions of the small intestine and the remainder by an active transport process in the distal ileum. Salts make the entire circuit 17 times before being carried out in the feces. Role of Secretin in Controlling Bile Secretion Secretion of a sodium bicarbonate–rich watery solution Secretory Functions of the Alimentary Tract Liver Secretion of Cholesterol and Gallstone Formation • Cholesterol is almost completely insoluble in pure water, but the bile salts and lecithin in bile combine physically with the cholesterol to form ultramicroscopic micelles in the form of a colloidal solution. Secretory Functions of the Alimentary Tract Secretions of The Small Intestine • • • • Secretion of mucus by Brunner’s Glands in the duodenum Secrete large amounts of alkaline mucus in response to • Tactile or irritating stimuli • Vagal stimulation • Secretin Protect the duodenal wall from digestion by the highly acidic gastric juice. the mucus contains a large excess of bicarbonate ions. Brunner’s glands are inhibited by sympathetic stimulation (peptic ulcers) Secretory Functions of the Alimentary Tract Secretion of Intestinal Digestive Juices by The Crypts of Lieberkühn • The surfaces of both the crypts and the villi composed of two types of cells: • • • • Moderate number of goblet cells, which secrete mucus Large number of enterocytes, secrete large quantities of water and electrolytes. Secretions by the enterocytes about 1800 ml/day. pH 7.5 - 8.0 Secretory Functions of the Alimentary Tract Digestive Enzymes in the Small Intestinal Secretion Peptidase Disaccharidase Intestinal lipase • Splitting small peptides into amino acids • Sucrase • Maltase • Isomaltase • Lactase • Splitting neutral fats into glycerol and fatty acids Secretory Functions of the Alimentary Tract Secretion of Mucus By The Large Intestine • • •  • • • The mucosa of the large intestine, has many crypts of Lieberkühn. It contains no villi. The epithelial cells secrete almost no digestive enzymes. Regulation of mucus secretion Direct, tactile stimulation of the epithelial cells Local nervous reflexes Stimulation of the pelvic nerves from the spinal cord Secretory Functions of the Alimentary Tract Secretion of Mucus By The Large Intestine  Functions of the Mucus in the large intestine • • • • 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 (a pH of 8.0 caused by large amounts of sodium bicarbonate) provides a barrier to keep acids formed in the feces from attacking the intestinal wall.