The gallbladder is a thin walled green muscular sac on the inferior surface of the liver. The gallbladder stores bile that is not immediately needed for digestion and concentrates it. When the muscular wall of the gallbladder contracts bile is expelled into the bile duct. Bile secretion: Bile is necessary for digestion and absorption of lipids in the small intestine. Bile is a mixture of bile acids, bile pigments and cholesterol. Bile is produced and secreted by liver, stored in gallbladder, and ejected into the lumen of small intestine. It emulsify lipids to prepare them for digestion. When chyme reaches the small intestine CCK is secreted which stimulates contraction of gallbladder and relaxation of sphincter of Oddi, to flow bile into the lumen of the duodenum. When lipid absorption is complete, bile acids are recirculated to liver. figure(11) Figure (11)bile secreon The function of the gall bladder is :- 1-to store bile 2- to concentrate bile ( epithelial cells of gall bladder absorb ions and water)
3 -to eject bile into the lumen of the small intestine. l. Bile is constituents of *bile acid (50%) *bile pigments as billirubin (2%). *cholesterol (4%) *and phospholipids (40%). *Bile contains electrolytes and water.. The main stimulus for ejection of bile is CCK which is secreted by I cell in response to amino acids and fay aci d. 95% of secret ed bi ll acids are recirculated to the liver, bile acid excreted in feces 5%, must be replaced by synthesis of new bile acids. Role of bile secretion A-detergent action : Bile secretion decrease the surface tension of the substance so it will brake them into smaller particle called micelles to increase the surface area to about 1000 me s original one in which lipase will do it s effect. B hydrotropic effect : Which means makes lipid water soluble to pass easily via epithelial cell membrane bile secretion regarded as amphipathic molecule which means that it has 2 poles one is the hydrophilic pole which is formed from dissociate carboxyl group or hydroxyl group and the other is the methyl group which represent the hydrophobic pole of the bile salts.bile salts arrange themselves around the lipid molecule in such away that the hydrophilic group in the surface and the hydrophobic toward the lipid particles so the surface of lipid will be water soluble. Digestion
I t is chemical breakdown of ingested food into absorbable molecules, The digestive enzymes are secreted by salivary gland, gastric gland, pancreatic glands, and apical membrane of intestinal epithelial cells. Digestion of carbohydrates: Three major sources of carbohydrates exist in the normal human diet. These are sucrose (table sugar), lactose (disaccharide) in milk and starches (polysaccharides) present in all non animal foods, and grains. Other carbohydrates ingested are glycogen, alcohol and other. Carbohydrate digestion and absorption:- *Carbohydrates start breaking down in the mouth. *The enzyme salivary amylase starts breaking up the starches reducing it into smaller glucose molecules where in the stomach it continues to be broken down further. *Upon entering the small intestine the pancreas releases the enzyme pancreatic amylase to help complete the hydrolysis of starch into smaller chains of glucose molecules monosaccharide's, which is 1 molecule of sugar. * The monosaccharide's are absorbed into the small intestine and delivered to the liver by way of the hepatic portal vein. *After the liver processes the nutrients, the nutrients enter into the blood stream circulating throughout the body. Lipid or fat digestion and absorption:- In the stomach fats are broken down into glycerol and fatty acids. * The digestion of fats starts in the stomach when mixed with the *enzyme lipase. *The major part of the breakdown takes place in the small intestine. *In the duodenum the enzyme pancreatic lipase furthers the process by breaking the fats down from triglycerides to monoglycerides (which is 1 fay aci d ins tead of 3 fay aci ds connected to a glycerol molecule).
*Bile is produce in the liver and secreted by the gall bladder which increases the lipids solubility, breaking it down into droplets, making it easier for the small intestine to absorb. *When foods with high lipid content enter the stomach, the hormone gastric inhibitory peptide is released, slowing down movement flow out of the stomach. *This is why we feel full after eating high fat foods. Protein Digestion and Absorption:- *Proteins are split into linked amino acids called peptides and then into individual amino acids. *In the stomach the enzyme pepsin starts the breakdown of proteins into smaller units called polypeptides and peptides. *In the duodenum of the small intestine the pancreatic enzymes trypsin and chymotyrpsin also split proteins into polypeptides and peptides. *In the jejunum of the small intestine an enzyme created by the small intestine called peptidase splits the large peptides into smaller peptides and than into amino acids. *All of these smaller protein fragments go directly to the liver by the hepatic portal vein. Once in the liver one of three things happens to the proteins: 1. It converts to glucose, 2. It converts to fat or 3. It is directly released into the blood as amino acids. Glossary of Terms
Duodenum long. Jejunum long. -2 meters Ileum The third secon of the sma l l int es ne. It i s 1.5 met ers l ong. Hepatic Portal Vein The vein that delivers nutrients rich blood to the liver. Esophageal Sphincter A gateway separating the esophagus and the stomach, controlling the entry of food. Pyloric sphincter A gateway separating the stomach and the small intestine, controlling the entry of chyme. Chyme A slushy acidic mixture of food and digestive juices. Enzymes Protein molecules that increase the speed of chemical reactions in the body. They work by combining with and altering the molecules of other chemical substances. The digestive enzymes split large molecules of food into smaller units for absorption. Villi Projections along the small intestinal wall increasing area surface for greater absorption. Hydrolysis splitting of substances by adding water Glycerol the combination of fats and oils. Absorption It is movement of nutrients, water and electrolytes from the lumen of the intestine into the blood. There are two path for absorption, a cellular path and paracellular path. In the cellular path the substance must cross the apical (luminal) membrane, then enter the tight junction intestinal epithelial cell,
and be extruded from the basolateral membrane. In paracellular, through intercellular spaces between intestinal epithelial cells, and to the blood. The structure of intestinal mucous is suited for absorption of large quantities of nutrients though villi and microvilli which increase the surface of a small intestine. The villi are largest in the duodenum, where digestion and absorption occur and shortest in the terminal ileum. The surface of the villi are covered with epithelial cells, the apical surface of epithelial cells is further expanded by tiny enfolding called microvilli, this microvilli surface is called the brush border because it is brush like appearance, under light microscopy. The villi and microvilli increase total surface area by 600 fold. The epithelial cells of small intestine are replaced every 3-6 days.