 In this video I will describe the mechanisms regulating the secretion of bile, describe the enterohepatic circulation of bile salts, describe the function of bile salts and lipase, and then describe the digestion and absorption of lipids. Bile is an exocrine secretion produced by hepatocytes in the liver. Bile secreted through hepatic ducts and then stored into the gallbladder. Within the gallbladder bile becomes concentrated and then bile will exit the gallbladder through the cystic duct and common bile duct where it will then travel down into the duodenum. Bile acids are produced from cholesterol by hepatocytes within the liver. These bile acids are then conjugated in the liver producing the conjugated bile acids that are also known as bile salts. The bile salts are amphiphilic molecules that have a hydrophilic region and a hydrophobic region and this is important for the function of bile enabling emulsification of fat to break down large fat droplets and facilitate the chemical digestion of triglycerides. Bile is reabsorbed by the intestines so that the bile salts and bile acids can be recycled so the enterohepatic circulation is the absorption of these bile salts and bile acids from the intestine into the blood that travels back to the liver where the bile salts and bile acids can then be reprocessed into bile and secreted again into the intestines. Bacteria within the intestines can catalyze chemical reactions to modify the bile acids. A decarboxylation reaction is one of those reactions that's carried out by enzymes within gut bacteria. As we can see here the conversion of the bile acid-collic acid to a secondary bile acid-deoxy-collic acid requires the bacteria within the gut. This just adds more variety to the number of bile acids and bile salts that are participating in the emulsification of lipids facilitating chemical digestion and absorption of lipids. So the bile salts and bile acids are amphiphilic molecules that have a hydrophilic side and a hydrophobic side and this enables the emulsification of fat droplets as the hydrophobic side is able to mix with a lipid droplet. But the hydrophilic side enables this lipid droplet to become suspended within an aqueous solution. Large droplets of fat will be broken down in the process of emulsification to produce smaller particles that are known as miceels. A miceel is a tiny molecular sphere with a hydrophilic exterior and a hydrophobic interior. The secretion of bile is regulated through nervous as well as endocrine mechanisms. Bile acids and bile salts are continuously being produced by hepatocytes in the liver and this bile drains through the hepatic ducts but will not enter into the duodenum as long as the sphincter of odi remains closed bile will back up into the gall bladder where the bile can then be stored in the gall bladder and become concentrated. So the nervous system regulates the secretion of bile by stimulating contraction of the gall bladder. The parasympathetic division of the autonomic nervous system will stimulate contraction of the gall bladder. The primary endocrine mechanism that stimulates the secretion of bile is the hormone CCK, cholecystokinin. When you consume a meal that's rich in fat, that fat is detected by chemoreceptors within the duodenum. The eye cells, the endocrine cells that secrete CCK, so then CCK travels through the blood and then binds to receptors on cells in the gall bladder to stimulate contraction of the gall bladder. CCK will also cause the sphincter of odi, the smooth muscle surrounding the opening from the common bile duct to the duodenum to relax. Then bile will flow down from the gall bladder and from the hepatic ducts of the liver through the common bile duct into the duodenum where bile can then mix with chym and the pancreatic juice and facilitate the chemical digestion and then absorption of lipid soluble nutrients. The interohepatic circulation of bile salts refers to the reabsorption of bile salts and bile acids by the blood vessels in the small intestine, primarily in the distal end of the ilium. Bile salts are reabsorbed into the blood of the hepatic portal system, the hepatic portal vein carries blood from the intestines into the liver, and then the hepatocytes can remove the bile salts and bile acids from this blood and then secrete the bile salts and bile acids into the bile ducts, So the majority of bile that is secreted will be reabsorbed, about 95% of bile salts and bile acids are reabsorbed through the interohepatic circulation, and only about 5% of the bile salts and bile acids will be lost in the feces. So bile enables the emulsification of fat droplets forming smaller fat droplets that are dispersed within the aqueous solution inside of the duodenum. This will increase the surface area that can be exposed to the enzyme lipase. So lipase is the enzyme that catalyzes hydrolysis of triglycerides. The pancreatic lipase is the primary enzyme responsible for chemical digestion of triglycerides, converting triglycerides to fatty acids, and monoacial glycerides, or diacial glycerides. So when a triglyceride is chemically digested within the duodenum, it will usually not be broken down all the way to glycerol and three free fatty acids. Instead, one or two fatty acids will be removed from the triglyceride, and with two fatty acids removed, most of the triglycerides are converted to monoglycerides. Then these monoglycerides are amphiphilic molecules that will help contribute to emulsification. So fatty acids and monoglycerides join the mycelium with the hydrophilic regions of those molecules facing the aqueous solution and the hydrophobic regions of those molecules facing the interior of the mycelium. So lipase is the enzyme that catalyzes the hydrolysis of triglycerides. A triglyceride can be broken down to produce one monoglyceride and two free fatty acids. Following chemical digestion of triglycerides, lipid-soluble nutrients like fatty acids and monoglycerides are absorbed across the apical surface of enterocytes by simple diffusion. Then once inside of the enterocyte, fatty acids and monoglycerides are joined back together forming triglycerides, and then these triglycerides are packaged into lipoprotein particles called chylomicrons. The Golgi apparatus packages lipid-soluble nutrients into chylomicrons, then chylomicrons are transported through a secretory vesicle and excreted by exocytosis at the basolateral surface of the enterocyte. The chylomicrons are too large to enter the blood at the capillaries inside of the small intestine. Instead, chylomicrons will enter lacteals, which are lymphatic capillaries. The lacteals will drain into larger lymphatic vessels and lymphatic ducts that eventually join to form the thoracic duct, which drains lymph into the blood at the subclavian veins. Here we see the structure of a chylomicron, which is a lipoprotein particle with an outer layer of phospholipid and embedded apolipoproteins, then deep within the interior of the chylomicron are the triglycerides and other lipid-soluble nutrients. The apolipoproteins on the surface of a chylomicron bind to receptors on the surface of cells throughout the body. In organs like the liver, these chylomicrons will be taken up and processed in order to store nutrients and regulate the distribution of lipid-soluble nutrients throughout the body. Another major tissue that would take up lipoprotein particles in order to store lipid-soluble nutrients is the adipose tissue. The adipose site's fat cells within adipose tissue will also store triglycerides until they're needed to be broken down to release energy.