NCERT Solutions for Class 11 Biology Role of Liver in the Digestion
Biology notes for Class 11: The Liver is a vital organ, only found in vertebrates. In humans, it is located in the upper right of the abdomen, below the diaphragm. The liver has a wide range of functions, including detoxification of various metabolites, protein synthesis, and the production of biochemicals necessary for digestion. It also plays a role in metabolism, regulation of glycogen storage, decomposition of red blood cells and hormone production, but its main function within the digestive system is to process the nutrients absorbed from the small intestine. The liver is a gland. It is an accessory digestive gland and produces bile, an alkaline compound which aids in digestion via the emulsification of lipids. The gallbladder, a small pouch is located under the liver, stores bile produced by the liver.
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Role of Liver in the Digestion
The main functions of the liver as an accessory digestive organ within the human digestive system are:
The secretion of bile and bile salts: In addition to pancreatic and intestinal secretions, a third secretion that is poured into the duodenum is the bile. Bile or gall is a dark green to yellowish-brown fluid and alkaline in nature, produced by the liver and aids the digestion of lipids in the small intestine. In humans, bile is produced continuously by the liver (liver bile) and stored and concentrated in the gallbladder (gallbladder bile). The composition of gallbladder bile is 97% water, and rest is made up of bile salts(the sodium salts secreted in bile, sodium taurocholate and sodium glycocholate, which greatly lower surface tension and are important in emulsifying fats), bile pigments (the complex, highly coloured waste products of the hemoglobin of old red blood cells, found in the bile, bilirubin , biliverdin, their derivatives; urobilinogen, urobilin, bilicyanin, and bilifuscin), and stercobilin). Other constituents are cholesterol, fatty acids and lecithin, and inorganic salts.
Bile acts to some extent as a surfactant(any substance that tends to reduce the surface tension of a liquid in which it is dissolved) and helps to emulsify the lipids in food. Bile salt anions are hydrophilic on one side and hydrophobic on the other side; hence they tend to aggregate around droplets of lipids (triglycerides and phospholipids) to form micelles, with the hydrophobic sides towards the fat and hydrophilic sides facing outwards. The hydrophilic sides are negatively charged, and this charge prevents fat droplets coated with bile from re-aggregating into larger fat particles.
The dispersion of food fat into micelles increases the surface area for the action of the enzyme pancreatic lipase, which actually digests the triglycerides, and is able to reach the fatty core through gaps between the bile salts. A triglyceride is broken down into three fatty acids and a monoglyceride, which are absorbed by the villi on the intestine walls. Thus, bile increases the absorption of fats, it is an important part of the absorption of the fat-soluble substances, such as the vitamins A, D, E, and K.
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Regulation of Bile Secretion
When the food enters the duodenum, the liver is stimulated to produce more bile. The contraction of the walls of the gallbladder (stores bile) takes place to release the bile in the duodenum. The hormone secretin stimulates the liver to produce bile. The evacuation of the gallbladder is controlled by a hormone called cholecystokinin produced by the duodenal mucosa.
Phagocytosis of bacteria and dead or foreign materials: Within the liver, blood passes through spaces called sinusoids, instead of through capillaries (as elsewhere in the body). A special type of cell called Kupffer’s Cells, which are also known as stellate reticuloendothelial cells, are located in the sinusoids and destroy many types of unwanted particles present in the bloodstream through the liver. Such particles include bacteria,
antigens, i.e. other substances from outside of the body (sometimes called “foreign matter”), imperfect or no-longer functioning blood cells (e.g. damaged leucocytes and erythrocytes).
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