The process by which digested food enters the blood and lymph from the alimentary canal is called absorption or absorption of food. The small intestine is the main site of absorption. Small intestine absorbs about 90% of food. 10% is absorbed by the stomach and colon. The lining of the small intestine folds to form a finger-like projection. It is called Vilai. Vilai increases the absorption capacity of the intestine by 40 times. There are about 500,000 villi in the human small intestine. The fine projections in villi cells are called microvilli. Microvilli join together to form a brush border. Controls the brush border absorption process. Microvilli increase the absorption capacity of the intestine by 60 times. Food is absorbed by endocytosis along with vitamins, minerals and water.
1. Protein Absorption: Proteins are digested to produce amino acids. Protein is absorbed in the small intestine as amino acids. However, semi-complex proteins like peptones, proteoses, peptides etc. are also absorbed in very small amounts. Egg whites, shrimp, crab etc. are absorbed unchanged. Amino acids are absorbed by the villi of the duodenum and jejunum through the process of active absorption, diffusion and phagocytosis. Normally, L-amino acids are absorbed by the active pathway and D-amino acids by absorption. The cytoplasmic tubule of Vilai functions in the absorption of proteins. Thyroxine hormone regulates protein absorption.
2. Absorption of sugars: Sugars are digested into monosaccharides like glucose, fructose, galactose, lactose, sucrose, xylose, mannose etc. The small intestine absorbs sugars as monosaccharides. However, some sugars are also absorbed as disaccharides. Of the monosaccharides, 80% are absorbed as glucose and 20% as fructose and galactose. With the help of Na+ and ATP, the tubules of the villi of the jejunum absorb glucose. Fructose is absorbed in the digestive process. Sucrose and lactose are absorbed in the duodenum and small intestine.
3. Lipid Absorption: Lipids are digested into fatty acids, glycerol, monoglycerides and cholesterol. Fatty acid absorption is a complex process. Fatty acids, monoglycerides and bile salts bind together to form micelles. Micelle particles are absorbed by the duodenum and mid-small intestine. Under the influence of thiokinase enzyme, fatty acids are converted into active fatty acids. Active fatty acids, glycerol and proteins join together to form triglycerides. These are called chylomicron particles. Chylomicron particles enter the lymphatic vessels of the small intestine. If the branches of the lymphatic duct are white, it is called lactial. Chylomicrons enter the blood plasma through lymphatic vessels. Enzymes located in the plasma break down chylomicrons or lipoproteins to form fatty acids and glycerol.
4. Water Absorption: Water is absorbed by the epithelial cells of the villi of the small intestine. Normally 200-400 ml of water is absorbed per hour.
5. Mineral Salt Absorption: Mineral salts are absorbed by the epithelial cells of the villi of the colon.
6. Absorption of Vitamins: Vitamins are absorbed when dissolved in water or fat by the epithelial cells of the villi of the small intestine.
Absorption of nutrients is a very important process for the body. Digestion produces amino acids from proteins, glucose and fructose from sugars and fatty acids and glycerol from lipids. The small intestine absorbs all the digested food. Then it reaches different parts of the body through the blood. Body cells take all these nutrients and stay healthy, strong and fresh.