Absorption and Assimilation of Digested Products in Humans
Absorption is the passage of digested food through the lining of the intestine into the blood or lymph. Almost all absorption takes place from the small intestine. The absorption of water takes place mainly in the colon (large intestine).
During the course of digestion, carbohydrates are broken down into simple sugars. These sugars pass into the blood capillaries of the intestinal villi. The products of digestion of fat are fatty acid and glycerol. These are absorbed through the intestinal wall by lacteals. The final products of protein digestion are the amino acids and in this form they are absorbed into the blood. The large intestine harbours many bacteria and is the site of synthesis of vitamin B. The vermiform appendix in the human has no known function to perform.
Mechanism of Intestinal Absorption
Substances are absorbed from the intestine by various mechanisms. These include passive diffusion, osmosis and active transport. Of these, diffusion and osmosis depend on concentration gradients, and active transport takes place against concentration gradients using metabolic energy.
Generally water is absorbed by the intestinal cells and from there to the blood by osmosis. The concentration of solutes in the blood is higher than that in intestinal lumen. Water is also absorbed in large quantities whenever there is increased absorption of minerals.
Most of the absorption of digested materials is through active transport. The process of active transport uses ATP to push materials 'uphill' from an area with a relatively low concentration of small molecules (such as the intestinal lumen) to one with relatively high concentration of small molecules (such as an intestinal mucosal cell).
The products of fat digestion such as, monoglycerides, diglycerides and fatty acids are insoluble in water. Because of this, they are not directly absorbed from the intestine. They are at first incorporated into small spherical water soluble droplets called micelles. The formation of micelles is greatly influenced by bile salts and phospholipids in the intestinal lumen. From these micelles which are aggregates of many molecules, the fatty acids, glycerides, sterols and vitamins are absorbed into the intestinal cells. Any obstruction of bile from entering into the small intestine results in prevention of the formation of micelles. This leads to lack of absorption of lipid materials (obstructive jaundice). The unabsorbed materials are excreted along with faeces.
Assimilation of Food
In the body, the assimilation of digested food takes place in the following manner:
- The final products of fat digestion are the fatty acids and glycerol. These substances are absorbed into the blood stream through the lacteals. In the body they are again converted into fats, which are stored in the adipose tissue. This stored fat can be utilised by the body when there is a greater metabolic need. Then once again the fat is broken down into fatty acid, and glycerol by the action of lipase;
- The simple sugars absorbed through the intestinal villi reach the blood and the tissues. Simple sugars are metabolized to produce the necessary energy for various activities of the animals. The extra sugar is converted into a complex polysaccharide, glycogen, in the liver. This stored glycogen is mobilised and utilised during conditions of stress. The excess amount of fat can also be converted into carbohydrates by a process called gluconeogenesis. These carbohydrates are also stored as explained above;
- The amino acids absorbed from the intestine are utilised to synthesise various type of proteins in the body. The body needs to synthesise various tissue proteins and various types of enzymes.
The Hormonal Control of Digestive Secretions: (+) Activation, (-) Inhibition