 |
| Absorption |
|
| After the food is broken down into a soluble form, the intestinal walls of ileum absorb the nutrients and also water. Due to its larger surface area, thin wall lining the cavity and large number of finger-like projections, called the villi, maximum absorption takes place in this portion of the alimentary canal. The food that is absorbed passes through the epithelial cells of the villi and then into the blood capillaries in the villi. From the capillaries, the fat-soluble substances pass into the lymph. These substances are then transported throughout the body by the lymphatic system, which drains them into the blood near the heart. |
| |
| Absorption takes place in two ways: Passive absorption and active absorption |
| |
| Passive absorption |
| |
| The absorption of certain nutrients and water takes place by passive absorption. In this process, there is no expenditure of energy. The absorption depends on the concentration gradient across the cell membranes of the villi. If the nutrients are more in concentration in the lumen they are absorbed into the intestinal walls. This process goes on as long as the concentration of the nutrients remains more. The entry of solute particles (nutrients) increases the osmotic pressure. This makes the water in the intestinal lumen enter the cells of the villi by osmosis (that is, from the region of higher concentration to the region of lower concentration). These are slow processes. Slightly faster is facilitated diffusion that involves the absorption of nutrients along the concentration gradient but with the help of a carrier molecule. |
| |
| Active absorption |
| |
| This process is much faster and requires energy. A membrane pump operates across the cell membranes which transports the nutrients from the lumen of the intestine into the cells. This transport occurs with the help of ATP molecules. The carrier proteins in the membrane act as ATPases and hydrolyse, the ATP molecule to provide energy. This energy is used to transport nutrient molecules across the membrane. The transport occurs irrespective of the concentration of the nutrients in the lumen. Nutrients like sodium ions, glucose, galactose and amino acids are transported actively. Absorption of sodium ions is necessary for the absorption of the other three types of nutrients. This type of absorption results in fast and almost complete absorption of nutrients from the intestinal lumen into the blood. |
| |
 |
| |
| Absorption of digested food (a) Amino acids and monosaccharides, and (b) Fatty acids |
| |
| Fat absorption |
| |
| Fats are broken down into glycerides and fatty acids. These are insoluble in water and therefore cannot be absorbed by the intestinal cells directly. Sterols and fat-soluble vitamins also belong to this category of molecules. They are first acted upon by the bile salts and phospholipids to form small, spherical droplets called micelles that are water-soluble molecules and then absorbed. |
| |
| The reconstructed triglycerides combine with phospholipids and cholesterol in the intestine. They release protein coated water soluble fat globules or droplets called the chylomicrons into the lymph. These droplets are sent to blood through the central lacteal of the lymphatic system for distribution throughout the body. |
| |
| The undigested food then passes into the large intestine where it is temporarily stored. More water is absorbed from the waste matter to form solid faeces. |
| |
| The faeces are egested out of the anus by the movements of the colon. |
| |
