The small intestine is described as the seat of absorption, This is because most of the end –product of digestion are absorbed there. The monosaccharides, amino acids and dipeptides are absorbed there either by simple diffusion or active transport(because this often occurs against their concentration gradients). The mineral salt, vitamin and water are also absorbed by diffusion.
All of these substances across the intestinal wall into blood capillaries. These capillaries converge to form the hepatic portal vein, which conveys the products to the liver.
The route taken by the fatty acids and glycerol is different. These enter the columnar epithelial cells of the villi and are reconverted into fats, which enter the lacteals. At this point, they are often coated by proteins to form lipoproteins and chylomicrons.
They are carried along lymphatic vessels, and at the thoracic lymphatic duct, they enter the blood stream.
Adaptation of the small intestine for absorption
1. Large surface area due to it enormous length
2. Numerous villi further increase the surface area
3. Large number of blood capillaries and lacteals
4. Wall of lacteal vessels and capillaries only one cell thick
Through the ilescaecal sphincter, the contents of the small intestine pass from the ileum into the large intestine.