1. During the process of digestion, large food molecules are broken down into small components that can be absorbed into cells that form the lining of the small intestine. Digestion begins in mouth, where large food molecules are broken down mechanically (teeth) and then enzymatically (saliva). Proteases, lipases, amylases, maltases are enzymes that catalyse the breakdown of macromelocules into the basic molecule units (such as aminoacids, fatty acids glucose) so that they can be absorbed.
2. Circular folds, villi, and microvilli--tiny projections from the surfaces of cells--increase the surface area for absorption. Those structures can increase the surface area even 30 times. The microvilli also secrete the enzymes which help in the process of breakage. Motor proteins in the microvilli makes them move and this also helps in the process of absorption.
3. After moving into cells of the intestinal lining, fatty acids and glycerol are recombined into fats, coated with proteins, and transported into lymph vessels, which eventually empty into large veins. These lymphatic vessels are called lacteals. Fatty acids are transported from the intestinal lumen to the enterocyte where they are packed into chylomicrons (combined with proteins). The chylomicrons pass into the lacteals in the form of chyle and finally transported to the bloodstream.
4. Sugars and amino acids pass from the intestinal epithelium and into blood capillaries. This is a two-stage process:
• from the lumen into intestinal epithelial cells via membrane transporters (Na-symporters)
• from the cells into the blood (capillaries)
This is possible because epithelial cells of the intestine are polarized, meaning that apical and basolateral ends are different (contain different transporters).
5. The nutrient-laden blood from the intestines is carried in hepatic portal vein the to the liver. There is a whole system involved in the transport of blood from parts of the gastrointestinal tract to the liver and it is called the portal venous system. The hepatic portal vein is a blood vessel part of that system that carries blood rich in nutrients extracted from digested contents.
6. The liver removes excess glucose from the blood and stores it as glycogen. There are a few roles of the liver that are involved in the carbohydrate metabolism. The major is the production of glycogen from glucose via glycogenesis. There is also the opposite process, when the organism needs glucose, so it is released from the glycogen via the process glycogenolysis.
7. The liver also converts nutrients to other essential substances, such as plasma proteins, cholesterol, and fats.
The liver is responsible for many synthesis processes (anabolic processes) such as synthesis of proteins (aminoacids), clotting factors, cholesterol, lypoproteins. It is involved in lipid metabolism: lipogenesis, and the production of triglycerides. One of the most important proteins synthesized in the liver are thrombopoietin which regulates the production of platelets by the bone marrow and insulin-like growth factor 1 which plays an important role in childhood growth.
