Glucose is a vital energy source for cells and levels in the blood stream, which must remain constant. The liver is central to blood glucose management because the liver is the only organ that can store and release glucose into the blood for use by other organs. Glycogen is a complex molecule composed of thousands of glucose units. Hepatocytes store glucose as intracellular glycogen granules. When energy intake exceeds energy output, the liver stores the surplus glucose as glycogen. When energy output exceeds energy intake, the liver releases glucose into the blood by performing glycogenolysis, the breakdown of glycogen into glucose. These processes response to the pancreatic hormones insulin and glucagon.
The liver is also responsible for gluconeogenesis, which is the synthesis of glucose from certain amino acids, lactate or glycerol. Adipose and liver cells produce glycerol by breakdown of fat, which the liver uses for gluconeogenesis.
The liver plays several roles in lipid metabolism: it performs cholesterol synthesis, lipogenesis, the production of triglycerides, and a bulk of the body’s lipoproteins are synthesized in the liver, including: chylomicrons remnants, very low density lipoproteins (VLDL), low density lipoproteins (LDL), high density lipoproteins (HDL) and fatty acids .
The liver also uses dietary amino acids and those released during normal tissue catabolism to synthesize its own proteins and enzymes as well as plasma proteins. Plasma proteins produced by hepatocytes include: albumin, fibrinogen, prothrombin, a-fetoprotein, a2-macroglobin, hemopexin, transferrin, complement components C3, C6 and C1, al-antitrypsin, caeruloplasmin . Up to 50% of the livers’ energy requirements can be supplied by amino acid oxidation. Oxidative deamination breaks amino acids into keto acid and an ammonia molecule. The keto acid is used in the Kreb’s cycle to produce adenosine triphosphate (ATP). The liver combines ammonia with CO2 to form urea and H2O.
The liver also plays an important role in vitamin and mineral storage. About 80% of the body’s vitamin A stores are concentrated in fat droplets within the stellate cells of the liver. In pathological conditions like hepatic fibrosis or liver cirrhosis the stellate cells lose vitamin A, transform into fibroblasts or myofibroblasts and begin producing large amounts of collagen and adhesive glycoproteins . Normal vitamin A reserves are enough to prevent a deficiency for about 10 months. The liver also contains about a year supply of B12. Vitamin D stores equal about 3-4 months. Small amounts of Vitamins E and K and Vitamin C are stored in the liver to facilitate liver functions.