It's related to the hepatic portal system. This hepatic portal system is a system of blood vessels that diverts blood from the small intestine to the liver. So basically what happens is nutrient-rich blood is delivered from the small intestine, through the hepatic portal system, back up to the liver. From there, nutrients that are in that blood from digestion are either processed or stored, or they can be used for synthesis of proteins, or used by cells to make ATP. The liver will determine what needs to happen with these substances. If there, for example, is too much glucose in that blood that's delivered to the liver, it will store some of that glucose, as I mentioned, as glycogen. If there's a certain vitamin or mineral that happens to be really high in the blood at that time, it can also store some of those extra vitamins and minerals, such as iron, for example, in the liver until it's needed later, as well.
The main job of the liver is to filter blood from the digestive tract before passing it to the rest of the body. The liver may be responsible for up to 500 separate functions, usually in combination with other systems and organs. It detoxifies chemicals and metabolizes drugs, secreting bile that ends up back in the intestines while doing so. The liver makes proteins that are important for blood clotting and other functions. It is involved in carbohydrate metabolism, protein metabolism (synthesis and degradation), and amino acid synthesis. In lipid metabolism, the liver is involved in cholesterol synthesis, the production of triglycerides (fats), and the synthesis of the bulk of lipoproteins. The liver breaks down insulin and other hormones, and converts ammonia to urea (urea cycle). It also stores glucose, in the form of glycogen, as well as vitamins A, D, B12, K, iron, and copper. With all of these functions to perform, it is easy to understand how liver disease can have such a debilitating effect on the entire body.
The enzymes involved in respiration, photosynthesis and protein synthesis work inside cells. Other enzymes are produced by specialised cells and released from them; the digestive enzymes are like this. They pass out into the gut, where they catalyse the breakdown of food molecules.
The exocrine function of the pancreas results in a pancreatic juice that contains enzymes capable of breaking down the three major components of food-carbohydrates, fats, and proteins. The pancreas is divided into units called acini (acinus is Latin for berry). Each acinus is spherical, with enzyme-secreting cells surrounding a central space (Figure 50). Every enzyme-secreting cell synthesizes all the pancreatic enzymes. The enzymes pass into the center of the acini, entering the narrowest ducts of the branching secretory system. The enzymes pass by larger and larger ducts, eventually reaching the single pancreatic duct, or duct of Wirsung. The cells that line the duct system secrete water and bicarbonate ions, adding them to the enzymes. Consequently, the final pancreatic juice is alkaline. The volume of juice (approximately 2000 ml per 24-hour day) that is secreted is precisely enough to neutralize the acid contents of the stomach as they both enter the duodenum. Pancreatic enzymes are most effective when the contents of the duodenum are at a neutral pH. Chronic disease of the exocrine component of the pancreas results in deficiency of these pancreatic enzymes, giving rise to poor absorption of foodstuffs. The most conspicuous feature of malabsorption is the excretion of fats in feces.
External pancreatic secretion is regulated by hormones, primarily secretin and cholecystokinin. The homeostasis that results when alkaline pancreatic juices mix with acidic gastric juices causes the release of the hormone secretin from cells in the walls of the duodenum. Secretin passes into the bloodstream and stimulates the production of water and bicarbonate ions from the duct system of the pancreas (Figure 51). A greater volume of acid gastric juice passing into the duodenum triggers a greater volume of bicarbonate-rich juice provided by the pancreas to keep the duodenal contents neutral. The pH balance is maintained, and the pancreatic enzymes can perform at their most effective level. The hormone cholecystokinin, which means “gall bladder mover”, is also synthesized in the walls of the duodenum, and is released in response to the presence of amino acids and fatty acids, as partly-digested food starts arriving from the stomach. Cholecystokinin passes around the circulation and causes enzyme secretion by the pancreatic acinar cells. This enzyme secretion increases the ability of the pancreatic juice to break down more fats and proteins. Cholecystokinin also causes contraction of the gall bladder, which provides bile that promotes the absorption of fatty acids and glycerol. Bile from the liver, as well as pancreatic juices, are necessary for the absorption of fat. The common bile duct and the pancreatic duct that bear these fluids converge, so the fluids enter the duodenum together.
The first component protein of an enzyme of fatty acid biosynthesis, fatty acidsynthase, was purified ().
Polyprenol diphosphates were shown to be involved in the biosynthesis ofpolysaccharides in () and in the biosynthesis of peptidoglycans in ().
The structure of juvenile hormone of insects was elucidated by Roller H ().
Among activators, a variety of endogenously present lipids were described ().
An antiphosphatidylcholine antibody was described in the serum of a patient diagnosed with hemolytic anemia ().
Phosphatidylserine was shown to be recognized by specific antibodies in patients with lupus anticoagulant ().
The "Nod factors" produced in legumes by as a class of signalling molecules was identified as lipo-chitooligosaccharides ().
First demonstration of phosphatidylinositol anchor on the lipopolysaccharides of a prokaryotic species, ( ).
Demonstration of the induction of the synthesis of proteinase inhibitors by methyl jasmonate in a wounded plant but also in nearby plants through the atmosphere ().
First clear demonstration that fatty acids are as unfavorable as saturated fatty acids in raising LDL cholesterol and lowering HDL cholesterol levels ().