The liver is located in the upper right of the abdomen, under the diaphragm and above the stomach, right kidney, and intestines. It consists of two lobes, each in turn composed of numerous lobules. All the lobules are connected to a network of ducts that ultimately combine to form the hepatic duct. The liver receives blood from both the hepatic artery and the hepatic portal vein. It can hold up to 13 percent of the body's blood supply at any one time, and all blood leaving the digestive tract passes through the liver.
The liver's function include regulating the levels of chemicals in the blood; breaking down nutrients and other molecules being carried in the blood; producing blood plasma proteins; producing cholesterol and fat carrier proteins; storage of excess glucose in the form of glycogen; storing iron extracted from haemoglobin; converting ammonia to urea; clearing drugs and toxins from the bloodstream; and producing bile, which is transported to the galbladder and duodenum through the hepatic duct. Bile helps to break down fats prepatory to digestion and absorption.
One of the most important functions in to reduce the fluctuations in nutrient availability caused by the feeding-fasting cycle. During fasting (e.g. between meals) enzyme balances shift; fewer enzymes are produced for lipogenesis (fat synthesis) and more for gluconeogenesis (glucose synthesis), increasing the body's glucose-synthesising efficiency. It also induces the activity of pyruvate dehydrogenase, which ultimately results in the conservation of lactate, pyruvate, and amino acid carbon skeletons, all of which can be used to make glucose. Other enzymes boost fatty acid oxidaton, ketogenesis, and amino acid degradation. When needed, amino acids broken down in muscle are converted to glutamate and alanine and transported to the liver for gluconeogenesis from their carbon skeletons; this releases ammonia, and produces NADH and alpha-ketoglutarate. The ammonia is converted to urea and exported to the kidneys.
After feeding, dietary glucose is transported to the liver through the portal vein, along with partially-metabolised amino acids. Glucose can be stored as glycogen (glycogenesis) or transformed to lactose and pyruvate (glycolysis), or enter the pentose phosphate pathway to generate NADPH for biosynthetic processes. Much of the glucose passes through the liver on its way to other organs, e.g. the brain, which is completely dependent on glucose to function.
The liver is constituted of two types of hepatocytes (liver cells): periportal hepatocytes near the hepatic arteriole and portal venule; and perivenous scavenger hepatocytes near the central venule. Blood enters the liver via the portal vein and hepatic artery and leaves via the central vein. The amino acid glutamine enters the periportal cells and is hydrolysed (broken down). Stray ammonium ions, which are toxic, are converted to glutamine by the scavenger cells, exit through the central vein and circulate back to the liver. Another toxin removed by the liver is ethanol; the first two steps of its oxidation take place in the liver. Ethanol is converted to acetaldehyde, and the acetaldehyde to acetate.