The liver is an extremely active organ. Those functions already described are only mentioned here.
Carbohydrate metabolism.The liver has an important role in maintaining plasma glucose levels. After a meal when levels rise, glucose is converted to glycogen for storage under the influence of the hormone insulin. Later, when glucose levels fall, the hormone glucagon stimulates conversion of glycogen into glucose again, keeping levels within the normal range (see Fig 12.40).
Fat metabolism.Stored fat can be converted to a form in which it can be used by the tissues to provide energy (see Fig. 12.45).
Deamination of amino acids. This process:
• removes the nitrogenous portion from the amino acids not required for the formation of new protein; urea is formed from this nitrogenous portion which is excreted in urine
• breaks down nucleic acids (genetic material, see p. 433) to form uric acid, which is excreted in the urine.
Transamination. Removes the nitrogenous portion of amino acids and attaches it to other carbohydrate molecules forming new non-essential amino acids (see Fig. 12.43).
Synthesis of plasma proteins and most blood clotting factors from amino acids.
Breakdown of erythrocytes and defence against microbes.This is carried out by phagocytic hepatic macrophages (Kupffer cells) in the sinusoids.
Detoxification of drugs and noxious substances.These include ethanol (alcohol) and toxins produced by e.g. microbes.
Inactivation of hormones.These include insulin, glucagon, Cortisol, aldosterone, thyroid and sex hormones.
Production of heat.The liver uses a considerable amount of energy, has a high metabolic rate and produces a great deal of heat. It is the main heat-producing organ of the body.
Secretion of bile.The hepatocytes synthesise the constituents of bile from the mixed arterial and venous blood in the sinusoids. These include bile salts, bile pigments and cholesterol.
Storage.The substances include:
• glycogen (see p. 312)
• fat-soluble vitamins: A, D, E, Ê
• iron, copper
• some water-soluble vitamins, e.g. vitamin B12.
Composition of bile
About 500 ml of bile are secreted by the liver daily. Bile consists of:
• mineral salts
• bile pigments, mainly bilirubin
• bile salts, which are derived from the primary bile acids, cholic acid and chenodeoxycholic acid
Bile ducts̉he right and left hepatic ducts join to form the common hepatic duct just outside the portal fissure. The hepatic duct passes downwards for about 3 cm where it is joined at an acute angle by the cystic duct from the gall bladder. ̉he cystic and hepatic ducts merge forming the common bile duct, which passes downwards behind the head of the pancreas. This is joined by the main pancreatic duct at the hepatopancreatic ampulla and the opening into the duodenum is controlled by the hepatopancreatic sphincter (of Oddi). The common bile duct is about 7.5 cm long and has a diameter of about 6 mm.
The walls of the bile ducts have the same layers of tissue as those described in the basic structure of the alimentary canal (Fig. 12.2). In the cystic duct the mucous membrane lining is arranged in irregular circular folds, which have the effect of a spiral valve. Bile passes through the cystic duct twice - once on its way into the gall bladder and again when it is expelled from the gall bladder into the common bile duct and then on to the duodenum.
The gall bladder is a pear-shaped sac attached to the posterior surface of the liver by connective tissue. It has a fundus or expanded end, a body or main part and a neck, which is continuous with the cystic duct.
The gall bladder has the same layers of tissue as those described in the basic structure of the alimentary canal with some modifications.
Peritoneum covers only the inferior surface. The gall bladder is in contact with the posterior surface of the right lobe of the liver and is held in place by the visceral peritoneum of the liver.
Muscle layer. There is an additional layer of oblique muscle fibres.
Mucous membrane displays small rugae when the gall bladder is empty that disappear when it is distended with bile.