The liver is divided into four lobes, the greatest part being the right lobe, with a small left lobe, while the quadrate and caudate lobes are two small divisions on the back and under-surface. Around the middle of the under-surface, towards the back, a transverse fissure (the porta hepatis) is placed, by which the hepatic artery and portal vein carry blood to and from the liver, and the right and left hepatic ducts emerge, carrying the BILE formed in the liver to the GALL-BLADDER, situated under the right lobe, in which it is stored.

Occupying the right-hand upper part of the abdominal cavity, the liver is separated from the right lung by the DIAPHRAGM and the pleural membrane (see PLEURA). It rests on various abdominal organs, chiefly the right of the two KIDNEYS, the suprarenal gland (see ADRENAL GLANDS), the large INTESTINE, the DUODENUM and the STOMACH.

The blood supply differs from that of the rest of the body, in that the blood collected from the stomach and bowels into the PORTAL VEIN does not pass directly to the heart, but is first distributed to the liver, where it breaks up into capillary vessels. As a result, some harmful substances are filtered from the bloodstream and destroyed, while various constituents of the food are stored in the liver for use in the body's metabolic processes. The liver also receives the large hepatic artery from the coeliac axis. After circulating through capillaries, the blood from both sources is collected into the hepatic veins, which pass directly from the back surface of the liver into the inferior vena cava.

The liver is enveloped in a capsule of fibrous tissue from which strands run along the vessels and penetrate deep into the organ, binding it together. Subdivisions of the hepatic artery, portal vein, and bile duct lie alongside each other, finally forming the interlobular vessels, which lie between the lobules of which the whole gland is built up. Each is about the size of a pin's head and forms a complete secreting unit; the liver is built up of hundreds of thousands of such lobules. These contain small vessels, capillaries, or sinusoids, lined with stellate KUPFFER CELLS, which run into the centre of the lobule, where they empty into a small central vein. These lobular veins ultimately empty into the hepatic veins. Between these capillaries lie rows of large liver cells in which metabolic activity occurs. Fine bile capillaries collect the bile from the cells and discharge it into the bile ducts lying along the margins of the lobules. Liver cells are among the largest in the body, each containing one or two large round nuclei. The cells frequently contain droplets of fat or granules of GLYCOGEN – that is, animal starch.

Postions of liver, gall-bladder and connecting bile ducts.

The liver is, in effect, a large chemical factory, and the heat it produces contributes to the general warming of the body. The liver secretes bile, the chief constituents of which are the bile salts (sodium glycocholate and taurocholate), the bile pigments (BILIRUBIN and biliverdin), CHOLESTEROL, and LECITHIN. These bile salts are collected and formed in the liver and are eventually converted into the bile acids. The bile pigments are the iron-free and globin-free remnant of HAEMOGLOBIN, formed in the Kupffer cells of the liver. (They can also be formed in the spleen, lymph glands, bone marrow and connective tissues.) Bile therefore serves several purposes: it excretes pigment, the breakdown products of old red blood cells; the bile salts increase fat absorption and activate pancreatic lipase, thus aiding the digestion of fat; and bile is also necessary for the absorption of vitamins D and E.

The other important functions of the liver are as follows:

• In the FETUS it forms red blood cells, while the adult liver stores vitamin B12, necessary for the proper functioning of the bone marrow in the manufacture of red cells.

• It manufactures FIBRINOGEN, ALBUMINS and GLOBULIN from the blood.

• It stores IRON and copper, necessary for the manufacture of red cells.

• It produces HEPARIN, and – with the aid of vitamin K – PROTHROMBIN.

• Its Kupffer cells form an important part of the RETICULO-ENDOTHELIAL SYSTEM, which breaks down red cells and probably manufactures ANTIBODIES.

• Noxious products made in the intestine and absorbed into the blood are detoxicated in the liver.

• It stores carbohydrate in the form of glycogen, maintaining a two-way process: glucose glycogen.

• CAROTENE, a plant pigment, is converted to vitamin A, and B vitamins are stored.

• It splits up AMINO ACIDS and manufactures UREA and uric acids.

• It plays an essential role in the storage and metabolism of FAT.