The word 'liver' seems to come from old Norse, lifr. The adjective 'hepatic', describing things to do with the liver, comes from the Greek hepatos. The adult human liver weighs 1-1.5 kg and lies immediately under the diaphragm. It is supplied with blood from below through two major vessels: the hepatic artery
(which supplies about 20% of the blood) and the hepatic portal vein, often called simply the portal vein. The portal vein carries blood which has passed through the complex system of blood vessels around the intestinal tract (see Fig. 3.4). This unusual feature - that the liver receives its major blood supply via a vein - gives the liver a special role in metabolism.
The portal vein is short - about 7-8 cm long. It is formed by the joining of veins coming from different parts of the intestinal tract including the stomach, and also from the spleen. These veins carry the substances absorbed from the intestinal tract into the blood - particularly, from the point of view of energy metabolism, monosaccharides and amino acids. Thus, the water-soluble substrates arising from the diet are transported first to the liver, before entering the general circulation.
Another important, although small, group of veins joins the portal vein just before it enters the liver - the pancreatic veins. These veins carry blood from the endocrine part of the pancreas (described in more detail in Chapter 5), containing the pancreatic hormones insulin and glucagon. These hormones therefore exert their effects first on the liver, before being diluted in the general circulation.
Blood leaves the liver through a number of hepatic veins, which enter the inferior vena cava, the main blood vessel returning blood from the lower part of the body up towards the heart.
There is one other important system of vessels associated with the liver - those that carry bile to the gall bladder. Bile (see Section 220.127.116.11) contains the bile salts (Box 3.1), which are essential to the digestion and absorption of fats from the intestine. It is also a route for excretion of organic compounds detoxified in the liver, and having a molecular mass greater than 400 Da. The 500-1000 ml of bile produced each day travels through a system of hepatic ducts to the gall bladder, a pear-shaped organ (about 8 cm long by 2-3 cm in diameter) located immediately under the liver (see Fig. 3.5). Here it is stored between meals, and emptied during digestion through the common bile duct to the duodenum.
The major part of the liver (80% by volume) is composed of one cell type, the hepatocyte. Other cell types include the phagocytic Kupffer cells (macrophages, see Section 4.7.3) and endothelial cells. These other cell types are generally smaller than hepatocytes and so may make up a larger proportion of total cell number. Hepatocytes are arranged in a very characteristic manner (Fig. 4.1) which appears in cross-section as hexagonal units or lobules, each around 1 mm across. At each corner of the hexagon is a triad of three vessels: tiny branches of the portal vein, the hepatic artery and the bile duct. In the centre of the lobule is a branch of a hepatic vein, carrying blood away. The hepatocytes radiate out from the central vein. Blood flows from the triads towards the central vein in small passages between the hepatocytes, the sinusoids. The sinusoids are therefore the equivalent of the capillaries found in other tissues, and are lined by flat endothelial cells, as are all capillaries. The
blood in the sinusoids is in intimate contact with the hepatocytes. Bile formed in the hepatocytes passes out to the bile duct branch in the triad along the lines of hepatocytes in fine tubes, the bile canaliculi (little canals).
The precise arrangement of hepatocytes within the liver is closely related to the function of the cells; this is known as metabolic zonation of hepatic metabolism. The hepatocytes on the 'outside' of each lobule (periportal hepatocytes) are exposed to blood which has recently arrived at the liver in the portal vein and hepatic artery. Thus, these cells are well oxygenated and supplied with substrates, and oxidative metabolism predominates. The synthesis of glucose (gluconeogenesis) occurs mainly in these cells, whereas the cells nearer the centre of each lobule (perivenous hepatocytes) are more involved in glycolysis and also ketone body production. It appears that this arrangement is quite flexible, and each individual cell can perform either function depending upon its location in the lobule and the prevailing physiological circumstances.
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