The liver is a large, reddish-brown organ which lies just be- low the diaphragm and partly overlaps the stomach. In addition to a supply of oxygenated blood from the hepatic artery, it receives all the blood which leaves the alimentary canal. It has a great many important functions, some of which are described below.
1. Regulation of blood sugar. The liver is able to convert glucose, amino acids and other substances to an insoluble carbohydrate, glycogen. Some of the glucose so converted may be taken from the hepatic portal vein carrying blood, rich in digested food, from the ileum to the liver. About 100 g glycogen is stored in the liver of a healthy man. If the concentration of glucose in the blood falls below about 80 mg/100 cm3 blood, some of the glycogen stored in the liver is converted by enzyme action into glucose and it enters the circulation. If the blood sugar level rises above 160 mg/100 cm, glucose is excreted by the kidneys. A blood glucose level below 40 mg 100cms affects the brain cells adversely, leading to convulsions and coma. By helping to keep the glucose concentration between 80 and 150 mg the liver prevents these undesirable effects and so contributes to the homeostasis of the body.
2. Formation of bile. Green and yellow pigments are formed when the red blood cells break down. These pigments are removed from the blood by the liver and excreted in the bile. The liver also produces bile salts which play an important part in the emulsification and subsequent absorption of fats.
Bile is produced continuously by the liver cells, but stored and concentrated in the gall bladder. It is discharged group the bile duct into the duodenum when the acid chyme arrives there, Bile is reabsorbed with the fats it emulsifies and even ally returns to the liver.
3. Storage of iron. Millions of red blood cells break up every the liver their decomposition is completed and the i from the haemoglobin is stored.
4. Deamination. Excess amino acids are not stored I'm the Dody. Amino acids which are not built up into proteins used for growth and replacement are converted to Carbohydrates by the removal from the molecule, which contains the nitrogen. The residue can be co vertex to glycogen, being stored or oxidized to release energy he nitrogen of the amino group is converted in the to urea, an excretory product that is constantly eliminated by the kidneys.
5. Manufacture of plasma proteins. The liver makes most of the proteins found in blood plasma, including fibrinogen which plays an important part in the clotting action of the blood.
6. Body heat. The above list shows that a great many chemical changes go on in the liver and many of them release energy inn the form of heat. This heat is distributed throughout the body by the circulatory system and helps to maintain the body temperature.
7. Use of fats in the body. When fats stored in the body are required for use in providing energy, they travel in the blood stream from the fat depots. When they reach the liver they are converted to substances which can be readily oxidized by other tissues to release energy.
8. Detoxication. Poisonous compounds, produced in the large intestine by the action of bacteria on amino acids, enter the blood, but on reaching the liver are converted to harmless substances, later excreted in the urine. Many other chemical substances normally present in the body or introduced as drugs are modified by the liver before being excreted by the kidneys.
The hormones, for example, are converted to inactive compounds in the liver so limiting their period of activity in the body.
9. Storage of vitamins. The fat soluble vitamins A and D are stored in the liver. This is the reason why animal liver is a valuable source of these vitamins in the diet. The liver also stores a product of the vitamin B,. This product is necessary for the normal production of red cells in the bone marrow
HOMEOSTASIS
A complete account of the functions of the liver would involve a very long list. It is most important, however, to realize that the one vital function of the liver, embodying all the details outlined above, is that it helps to maintain the concentration and composition of the body fluids, particularly the blood Within reason, a variation in the kind of food eaten will not produce changes in the composition of the blood If this internal environment, as it is called, were not so constant, the chemical changes that maintain would become erratic and unpredictable so that with quite slight changes of diet or activity the whole organization might break down The regulation of the internal environment is called homes stasis and is discussed again.