Liver cell structure and function

he liver is the largest gland in the body, and is situated slightly below the diaphragm and anterior to the stomach. It consists of two lobes which are wedge-shaped. Two blood vessels enter the liver, namely the hepatic portal vein with dissolved food substances from the small intestine, and the hepatic artery, with oxygenated blood from the lungs. Two ducts originate in the liver, and these unite to form the common hepatic duct which opens, with the pancreatic duct, in the hollow side of the duodenum (the first section of the small intestine). The gall bladder lies inside the liver, and is the storage place for bile, which is formed by the liver cells.

The right lobe of the liver is larger than the left lobe. Each lobe is further divided into many small lobules, each being about the size of a pin-head, and consisting of many liver cells, with bile channels and blood channels between them. Permeating the entire liver structure is a system of blood capillaries, bile capillaries and lymph capillaries.

The liver cells secrete the bile, and this collects in the bile capillaries, which then unite, forming bile ducts. These bile ducts all eventually unite, forming the main hepatic duct, which gives off a branch, the cystic duct, on its way toward the hepatic duct. The cystic duct leads into the gall bladder. Where a cystic duct joins the hepatic duct, the two continue as the general bile duct, which then joins the pancreatic duct, forming a common duct that opens into the duodenum.

The functions of the liver are varied, working closely with nearly every fundamental system and process in the human body, in particular homeostasis and the regulation of blood sugar.

1. Regulation of blood sugar: The level of blood sugar stays at around 0.1%, and excess coming from the gut is stored as glycogen. The hormone called insulin – excreted by the pancreas – causes the excess glucose to turn into glycogen.

2. Regulation of lipids: Lipids are extracted from the blood and changed to carbohydrates, etc. as required or sent to fat storage sites if not needed straight away.

3. Regulation of amino acids: a supply of amino acids in the blood is kept at a normal level. Any spare which has not been absorbed cannot be stored but is converted into the waste products, called urea when at the liver, and is then sent to the kidneys to be removed from the body as urine. The remainder of the amino acid molecule is not wasted; it is changed into a carbohydrate that can be used.

4. Production of heat: the liver is one of the hardest working regions of the body and produces a lot of waste heat. This is carried round the body in the blood and warms less active regions.

5. Forms bile: bile consists of bile salts and the excretory bile pigments. It is important to speed up the digestion of lipids.

6. Forms cholesterol: this fatty substance is used in the cells. Excess amounts in the blood can cause the blood vessels to become blocked, leading to heart attacks, etc.

7. Removals of hormones, toxins, etc. The liver extracts many harmful materials from the blood and excretes them in the bile or from the kidneys.

8. Formation of red blood cells in the young embryo while it is developing in the womb.

9. Making heparin: this is a substance that prevents the blood from clotting as it travels through the blood system.

10. Removal of hemoglobin molecules: when red blood cells die, the hemoglobin is converted into bile pigments and the iron atoms are saved for future use.

11. Storage of blood: the liver can swell to hold huge amounts of blood which can be released into the circulation if the body suddenly needs more, e.g. if it is wounded.

12. Forms plasma proteins: the plasma proteins are used in blood clotting and in keeping the blood plasma constant. The main blood proteins include fibrinogen, prothrombin, albumens and globulins.

13. Storage of vitamins such as vitamin A and D. Vitamin A is also made in the liver from carotene, the orange-red pigment in plants. Vitamin B12 is also stored in the liver.