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As well as being involved in the control of blood glucose levels, the liver has other extremely important functions. To be able to fully understand these, the structure of the liver must first be understood.
The liver is made up of numerous lobules which are packed with virtually identical cells called hepatocytes. The liver is supplied with blood flowing in from the hepatic artery (bringing oxygen) and the hepatic portal vein (bringing blood from the gut).
The blood in this vein carries lots of products from digestion for example; glucose, amino acids, lipids, cholesterol, plasma proteins, urea, carbon dioxide.
As the blood flows through the sinusoids, the hepatocytes take in what they need from it and shed their waste into it.
You should already be familiar with these two liver functions as they have been covered in previous Learn Its:
- Carbohydrate metabolism: gluconeogenesis, glycolysis and glycogenesis all occur in the hepatocytes.
- Deamination: when excess proteins have the NH2 group removed to make ammonia. This is then converted into urea and released into the blood to be taken to the kidney for excretion.
Other functions include:
★ Fat metabolism: if there is not enough carbohydrate for respiration, fats are used. In fact, the preferred respiratory substrate of the heart is fatty acids so the liver is an efficient converter of fats into fatty acids.
★ Making cholesterol: rather than being 'bad', the chemical cholesterol is actually essential for your body. It is needed to make:
- Cell membranes, particularly of neurones,
- Some hormones e.g. testosterone and oestrogen,
- Vitamin D,
Excess cholesterol however is not good. It may precipitate in the gall bladder or bile duct to form gallstones.
This can be very painful and stops the flow of bile to the small intestine. It may also form deposits on the walls of blood vessels.
This atherosclerosis narrows the arteries and hardens them and may lead to clots blocking the flow of blood thus killing the cells that are normally supplied by it.
★ Bile production: bile salts are made from cholesterol by the hepatocytes and then secreted into the canaliculi to be taken to the bile duct and duodenum or to be stored in the gall bladder.
★ Detoxification: hepatocytes break down and chemically change many harmful substances and drugs (e.g, alcohol and antibiotics) as well as some hormones (e.g, testosterone and oestrogen).
★ Storage of Vitamins: Vitamin A (for sight), D (needed to make a hormone for the absorption of calcium and phosphate into the blood from food eaten and for their metabolism) and B12 (for the formation of red blood cells).
★ Breakdown of haemoglobin (Hb): red blood cells are broken down in the spleen and the Hb is released into the plasma. Phagocytic cells of many parts of the body, including the liver take it up. These cells which line the blood vessels in the liver are called Kuppfer cells. Having ingested the Hb, they remove the iron from it.
★ The iron is then combined with a plasma protein called transferrin. This complex may be taken up by the bone marrow cells for new Hb and red blood cell production or it may be taken up by the hepatocytes for storage. The non-iron part of the Hb is converted into a green-yellow pigment and is released into the bile as an excretory product.
★ Synthesis of plasma proteins: e.g, fibrinogen and prothrombin (used in blood clotting), globulins (used in transporting substances that could not otherwise travel in the plasma, e.g, lipids), albumin - keeps the osmotic concentration of the blood high (see Learn It on Transport in Animals) and also used for transport like globulins.