Know your liver

What’s so great about it?

Your liver is for life. You have only one and though it is unique in its ability to regenerate, it can only survive limited damage - so it does need looking after.

 

What is its position?

Place your right hand over the area under your ribs on the right side of your body and it will just about cover the area of the liver. The liver is the largest gland and solid organ in the body, weighing some 1.8 kg in a men and 1.3 kg in women. It holds approximately 13 per cent (about one pint or 0.57 litres) of the total blood supply at any given moment, and it has over 500 estimated functions.

 

What does it look like?

The liver is dark reddish brown in colour and is divided into two main lobes (the much larger right and the smaller left), which are further subdivided into approximately 100,000 lobules. About 60 per cent of the liver is made up of liver cells (hepatocytes), and each of these has an average lifespan of 150 days. In every milligram of liver tissue there are approximately 202,000 cells. Two-thirds of the body of the liver is the parenchyma, which contains the hepatocytes, and the remainder is the biliary tract. The liver receives its blood supply via the hepatic artery and portal vein which transports nutrients from the intestine (gut).

 

The liver and biliary tract

The biliary tract contains the right and left hepatic ducts, which meet to form the common hepatic duct. This is joined by the cystic duct from the gall bladder, which then forms the common bile duct. The common bile duct joins the intestine at the duodenum through a valve called the Sphinter of Oddi.

 

The gall bladder is a pear-shaped bag 9 cm long with a capacity of about 50 ml. Breakdown products, such as bile salts, bilirubin, cholesterol, phospholipids, proteins, electrolytes and water, are secreted by hepatocytes, and they are eventually transported down the bile ducts (this is bile and it is modified by cholangiocytes lining the bile ducts). The gall bladder stores bile, a greenish-yellow coloured liquid, which is delivered during a meal into the gut to assist with the breakdown (emulsifying) of fat in the food digested to allow easier absorption of fat and vitamins A, D, E and K. The liver produces approximately one pint (or 0.57 litres) of bile a day.

 

What is its role?

The liver is a complex chemical factory that works 24 hours a day. Virtually all the blood returning from the intestinal tract to the heart passes through the liver. This means everything you swallow that is absorbed into the bloodstream passes through the liver.

 

The liver functions can be divided into four basic categories:

1. Regulation, Synthesis and Secretion

  1. Glucose - The liver plays a key role in the homeostatic control of blood glucose, by storing or releasing it as needed in response to the pancreative hormones insulin and glucagons.
  2. Proteins - Most blood proteins (except antibodies) are synthesised and secreted by the liver, e.g. Albumin; decreased amounts of serum albumin may lead to oedema - swelling due to fluid accumulation in the tissues. The liver also produces most of the proteins responsible for blood clotting called clotting factors.
  3. Bile - Bile is both excretory and secretory. In addition to bile salts, it contains cholesterol phospholipids and bilirubin (from the breakdown of haemoglobin). Bile salts acts as ‘detergents' that aid in the digestion and absorption of dietary fats.
    1. Lipids - The liver synthesises cholesterol, a lipid that is an essential component of cell membranes. Cholesterol then circulates in the body to be used or excreted into bile for removal. The liver also synthesises lipoproteins, which circulate in the blood and shuttle cholesterol and fatty acids between the liver and body tissues.

 

2. Storage

The liver stores glucose in the form of glycogen, and also fat-soluble vitamins (A, D, E and K), vitamins B6 and B12 and minerals such as copper and iron. However, excessive accumulation of certain substances can be harmful.

 

3. Purification, Transformation and Clearance

The liver removes harmful substances from the blood and breaks them down into less harmful compounds. It also converts most hormones and drugs to less active products.

  1. Ammonia - The liver converts ammonia to urea which is excreted in urine by the kidneys through a process called deamination. In another process called transmination, the liver can convert one amino aid into another (but not the eight essential amino acids) via the ‘citruline-ornithine pathway’.
  2. Bilirubin - Bilirubin is a yellow pigment formed as a breakdown product of red blood cell haemoglobin. The spleen, which destroys old red cells, releases bilirubin into the blood, where it circulates to the liver which excretes it in bile. Excess bilirubin results in jaundice, a yellow pigmentation of the skin and eyes.
  3. Hormones - The liver plays an important role in hormonal modification and inactivation of steroids testosterone and oestrogen.
  4. Drugs - Nearly all drugs are either modified or degraded in the liver. In particular, oral drugs are absorbed by the gut and transported to the liver, where they may be modified or inactivated before they enter the blood. Alcohol, in particular, is broken down by the liver, and long-term exposure to its end products can lead to cirrhosis.
  5. Toxins - The liver is generally responsible for detoxifying chemical agents and poisons.

 

4. Fighting Infections

The liver plays a vital role in fighting infections, particularly infections arising in the bowel. It does so by mobilising part of the body’s defence mechanism called the macrophage system. The liver contains over half of the body’s supply of macrophages (known as Kuppfer cells), which destroy any bacteria that they come into contact with.

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