Classed as part of the digestive system, the liver's roles include detoxification, protein synthesis and the production of chemicals necessary for digestion.1
This article will cover the main roles of the liver, how the liver regenerates, what happens when the liver does not function correctly, and how to keep the liver healthy.
Here are some key points about the liver. More detail and supporting information is in the main article.
- The liver is classed as a gland
- The liver carries out more than 500 roles in the human body
- It is the only organ that can regenerate
- The liver is the largest solid organ in the body
- Alcohol abuse is one of the major causes of liver problems in the industrialized world
Structure of the liver
The liver has a rubbery texture to the touch and appears reddish-brown.
Weighing 1.44–1.66 kilograms, the liver is reddish-brown in color with a rubbery texture; it is situated above and to the left of the stomach and below the lungs. The skin is the only organ that is heavier and larger.
The liver is roughly triangular in shape and consists of two lobes, a larger right lobe and a smaller left lobe. The lobes are separated by the falciform ligament — a band of tissue that keeps it anchored to the diaphragm.
A layer of fibrous tissue called Glisson's capsule covers the outside of the liver. This capsule is further covered by the peritoneum, a membrane that forms the lining of the abdominal cavity; this helps hold the liver in place and further protects it from physical damage.
Unlike most organs, the liver has two major sources of blood. Firstly, the portal vein brings it nutrient-rich blood from the digestive system. Secondly, the hepatic artery carries oxygenated blood from the heart.
The blood vessels divide into small capillaries, with each terminating in a lobule. Lobules are the functional units of the liver and consist of millions of hepatic cells (hepatocytes).
Blood is removed from the liver via three hepatic veins.
Functions of the liver
The liver is classed as a gland and has a great deal of functions associated with it. It is difficult to give a precise number, but textbooks often cite 500 distinct roles.
The major functions of the liver are:
- Bile production: bile helps the small intestine break down and absorb fats, cholesterol, and some vitamins. Bile consists of bile salts, cholesterol, bilirubin, electrolytes, and water.2
- Absorbs and metabolizes bilirubin: bilirubin is formed by the breakdown of hemoglobin. The iron released from hemoglobin is stored in the liver or bone marrow and used to make the next generation of blood cells.
- Assists in creating blood-clotting factors (coagulants): vitamin K is necessary to create certain coagulants, and to absorb vitamin K, bile is essential. Bile is created in the liver; if the liver does not produce enough bile, clotting factors cannot be produced.
- Fat metabolization: bile breaks down fats to make them easier to digest.
- Metabolizes carbohydrates: carbohydrates are stored in the liver where they are broken down into glucose and siphoned into the bloodstream to maintain normal glucose levels. They are stored as glycogen and released whenever a quick burst of energy is required.
- Vitamin and mineral storage: the liver stores vitamins A, D, E, K, and B12. It keeps significant amounts of these vitamins stored; in some cases, years-worth of vitamins are held as a back-up. Iron from hemoglobin in the form of ferritin is stored in the liver, ready to make new red blood cells. The liver also stores copper and releases it when needed.
- Helps metabolize proteins: bile helps break down proteins to make them digestible.
- Filters the blood: the liver filters and removes compounds from within the body, including hormones such as estrogen and aldosterone, and compounds from outside the body like alcohol and other drugs.
- Immunological function: the liver is part of the mononuclear phagocyte system. It contains high numbers of immunologically active cells called Kupffer cells; these cells destroy any pathogens that might enter the liver via the gut.3, 4
- Production of albumin: albumin is the most common protein in blood serum. It transports fatty acids and steroid hormones to help maintain the correct osmotic pressure and prevent blood vessels from becoming "leaky."5
- Synthesis of angiotensinogen: this hormone raises blood pressure via vasoconstriction when alerted by the kidney's production of renin.6
Regeneration of the liver
The liver is the largest solid organ in the human body.
Because of the importance of the liver and its functions, evolution has ensured that, if given a fighting chance, it can regrow incredibly rapidly. This ability is seen in all vertebrates from fish to humans.
The liver is the only visceral organ with the capacity to regenerate.
The liver can regenerate completely as long as a minimum of 25 percent of the tissue remains. One of the most impressive aspects of this feat is that regrowth to its previous size and ability can be achieved without any loss of function while it is doing so.
In mice, if two-thirds of the liver are removed, within 5-7 days, the remaining liver tissue can regrow to its original size. In humans, the process takes slightly longer, but regeneration can still occur in an incredible 8-15 days. Over the following few weeks, the new liver tissue is reorganized to become indistinguishable from the original tissue.7
This regeneration is facilitated by a number of compounds including growth factors and cytokines. Some of the most important compounds in the process appear to be hepatocyte growth factor, insulin, transforming growth factor-alpha, epidermal growth factor, interleukin-6, and norepinephrine.8
Diseases of the liver
As one might imagine, with an organ as complex as the liver, there are many problems that can occur. As with the other organs of the body, it works miraculously well; if it fails, however, the ramifications can be catastrophic.
Examples of liver disease include:
- Fascioliasis: caused by the parasitic invasion of a liver fluke of the Fasciola genus. Fascioliasis is a disease of the tropics; the flukes can lie dormant in the liver for months or years.
- Cirrhosis: fibrous tissue replaces liver cells (fibrosis). This condition can be caused by a number of factors, including toxins, alcohol and hepatitis. Eventually, fibrosis can lead to liver failure as the functionality of the liver cells is destroyed.9
- Hepatitis: caused by viruses (most commonly), toxins, or an autoimmune response. Hepatitis is characterized by an inflamed liver. In many cases, the liver can heal itself, but the worst-case scenario is liver failure.
- Alcoholic liver disease: excess alcohol consumption over long periods of time can cause liver damage — scarring and cirrhosis. It is the most common cause of liver disease in the Western world.10
- Primary sclerosing cholangitis (PSC): PSC is a serious inflammatory disease of the bile ducts that results in their destruction. No medical therapy has so far proven beneficial and the cause is currently unknown (an autoimmune response is suspected).11
- Fatty liver disease: usually in conjunction with obesity or alcohol abuse, vacuoles of triglyceride fat accumulate in liver cells. The condition is reversible and does not seem to cause too much ill effect.
- Gilbert's syndrome: a genetic disorder affecting 3–12 percent of the population. Bilirubin is not broken down adequately. Mild jaundice can occur, but the disorder is harmless.12
- Liver cancer: the most common forms are hepatocellular carcinoma and cholangiocarcinoma. The leading causes of liver cancer are alcohol and hepatitis. It is the sixth most common form of cancer and the second leading cause of cancer death.13
Maintaining liver health
Alcohol abuse can lead to cirrhosis of the liver.
Below are some recommendations to help keep your liver working as it should:14
- Diet: as your liver is responsible for digesting fats, an excess of lipids can overwork it and disturb it from other tasks. In addition, obesity can cause fatty liver disease.
- Alcohol: avoid drinking more than two drinks at a time. Excess consumption causes cirrhosis of the liver over time. The breakdown of alcohol produces chemicals that are toxic to the liver, such as acetaldehyde and free radicals. For serious damage to occur (in men), it takes the equivalent of a liter of wine every day for 20 years.15
- Mixing drugs: if some prescription drugs are mixed, they can interact negatively, even natural remedies: "herbal" does not mean "safe." Mixing drugs with alcohol puts significant pressure on the liver; alcohol and acetaminophen can lead to acute liver failure, for instance.
- Airborne chemicals: if painting or using strong cleaning or gardening chemicals, the area should be well ventilated or a mask should be worn. Because the liver has to deal with any toxins that enter the body, it can be damaged by airborne chemicals.
- Travel: if traveling to an area where hepatitis A or B might be a concern, vaccination is essential. Hepatitis C does not have a vaccination, so exercise caution in regards to safe sex, tattoos, and piercings. Malaria grows and multiplies in the liver, and yellow fever can lead to liver failure (the "yellow" in the name comes from one of its most obvious symptoms — jaundice). Both diseases can be prevented by oral medication and vaccination respectively.
- Safe sex: hepatitis C can be transmitted through sexual contact with someone infected with the virus (although the condition is more commonly spread through sharing needles and syringes, needlestick injuries in healthcare settings, and being born to a mother with the virus).16