The Functions of the Liver (Digestive System)

Reminder about the Liver:

The liver is an accessory organ within the human digestive system. This means that it assists with the digestive processes, e.g. by supplying substances useful to the digestive process - but that ingested material (i.e. food and drinks and the substances they are broken down into as they pass through the digestive system) does not pass through the liver.

The liver has over 500 functions (only some of which are mentioned on these pages).

1.

Secretion of bile and bile salts

Bile: Liver cells called hepatocytes secrete bile, which is a a yellow/green (though may appear as dark as brown) slightly alkaline liquid.
Bile Salts are also produced produced by the liver.

2.

Phagocytosis of bacteria and dead or foreign materials

Within the liver, blood passes through spaces called sinusoids - instead of through capillaries (as elsewhere in the body). A special type of cell called Kupffer's Cells, which are also known as stellate reticuloendothelial cells, are located in the sinusoids and destroy many types of unwanted particles present in the bloodstream through the liver. Such particles include:

• bacteria,
• antigens, i.e. other substances from outside of the body
  (sometimes called "foreign matter"),
• imperfect or no-longer functioning blood cells
  (e.g. damaged leucocytes and erythrocytes).

3.

Carbohydrate metabolism

(also Maintenance of normal blood glucose level)

4.

Lipid ("Fat") metabolism

Liver cells called hepatocytes perform several important roles concerning fat ("lipid") cells.
These include:

• Break-down of fatty acids - generating adenosine triphosphate (ATP), which is important for the contraction and relaxation of muscles.
• Synthesis of lipoproteins, which are important for the movement of fatty acids, cholesterol and triglycerides to and from cells.
• Storage of certain triglycerides
• Synthesis of cholesterol (as well as using cholesterol to produce bile salts).

5.

Protein metabolism

Liver cells called hepatocytes perform important roles re. the processing of protein cells.
These include:

• Synthesis of all plasma proteins except for -globulins.
  Plasma proteins produced in the liver include:
  albumin, lipoprotein, transferrin, caeruloplasmin, globulins (but not -globulins), -antitrypsin, -fetoprotein, fibrinogen, prothrombin, Factors V, VII, IX, X and XII, and XII.
• De-amination of excess amino acids, i.e. removal of the -NH₂ part (called the "amino group") from amino acids, enabling the remaining parts to be re-used, e.g. for conversion to ATP, carbohydrates, or fats.
• Conversion of the ammonia (NH₃) resulting from the de-amination of excess amino acids, into urea (via the ornithine cycle). That urea is ultimately excreted from the body as a part of urine.
  This is an important detoxification process because ammonia is more toxic than the urea it is converted to, for subsequent excretion via the urinary system.

6.

Processing drugs

The liver can detoxify substances such as alcohol - but is considered to be adversely affected by consumption of excessive quantities of alcohol over extended periods of time.
The liver is also understood to process various common drugs, e.g. penicillin, into bile.

7.

Processing hormones

The liver is able to chemically change "process" certain hormones, such as thyroid hormones and steroid hormones e.g. estrogen and aldosterone.

6. and 7. are identified in different ways in different textbooks and other teaching materials.
E.g. They may be described (collectively) as "biotransformation" and, in some cases as "detoxification" processes.

8.

Excretion of bilirubin

Bilirubin is a component of bile, which is produced by the liver.

The source of bilirubin is the heam of aged (i.e. no-longer optimally functioning) red blood cells, which also known as erythrocytes.

Following the liver secreting bilirubin as part of the fluid bile, it is eventually removed from the body (i.e. excreted) because most of the bilirubin in the bile is then metabolized by bacteria in the small intestines, then eliminated from the body in the faeces.

9.

Storage of vitamins and minerals

The liver stores several important chemicals, then releases them when they are needed somewhere else in the body. Such chemicals include:

• Glycogen,
• The fat soluble vitamins (A, D, E and K), the liver being the location of the body's main store of these.
• Vitamin B₁₂
• Minerals: Iron (Fe) and Copper (Cu).

10.

Activation of vitamin D

The liver is one of the parts of the body that, together with the skin and the kidneys, participate in forming the active form of vitamin D. (Vitamin D is necessary for absorption of the minerals calcium and phosphorous, and for regulation of the permeability of cell membranes.)

11.

Protection
(of the body)

Several processes that occur in the liver can be described as protecting the body, especially e.g. by helping to remove substances that will not serve a useful purpose. Some such processes are already mentioned above - such as phagocytosis (2.) and detoxification (incl. in 5. and 6.).
Another protective process performed by the liver is the filtration of portal blood, which removes certain toxins and microorganisms from the blood before it re-enters systemic circulation.

12.

Haematopoiesis

Haematopoiesis is the formation of the cellular components of blood.
The liver is the main site of embryonic haematopoiesis.
However, this function of the liver ceases before birth (bone marrow having been supplementing the haematopoiesis performed by the liver from about 5 months gestation).