Is 18 days.
6.2 The liver
The liver, weighing about 1.5 kg, makes up 3-5% of the body weight. It originated as a digestive organ but its functions are now much more diverse.
In an adult human, the liver is typically 28 cm × 16 cm × 9 cm although its exact size varies considerably according to the quantity of blood stored within it. It is found immediately below the diaphragm, to which it is attached. Blood is supplied to the liver by two vessels: the hepatic artery, which carries 30% of the liver's total blood supply, brings oxygenated blood from the aorta; the hepatic portal vein supplies 70% of the liver's blood and is rich in soluble digested food from the intestines. A single vessel, the hepatic vein, drains blood from the liver. In addition, the bile duct carries bile produced in the liver to the duodenum.
The branches of the hepatic artery and those of the hepatic portal vein combine within the liver to form common venules which lead into a series of channels called sinusoids. These are lined with liver cells or hepatocytes. Between the hepatocytes are fine tubes called canaliculi in which bile is secreted. The canaliculi combine to form bile ducts which drain into the gall bladder where the bile is stored before being periodically released into the duodenum.
Functions of the liver
The liver is the body's chemical workshop and has an estimated 500 individual functions. Some of these have been grouped under the following twelve headings:
1. Carbohydrate metabolism.
The liver's major role in the metabolism of carbohydrates is to convert excess glucose absorbed from the intestine into glycogen. This stored glycogen can later be reconverted to glucose when the blood sugar level falls. This interconversion is under the control of the hormones insulin and glucagon produced by the Islets of Langerhans in the pancreas.
2. Lipid metabolism.
Lipids entering the liver may either be broken down or modified for transport to storage areas elsewhere in the body. Once the glycogen store in the liver is full, excess carbohydrate will be converted to fat by the liver. Excess cholesterol in the blood is excreted into the bile by the liver, which conversely can synthesize cholesterol when that absorbed by the intestines is inadequate for the body's need. The removal of excess cholesterol is essential as its accumulation may cause atherosclerosis (narrowing of the arteries) leading to thrombosis. If in considerable excess, its presence in bile may lead to the formation of gall stones which can block the bile duct.
3. Protein metabolism.
Proteins are not stored by the body and so excess amino acids are broken down in the liver by a process called deamination. Transamination reactions, whereby one amino acid is converted to another, are also performed by the liver. All non-essential amino acids may be synthesized in this way, if they are deficient in the diet.
4. Synthesis of plasma proteins.
The liver is responsible for the production of vital proteins found in blood plasma. These include albumins and globulins as well as the clotting factors prothrombin and fibrinogen.
5. Production of bile.
The liver produces bile salts and adds to them the bile pigment bilirubin from the breakdown of red blood cells. With sodium chloride and sodium hydrogen carbonate, cholesterol and water, this forms the green-yellow fluid known as bile. Up to 1 dm3 of bile may be produced daily. It is temporarily stored in the gall bladder before being discharged into the duodenum.
6. Storage of vitamins.
The liver will store a number of vitamins which can later be released if deficient in the diet. It stores mainly the fat-soluble vitamins A, D, E and K, although the water-soluble vitamins B and C are also stored.
7. Storage of minerals.
The liver stores minerals, e.g. iron, potassium, copper and zinc.
8. Formation and breakdown of red blood cells - The fetus relies solely on the liver for the production of red blood cells. In an adult, this role is transferred to the bone marrow. The adult liver, however, continues to break down red blood cells at the end of their 120-day life span.
9. Storage of blood.
The liver, with its vast complex of blood vessels, forms a large store of blood with a capacity of up to 1500cm3. In the event of haemorrhage, constriction of these vessels forces blood into the general circulation to replace that lost and so helps to maintain blood pressure. In stressful situations, adrenaline also causes constriction of these vessels, creating a rise in blood pressure.
10. Hormone breakdown.
To varying degrees, the liver breaks down all hormones. Some, such as testosterone, are rapidly broken down whereas others, like insulin, are destroyed more slowly.
11. Detoxification.
The liver is ideally situated to remove, or render harmless, toxic material absorbed by the intestines. Foreign organisms or material are ingested by the Kupffer cells while toxic chemicals are made safe by chemical conversions within hepatocytes. Alcohol and nicotine are two substances dealt with in this way.
12. Production of heat.
The liver, with its considerable metabolic activity, can be used to produce heat in order to combat a fall in body temperature. This reaction, triggered by the hypothalamus, is in response to adrenaline, thyroxine and nervous stimulation.
Did you know?
In an average lifetime we spend
three and a half years eating.
Speaking
Заполните таблицу. Работая в парах, опишите процесс пищеварения с помощью вопросов и ответов (по таблице):
Division of the alimentary canal |
Organs and substances that act upon the foodstuffs |
The form of the mass after being treated |
Oral cavity |
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Stomach |
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Duodenum |
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Small intestine |
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Large intestine |
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Chapter 2. Why organisms need transport and exchange mechanisms
All organisms need to exchange materials between themselves and their environment. Respiratory gases and the raw materials for growth must pass into an organism and waste products must be removed.
Did you know?
The 700 million alveoli