A liquid tissue in many animal groups, with a wide range of transport and metabolic functions. In many groups the blood carries a respiratory pigment responsible for the carriage of oxygen and carbon dioxide (See haemoglobin); in others, such as insects, the blood carries little respiratory gas. In vertebrates the blood has a wide range of other functions, and in mammals forms part of the immune system.
An adult human has approximately 5 to 6 litres of blood. Mammalian blood is composed of blood plasma (approximately 55% by volume) and blood cells (approximately 45% by volume). The plasma is a pale, straw-coloured fluid containing approximately 10% dissolved materials in water. The dissolved materials include blood proteins such as fibrinogen to assist in clotting, and antibodies; soluble food substances such as glucose, amino acids and oil droplets; ions such as Na{TAG(tag=>sup)}+{TAG}, Cl{TAG(tag=>sup)}-{TAG} and HCO{TAG(tag=>sub)}3{TAG}{TAG(tag=>sup)}-{TAG}; waste substances such as urea and carbon dioxide; and hormones. Blood serum is blood plasma with the protein component removed. Also present may be traces of other substances derived from diet or lifestyle, such as alcohol or drugs. These and other dissolved substances can be detected by various diagnostic tests. Different types of blood cell are given in the table.
||Different types of blood cell.
Type
Number per mm{TAG(tag=>sup)}3{TAG}
Function
Erythrocyte|
5 000 000|
Carriage of oxygen and carbon dioxide|
Leucocytes Phagocyte|
7000|
Defence: actively seek and engulf bacteria, particularly in wounds|
Lymphocyte|
2500|
Antibody-producing cells|
Thrombocyte|
250 000|
Cell fragments which assist in blood clotting|
|
In vertebrates, blood is pumped around the body by the heart, which generates pressure to circulate blood to the body organs via an artery. Blood flows from the arteries into thin-walled, narrow vessels called capillaries where exchange of materials with body organs takes place. Blood returns to the heart in a vein. |
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