Blood has three main functions in the human body. These are: Transport of substances (like respiratory gases, oxygen and carbon dioxide; digested food or nutrients; waste products; hormones; enzymes and ions) from one part of the body to the other, Protection against disease, and Regulation of body temperature. We can now say that: The important functions of blood in our body are as follows:
1. Blood carries oxygen from the lungs to different parts of the body.
2. Blood carries carbon dioxide from the body cells to the lungs for breathing out.
3. Blood carries digested food from the small intestine to all the parts of the body.
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4. Blood carries hormones from the endocrine glands to different organs of the body (where they are needed).
5. Blood carries a waste product called urea from the liver to the kidneys for excretion in urine.
6. Blood protects the body from diseases. This is because white blood cells kill the bacteria and other germs which cause diseases.
7. Blood regulates the body temperature. This is because the blood capillaries in our skin help to keep our body temperature constant at about 37°C.
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The main transport system in human beings (or man) is the ‘blood circulatory system’ (which is sometimes called just ‘circulatory system’ for the sake of convenience). In the human circulatory system, blood carries oxygen, digested food and other chemicals like hormones and enzymes to all the parts of the body.
It also takes away the waste products (or excretory products) like carbon dioxide and urea produced in the body cells. The human blood circulatory system consists of the heart (the organ which pumps and receives the blood) and the blood vessels (or tubes) through which the blood flows in the body. In blood circulatory system, the blood flows through three types of blood vessels:
(i) arteries,
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(ii) veins, and
(iii) capillaries.
The blood vessels of the circulatory system are present in each and every part of the human body due to which the blood reaches all the parts of the body.
In addition to the blood circulatory system for the transport in human beings, there is another system called lymphatic system which also helps in the transport of materials in the human body.
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The liquid which circulates and carries materials in the lymphatic system is called lymph. Thus, in human beings, the various substances are transported through two liquids called ‘blood’ and ‘lymph’. We will first describe the blood circulatory system which is the main transport system in humans.