All living cells need oxygen for respiration, which is the process by which cells get energy to do their activities. Oxygen absorbed at the lungs, must reach all cells in the entire body.  The nutrients absorbed in the small intestine, and the water absorbed mainly by the large intestine, also must get to all body cells, for metabolic processes to occur.

Some substances formed as a result of body reactions, are known as metabolic waste. Metabolic waste like carbon-dioxide (formed in all living cells, from respiration) and urea (formed in the liver as a means of getting rid of excess proteins), are constantly being formed in the body.

Accumulation of these metabolic waste, is toxic to the body, and hence must be transported to organs which are designed for their – removal. Carbon-dioxide from lungs and urea as urine, from the bladder. All the above substances are carried by blood, which is the main transport medium in the body.

The system that carries blood, is the blood vascular  system, also known as the blood circulatory  system which consists of the heart and the blood vessels connected to it.

The heart, which is the size of a human fist, is the  muscular pump, which drives blood through the blood vessels. It has four chambers, namely two atria and two ventricles.
There are three types of blood
vessels.

1. Arteries that carry blood away from the heart,
2. Veins that bring blood to the heart,
3. Capillaries which are very thin blood vessels that run through the organs and connect arteries with veins.
The main largest artery, transporting oxygenated blood from the heart to all parts of the body, except the lungs, is the aorta. De-oxygenated blood from all parts of the body (except lungs) comes to the heart via the main veins called vena cavae.
The heart pumps this de-oxygenated blood to lungs, through the pulmonary artery. At the lungs, this blood gets purified by oxygenation, and is returned to the heart via the pulmonary vein.

By Kanthi Goonawardane

Kids world