The heart is a very vital organ which works incessantly from birth till death. It is divided into four compartments. The upper right compartment receives impure blood from all over the body, and sends it to the lower right one, from where it is sent for purification in the lungs. The purified blood from the lungs goes to the upper left compartment of the heart, and thence to the lower left one, from where it is pumped all over the body, through the arteries. The arteries are thickwalled tube-like structures which carry blood from the heart to various body-parts, through their branches and minute sub-branches. These minute sub-branches divide further into capillaries which are very thin-walled. The capillaries ramify into various muscles, organs, and tissues. Due to the thinness of their walls, the capillaries allow the nutrients and oxygen in the blood to come out of them, and be supplied to the tissues. In return, the carbon dioxide and other waste materials from the tissues diffuse into the capillaries. Minute sub-branches of veins start from where the capillaries end, and blood (with the waste materials in it) proceeds through the veins, and is ultimately returned to the heart. The veins are thin-walled. They have to raise the blood from the lower extremities, pelvic region. trunk, and abdomen against the force of gravity. This is achieved with the help of valves situated in the veins.The blood contains nearly 90% water, in which three kinds of bodies keep floating. These are: the red blood corpuscles, the white blood cells, and the blood platelets. The R.B.C.s contain a scarlet coloured substance called "haemoglobin", which is the carrier of oxygen. The W.B.C.s constitute the defence force in the body. They fight any foreign cells such as bacteria and other micro-organisms which, if allowed to infest the body, can cause many diseases. The blood platelets are supposed to play an important role in the coagulation of blood, when it comes out of the blood vessels due to cuts and wounds. The circulatory system is thus responsible for carrying out three main functions in the body, namely, supplying nutrients to various parts and collecting waste materials from them, maintaining the body temperature at the normal level, and supplying oxygen to every part and collecting carbon dioxide. In this last function it works in co-operation with the respiratory system.