The heart, a hollow organ with four chambers, can be imagined as an unbelievably strong pump. It is about the same size as a fist. Electrical signals coming from the brain cause the heart muscle to contract about 70 times in a minute in an adult. The contractions are more frequent in children and while we are playing sports. These contractions pump the blood through the whole body through the circulatory system. This system consists of two types of vessels: arteries with blood rich in oxygen and veins with blood poor in oxygen. While the arterial blood supplies oxygen to every cell in the body, the venous blood is sent to the lungs where it is purified by removing the waste product carbon dioxide and absorbing new oxygen.
