2. HEART The heart is a four-chambered muscular organs that pumps blood to all part of the body. It is able to do so by the rhythmic contraction and relaxation of its muscles. These are known as the heart beats. A normal heart beats about 60 to 80 times a minute. We can hear our heart beat with the help of an instrument called stethoscope.

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12. The walls of the auricles are thinner than those of the ventricles because the auricles have to pump blood into the ventricles,which is very near. The walls of the ventricles are thicker,since they have to push the blood to reach the farthest parts of the body.The blood in the right side of the heart remains completely separated from the left side with the help of valves.

14. The veins get deoxygenated blood to the right auricle from all parts of the body. This blood is pumped into the right ventricle, which in turns pumps it into the pulmonary artery that takes it to the lungs for oxygenation.

15. In the lungs carbon dioxide diffuses out into the alveoli and oxygen diffuses in. The oxygen-rich blood is carried from the lungs to the heart by the pulmonary vein. Oxygenated blood enters the left auricle. It is then pumped into the left ventricle. The left ventricle then pumps the oxygenated blood to all parts of the body.