Working of Circulatory system
The human Heart acts as a double pump. Blood comes to the heart twice on one complete circulation around the body. This is called double circulation.
In the right atrium, blood from the whole body comes back through two large veins called Vena Cava. It has less amount of oxygen. when the right atrium contracts blood is pushed into the right ventricle. From this chamber, the blood is pumped out through the pulmonary artery to the lungs. In the Lungs, blood collects oxygen from the inhaled air and gives up its carbon dioxide. This oxygenated blood from the lungs returns to the left atrium through pulmonary veins. When the left atrium contracts, it is pushed into the left ventricle. From this chamber of the heart, the blood through a large artery called the aorta is carried to the upper part and to the lower parts of the body.
"People who lived at high altitudes, where there is less oxygen, have more red blood cells per liter of blood than people who live at lower altitudes."
