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1. The cardiovascular system is the system of blood circulation. By the cardiovascular system we mean the heart, the arteries, the veins and the capillaries of the human body.
2. The centre of the circulatory system is the heart. The human heart is a cone-shaped organ, about 5 inches1 long and 3 1/2 inches broad. It weighs about 10 ounces2 in the adult male, 6 ounces in the female. It lies in the thoracic cavity, just behind the breastbone and between the lungs. The heart is a hollow muscle which has four chambers. The right heart consists of an upper chamber, the atrium or the auricle and a lower chamber, the ventricle. Between these two chambers is a one-way valve, the tricuspid valve. The left heart has two chambers, but the valve that separates its chambers we call the mitral valve. Although the heart is a unit, anatomically and functionally, we may think of it as of two pumps - the «right heart» and the «left heart». The right heart receives blood from the veins and pumps it into the lungs by way of the lesser circulatory system. In the lungs the blood receives oxygen. Then it moves into the left heart. From the left heart the well-oxygenated blood moves into a large artery, the aorta. The blood returns to the heart by means of the veins. The walls of the capillaries are so thin that the dissolved nourishment that has come from the digestive system and the oxygen that has come from the lungs can pass through them into the tissues of the body and so nourish it. The capillaries form a close network all over the body. They gradually join together and get larger, and become veins.
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