Since the heart pumps so much blood, it must be clear that the same blood must pass through the heart many times in the course of a day. This is true, for the round trip of blood from the heart to distnat parts of the body and back takes less than a minute. The round trip to nearer parts of the body takes an even shorter time.
The blood takes two main paths in its trip through the body. When the right ventricle of the heart contracts, blood is forced into a large artery that leds to the lungs. (An artery is an elastic tube that carries blood away from the heart.) Here the red cells of the blood take up oxygen from the air in the lungs. They also give up carbon dioxide. From the lungs, the blood flows through two veins that lead back to the heart. (A vein is an elastic tube that carries blood towards the heart.) The blood enters the left auricle and passes through the valve leading to the left ventricle. When the left ventricle contracts, the blood flows into an other large artery. This artery branches into smaller arteries that branches several times more into smaller and smaller arteries. The smallest arteries are in the tissues, and are called capillary arteries. From the capillaries, the blood transfers nourishment and oxygen to the cells and removes carbon dioxide and other wastes.
Capillary arteries connect with capillary veins. These tiny veins connect with larger and larger veins as they approach nearer to the heart. Blood flowing through the veins eventually reaches a large vein that enters the right auricle of the heart. From the right auricle, the blood flows through the valve leading to the right ventricle, and thus it ends a complete round trip through the body. The heart, the blood and the veins and arteries make up the body's circulatory system.
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