Circulatory System
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The Blood Vessels
3.6 The 1,00,000 km blood vessels of the circulatory system form a continuous system supplying oxygenated blood to the body’s 100 trillion cells, tissues and organs. There are three types of blood vessels. Arteries are the thick-walled vessels that carry blood from the heart to all parts of the body. Veins, which have thinner walls, carry blood back to the heart from all parts of the body. Veins also contain tiny valves that prevent blood from flowing downward from the pull of gravity. Capillaries, which are the smallest of the blood vessels, link the arteries to the veins. The wall of a capillary is only one cell thick, which allows oxygen and nutrients to leave the bloodstream easily and be taken up by the body’s cells. Because of their small diameter, blood cells must pass through capillaries in single file. No cell in the body is more than a few cells away from a capillary.
The Heart
3.7 A fist size human heart weighing about 340 grams, is the world’s most efficient and wonderous pump, pumping blood through more than 1,00,000 kilometers of blood vessels (arteries, veins and capillaries). The heart lies in the middle of the chest between the lungs. It is situated closer to the front of the chest than to the back and slightly to the left side. The power of the heart is less than a 100 watt bulb but has incredible efficiency of pumping nearly 7,600 litres of blood per day, equivalent to filling tanks of 300 cars; and that too without rest and break and year after year. Heart is made up of a special muscle consisting of several layers of muscles arranged in circles and spirals. Cardiac muscle is called involuntary because it works without our thinking about it. No other muscle in the body is as hardworking and as strong as the heart is. Heart is only 0.5% of body weight but needs 5% of total blood supply to take care of body’s nutritional needs. The work done by the heart is almost equal to the work we would perform if we had to lift about 4.50 kilograms weight 3 feet off of the ground and if we had to repeat this task twice every minute for our entire lives. The heart is the circulatory system’s pump. Actually, the heart is two pumps placed side-by-side. The septum, a muscular wall divides the right and left sides of the heart. It prevents the blood from crossing over from one side to the other. The right side of the heart pumps blood to the lungs, and the left side pumps blood to the rest of the body. The atria receive blood and pump it into the ventricles. The ventricles push the blood out of the heart. The ventricles’ job requires a greater force, so the ventricles are larger and more muscular. Valves in the heart allow blood to move through it in one direction only.
