Endocrine System

Print This Post Print This Post

 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 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.

3.8     Blood enters the right atrium of the heart and is pumped into the right ventricle. When the right ventricle contracts, blood is pumped through the pulmonary arteries to the lungs, where the blood gives off carbon dioxide and picks up oxygen. Oxygen-rich blood returns to the heart through the pulmonary veins and the left atrium. It is, then pumped to the left ventricle. The left ventricle contracts, pumping blood into the large artery, aorta, and out to various parts of the body. Oxygen-rich blood is carried by the branches of the aorta to cells in all parts of the body. When the cells use up all the oxygen, the veins carry the blood back to the heart. The arteries coming out of the right side of the heart carry this blood to the lungs, where it gets more oxygen. Then the oxygen-rich blood is carried to the left side of the heart, and the cycle begins all over again. Blood that does not get oxygen is purplish-red in colour. Blood that is rich in oxygen is scarlet or bright red in colour.

 Heartbeat and Cardiac Output

3.9     One complete contraction and relaxation of the heart muscle makes up one heartbeat. In an adult the heart beat rate is 60-80 times a minute. In children, it is faster – about 80-100 times. In babies, it is very fast. Exercise, fever, excitements and some kinds of infections increase the rate of heartbeat. During exercise, the muscles and the body organs need more oxygen and nutrients. The heart accommodates this need in two ways. First, it pumps blood out faster, increasing the heart rate. Second, the heart increases its stroke volume, the amount of blood pumped out of the heart

with each heartbeat. The amount of blood pumped with each beat multiplied by the number of beats per minute is called cardiac output. As a person exercises regularly and becomes more conditioned, the heart begins to work more efficiently and stroke volume increases. In very well-conditioned athletes, the heart itself enlarges in response to the increased demand. This type of enlargement is not harmful and disappears rapidly if the exercise level diminishes.

 Prev 1 2 ... 23 24 25 26 27 ... 50 51 Next

Advertisements