Circulatory System
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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.
Heart Arrhythmia
3.10 The heart beats in a regular rhythm as the walls of the atria and ventricles alternately contract and relax. This is called the cardiac cycle. If the cardiac cycle is disrupted and the heart gets out of rhythm, a condition called arrhythmia results. Arrhythmia can lead to serious consequences, including death. When a person begins to exercise, the heart adjusts to the greater demand for oxygen by increasing its rate. If the person exercising has a heart problem or is in a weakened condition, there is a chance that as the heart rate increases, the heart will get out of rhythm. When the heart’s atria get out of synch, a person can survive because blood is still able to move through the heart. However, if the ventricles are affected, sudden death may occur because blood does not get pumped to the vital organs. To help avoid exercise-related heart-rhythm problems, always warm up first so the heart can adjust slowly. There are other causes for arrhythmia. For example, the drug cocaine can cause arrhythmia by disabling the nerves that regulate the contractions of the heart.
What is Blood Pressure?
3.11 Blood pressure (BP) is determined by two key factors: how hard heart beats and how easily the blood flows through the arteries. The force exerted by the heart as it pumps blood into the arteries creates a pressure within them and this is called “blood pressure”. A certain level of blood pressure is thus essential to keep the blood circulating in the body. But when pressure becomes too high, it is called hypertension which is caused by constriction or narrowing of the small blood vessels.
