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According to their functions different organs of the human body are divided into several systems: the bones, the muscular system, the alimentary tract, the respiratory system, the urogenital1 system, the vascular system, and the nervous system.
The muscles and the bones are under the layer of subcutaneous2 fat. The muscles are connected with the bones.
The heart and the large blood vessels connected with it, as well as the lungs and the esophagus are in the thoracic cavity. The spleen, the liver and the stomach are in the abdominal cavity under the diaphragm. The small and large intestines are in the abdominal cavity lower than the stomach, the liver and the spleen. The kidneys are on the posterior side of the abdominal cavity.
Text A. Work of the Human Heart
The human heart contracts from the first moment of life until the last one. The contractions of the heart pump the blood through the arteries to all the parts of the body. Scientists have determined that the total weight of the blood pumped by the heart daily is about ten tons.
The rate of heart contractions is regulated by two groups of nerve fibers. It varies in different persons and at different age.
Physiologists have determined that in the adult the heart makes from 60 to 72 beats per minute. In children the rate of heartbeat is much higher. Research work of many scientists has helped to determine that the rate of heartbeat increases depending on different emotions.
Each beat of the heart is followed by a period of rest for the cardiac muscle. Each wave of contraction and a period of rest following it compose a cardiac cycle.
Research work has given physiologists the possibility to find out that the heart muscle works or contracts about one third of the time of the person’s life. The period of rest is shorter during greater physical exertion and longer when the body is at rest.
Each cardiac cycle consists of three phases: physiologists have called the first phase of short contraction of both atria—the atrial systole. They have called the second phase of a more prolonged contraction of both ventricles—the ventricular systole. The period of rest of the cardiac muscle is called the diastole.
The left ventricle discharges out the blood received by the left atrium from the pulmonary circulation through the aorta to the systemic circulation.
The blood received from the systemic circulation by the right atrium is discharged out of the right ventricle to the lungs through the pulmonary arteries.
Prolonged research work of many physiologists has given the possibility to estimate the role of the ventricles which serve as the main pump. The atria act as receiving chambers. The contraction of the atria which sends the final portion of the blood into the ventricle is considerably less.
Text B. The Circulation of the Blood
Now we know that the venous blood from the systemic and portal circulation is brought to the right atrium of the heart. When the pressure in the right atrium has increased the blood passes into the right ventricle from the right atrium.
During the systole of the ventricle the blood is pumped from the right ventricle into the pulmonary artery. When the right ventricle has pumped the venous blood into the pulmonary artery it enters the pulmonary circulation. The blood is brought to the lungs through the pulmonary artery. In the lungs the venous blood discharges out carbon dioxide. When the blood has discharged out carbon dioxide it takes in oxygen in the lungs.
The blood which has become oxygenated passes from the venous part of the pulmonary capillary system into the venules and veins. When the oxygenated blood has passed the four pulmonary veins it is brought to the left atrium of the heart.