Most cells in the human body are not in contact with the external environment and hence rely on the circulatory system for blood supply. Blood circulation is the movement of blood around the body pumped by the heart and involves a complicated process that supplies blood for efficient transportation of oxygen. Blood rich in oxygen is transported into the body while blood deprived of oxygen are brought to the heart for oxygenation. The heart has four chambers that serve as two pumps each of which has lower ventricle and upper atrium. The pathway of the blood to and from the heart follows a process where blood from the body enters the heart through the vena cava, into the right atrium and move into the right ventricle. From here, it is propelled into the pulmonary artery, goes into the lungs, and passes through the pulmonary vein, right ventricle and into the body through the aorta.
Deoxygenated blood from the body first enters the right part of the heart through two large veins called the vena cavae. Blood from the head and arms are returned by the superior vena cava while those from the other parts of the body goes to the heart through the inferior vena cava. This blood empties into the right atrium where there is low pressure. During relaxation of the heart between beats, the right atrium is filled with blood because of the pressure in the circulatory system. As the atria contracts, the pressure inside it rises forcing the opening of the tricuspid valve, and blood from the right atrium squeezes into the right ventricle as shown below.
Figure 1. Contraction of the atria and the blood fills the heart
After the emptying of the atrium, the pressure falls while the pressure in the ventricle rises so as to shut the atrioventricular valve thus impeding blood flowing back. The contraction of the right ventricle forces blood out through the semi-lunar valve into the pulmonary artery, causing the pressure inside it to fall below that in the pulmonary artery which in turn causes the shutting of the semi-lunar valve. Pulmonary artery, which is stretched outward due to the increase in blood pressure, provides a passage of the deoxygenated blood into the lungs where oxygen is added. Carbon dioxide from the blood is absorbed by tiny blood vessels called capillaries and replaced with oxygen. The oxygenated blood trickles down from the lungs into the heart through the pulmonary veins, which are the only veins that transport blood rich in oxygen.
The left atrium and the left ventricle relax and increase in volume so as to receive blood. The left atrium contracts, forcing the bicuspid valve to open hence allowing blood to flow into the left ventricle. A ring of muscle contracts simultaneously around each vein where it joins the atrium hence closes the veins and blood is stopped from leaving the heart. As the left atrium empties, the bicuspid valve closes, and the pressure of the left ventricle begins to rise. The blood here is forced at high pressure into the aorta through the semi-lunar valve. This high pressure is because the left ventricle has to pump blood to all parts of the body and hence has the thickest walls. The walls of the aorta enlarge, and muscles are stretched because of the blood in the left ventricle.
Figure 2. Ventricle contraction forcing blood into the lungs
As the left ventricle empties, the pressure inside it drops below that in the aorta, causing the semi-lunar valve to close as the walls of the aorta retracts thus inhibiting the blood flowing back into the heart. The aorta then carries the blood into the parts of the body apart from the lungs. At the aorta, the available options for the flow of blood are few. It can be pumped into the brain through the carotid artery, into the arms through the auxiliary articles or the aorta into the legs. The arteries, capillaries and the veins act as conduits for blood to pass through in the body and finally deoxygenated blood returns to the heart as summarized below.
Figure 3. Direction of blood flow in the heart
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