Blood circulation in human body

Blood circulation in human body

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Blood circulation in the human body is essential for carrying oxygen and nutrients to the cells for metabolism. Blood also carries carbon dioxide and waste products of metabolism back to excretory organs for disposal. Certain organs like brain are very much dependent on oxygenated blood so that brain loses its vitality within 4 minutes of loss of blood circulation. Other organs may be able to survive longer periods of loss of blood supply. Brain can also survive longer periods if body temperature is very low. This fact is used medically to protect the brain by cooling in a process known as therapeutic hypothermia.

Heart is the organ which pumps blood to various organs of the body and receives the blood returning from them. It pumps oxygen rich blood through blood vessels known as arteries and receives oxygen poor blood through blood vessels known as veins. As the pumping of blood occurs at high pressure, the walls of arteries are much thicker. Veins return blood passively to the heart and hence have thinner walls. Muscles of the body surrounding the veins act as an external pump by compressing the veins during limb movements. Role of muscle contraction is more important in the legs as the blood has to return against gravity.

Heart has four chambers and four valves to ensure flow of blood in one direction without backflow when the heart contracts. Upper chambers of the heart are called right atrium and left atrium. Lower chambers are called right ventricle and left ventricle. Left ventricle is much thicker than the right ventricle as it has to pump blood at much higher pressure, to the whole body. Right ventricle is thinner as it has to pump blood only to the lungs at lower pressure. Left ventricle is connected to the large blood vessel known as aorta, while the right ventricle is connected to the pulmonary artery. Pulmonary artery carries blood to the lungs for oxygenation.

Blood returns from the lungs after oxygenation to the left atrium. From the left atrium it enters the left ventricle through the mitral valve. Left ventricle pumps it through the aortic valve to the aorta. Blood returns from the body to heart through large veins known as vena cava. Superior vena cava returns blood from the head and neck and arms. Inferior vena cava returns blood from the legs and lower part of the body. Blood returning from the heart muscles comes through the coronary sinus. All these three veins join the right atrium. From the right atrium, blood moves to the right ventricle, across the tricuspid valve. Pulmonary valve guards the opening to the pulmonary artery and aortic valve guards the opening to the aorta.

Blood circulation to each organ is known after the name of the organ. Blood flow to the heart is known as coronary circulation. Cerebral circulation is the technical name for blood vessels of the brain. Pulmonary circulation denotes blood circulation in the lungs, primarily meant for oxygenation. There is another circulation in the lungs known as bronchial circulation which carries oxygenated blood for the metabolism of lung cells. Renal circulation refers to blood flow to the kidneys, meant primarily for filtration of waste products of metabolism. For this reason, blood flow of the kidneys is out of proportion to the size of the organs. Blood flow to the stomach and intestines is called splanchnic circulation. There is another system known as portal circulation which takes nutrients from the intestines to the liver. Liver has another blood supply like the lungs, known as hepatic circulation for providing oxygen to the liver cells.