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Circulation of Blood

Trace the flow of blood in the pulmonary and systemic circulations.

Circulation is the process by which blood goes to all parts of the body. The flow of blood in the circulatory system follows a definite pattern. The heart pumps blood. Blood that leaves the left side of the heart is rich in oxygen, so called oxygenated blood. Oxygenated blood enters the arteries and continues to travel through smaller arteries and arterioles until it reaches the capillaries in the organs of the body.

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Exchange of materials between the blood and the cells takes place across the capillary walls. Nutrients and oxygen are absorbed by the cells from the blood through the walls of the capillaries while wastes from the cells are absorbed by the blood.

Blood leaves the capillaries have little oxygen; it is called deoxygenated blood. Deoxygenated blood enters the venules, and then travels to the veins until it reaches the right side of the heart. From the right side of the heart, blood flows through the arteries to the lungs. In the capillaries of the lungs, the deoxygenated blood receives a fresh supply of oxygen; it becomes oxygenated again. At the same time, carbon dioxide (from the cells) leaves the blood and enters the air sacs of the lungs.

The oxygenated blood flows through veins back to the left side of the heart, ready to make another round-trip to all parts of the body and back.

Blood circulation consists two main parts: a bigger from the heart to all organs of the body and back; and a smaller circulation from the heart to the lungs and back. The bigger circulation is called systemic circulation, involving the whole body system. The smaller circulation is called pulmonary circulation, involving only the lungs.

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The systemic circulation begins at the left side of the heart. Contraction of the left ventricle sends the blood to the aorta and to the smaller arteries, arterioles and capillaries that nourish the different organs. Oxygen and nutrients pass from the blood to the cells and carbon dioxide as well as the other wastes passes from the cells to the blood. The very thin walls of the capillaries allow this exchange to take place.

The pulmonary circulation begins at the right side of the heart. Contraction of the right ventricle sends the blood to the pulmonary arteries and to the capillaries of the lungs where exchange of gases takes place. Carbon dioxide passes out of the blood while oxygen enters it. Blood coming from the lungs is oxygenated blood. It passes through the pulmonary veins to the left atrium, then to the left ventricle, from where it reenters the systemic circulation.

The heart muscle is supplied with oxygen and nutrients by its own blood vessels. The arteries of the heart, called coronary arteries, branch from the aorta. These arteries branch into arterioles and capillaries which supply oxygen and nutrients to the heart muscles. Deoxygenated blood from the heart muscle is collected by coronary veins and returned to the right atrium where it joins the deoxygenated blood from the systemic circulation.

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