The circulatory system
The circulatory system
The circulatory system has two interrelated, but distinct parts, the cardiovascular system which circulates blood around the body and the lymphatic system which returns excess fluid from the tissues to the cardiovascular system. The function of the cardiovascular system is to oxygenate blood in the lungs and distribute the oxygenated blood to the tissues of the body. At the same time, carbon dioxide that accumulates as a result of metabolism of oxygen within the tissues is removed from the tissues and transported to the lungs where it is released from the blood and exhaled. The cardiovascular system comprises the heart, a muscular pump, and blood vessels. Arteries convey blood to thin-walled capillaries where gaseous exchange takes place and veins return blood to the heart. The cardiovascular system is often described as two parallel systems; the pulmonary circulation moves blood through the lungs and the systemic circulation circulates blood through the body. Trace the circulation of blood through the two systems in Figure 4.1 by following the arrows from the side of the heart coloured red. It follows a figure-of-eight (8) pattern with the two systems interlinked at the heart, the upper loop representing the pulmonary circulation and the lower loop the systemic circulation. The heart is a muscular pump driving blood at considerable pressure through arteries that get progressively smaller in both circulations until capillaries are reached. Arteries are sometimes dismissed as mere plumbing, but they play a vital role in regulating the blood flow through organs and tissues. Capillary walls are only one cell thick, allowing for the efficient diffusion of gases and small nutrient molecules to and from tissues. Waste gases and metabolites are also returned to the circulatory system through capillaries and these unite to form veins carrying blood under comparatively low pressure back to the heart. The heart comprises two muscular pumps arranged in parallel and beating in unison. As you can see in Figure 4.1, these two pumps are designated as the right and left sides of the heart. Each pump consists of two chambers, a thin-walled atrium that receives blood from one or other circulation and a thick-walled ventricle that ejects blood into the circulations.
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