Your blood vessels – the pipes – are pretty amazing, too. This complex network of tubular passageways carries blood from the heart to tissues throughout the body and back again to the heart. Freshly oxygenated blood leaving the heart enters the aorta, the large artery connected to the left ventricle, which branches off into large, elastic arteries and smaller arterioles. Your vascular system contains more than 100,000 arterioles, many of which are less than 1/100 inch in diameter. Arteries have thick, elastic walls made of smooth muscle tissue that allows them to expand and contract in response to changes in pressure against them caused by the heart’s rhythmic pumping action. As the muscular arteries contract and relax, they act much like reservoirs and auxiliary pumps, keeping the blood flowing during the resting phase of the heartbeat.
Arterioles are the smallest of the arteries. They deliver oxygenated blood to capillaries and interconnecting branchlike capillary beds. These tiny vessels, sometimes collectively called the microvasculature, are extremely thin-walled and fragile. As fluids diffuse through the capillary walls, nutrients, gases, hormones, and other vital components are delivered to the tissues, and waste products of cellular metabolism are picked up.
The blood then begins its journey back to the heart, flowing from capillaries into small venules that merge to form larger veins. Veins have thinner, less muscular walls than arteries, but their diameter is wider and they are equipped with a complex series of valves. Unlike the blood flow through the arteries, which is assisted by the muscles in the vessels themselves, venous blood flow is assisted by pressure changes that occur when you breathe, which suck blood upward. This is known as the respiratory pump. In addition, as the skeletal muscles throughout your body contract and relax, they move venous blood toward the heart, a phenomenon called the muscular pump. (This explains why your feet and ankles swell when you sit or stand for long periods of time. The muscles in your lower extremities are not active enough to help move blood up through the veins, resulting in pooling of blood and swelling.) All the while, the valves in the veins prevent backflow.
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