There are approximately 100,000 miles of blood vessels in an adult human body -- enough to circle the planet four times. The arteries and veins are an intricate network that helps the organs get the nutrients they need. Veins are fragile and can collapse easily under certain circumstances. Arteries, on the other hand, are sturdier and thicker as they are built for the heavy workload. While these two types of vessels are similar, the key differences in their designs make the circulatory system one of the most efficient superhighways.
For the most part, oxygenated blood is carried away from the heart by the arteries, while the veins carry deoxygenated blood back to the heart. The one exception to this rule is pulmonary and umbilical blood flows. From the heart's right ventricle, deoxygenated blood travels to the lungs via the pulmonary artery. With the help of capillaries, the blood oxygenates through the respiration process and is returned to the heart via the pulmonary veins. In pregnancy, the artery passes deoxygenated blood to the placenta, and the umbilical vein inside the cord carries oxygenated blood back to the fetus.
Veins have to help blood travel against gravity, a practice that requires special mechanisms called valves. Deep veins use these unidirectional valves to ensure blood goes to the heart and doesn't backflow. Muscles contractions help blood move through the veins. Arteries don't need valves. They are strong and thick, which helps them to work with the pressure from a pumping heart that pushes blood in one direction.
Arteries branch off into arterioles, smaller, muscular blood vessels that lead to capillaries. The arteriole lumen, the internal open space in vessels through which blood flows, ranges between 100 and 300 micrometers in diameter. Venules are very small veins that pick up blood from capillary beds and move them to the larger veins. Venules range in diameter from approximately 8 to 100 micrometers.
Both arteries and veins have the same makeup when it comes to their layering. However, marked differences in each layer help them fulfill their functions. Tunica intima, also called Tunica interna, is the innermost layer or endothelium. Because of their smooth muscle constriction, the endothelium looks wavy in arteries and smooth in veins. Also, because arteries are larger and subject to blood flow pressures, their lining has an elastic membrane.
Surrounding the Tunica intima is the Tunica media. This middle layer is much thicker in arteries than in veins. Veins have collagen fibers and few elastic fibers while arteries have mostly elastic fibers. In this layer, vasa vasorum or small blood vessels and nervi vasorum -- nerves -- are present in veins but not arteries.
Also known as Tunica adventitia, this outer layer anchors blood vessels to surrounding tissues. It is thin in arteries, but is the thickest layer in veins. Both vessels have the nervi vasorum and vasa vasorum present at this level, but there are material differences. Veins have more smooth muscle fibers while arteries have few if any.
The Great Saphenous Vein is the longest vein in the body. It starts out as a superficial vein on top of the big toe and eventually deepens to join the femoral vein at the upper thigh. The length ranges between 39 and 55 centimeters. By comparison, the aorta is both the largest and longest artery. With an average length of 45 centimeters, it comes from the left ventricle of the heart and becomes the ascending aorta, aortic arch, thoracic aorta, and abdominal aorta.
There are three types of arteries: elastic, muscular, and arterioles. Elastic arteries need to accommodate the sudden surge from the heart contracting and ejecting blood. Muscular arteries are better at contracting and dilating as they adjust blood delivery, while arterioles deliver blood to capillaries. By contrast, in addition to pulmonary and systemic veins, there are superficial and deep veins. Superficial veins are just below the skin, which means they are visible and carry less blood. Deep veins are inside the muscles and pair with an artery of the same name.
Research has shown that in instances of blocked arteries, new arteries grow as a way to bypass the damage and provide oxygenated blood. If veins collapse due to damage, rest can enable them to heal and return to normal. However, severely damaged veins may be able to regrow just like arteries. The process of angiogenesis can occur thanks to the repair molecules VEGFR, VEGFR2, and NRP1, which bind together and stimulate the healing process.
Because arteries mostly carry oxygenated blood, they are bright red. Blood in veins is a darker red, but the superficial veins look blue or purple due to ocular perception. Subcutaneous fat reflects the light that penetrates the skin all the way to the vein, which the eyes perceive as blue or purplish blue.
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