Blood vessels, like the heart, are components of the cardiovascular system , also called the circulatory system. Although they are all vessels that transport blood, there are some basic differences between vein, artery and capillary.
→ Characteristic common to blood vessels
Blood vessels are usually formed by a few layers (tunics) in common: the tunica intima, tunica media and tunica adventitia. The tunica intima, the innermost layer, is formed by endothelial cells that are supported by connective tissue. Next, we have the tunica media, formed by smooth muscle cells and elastic tissue. On the outside, we have the tunica adventitia , formed mainly by dense non-patterned connective tissue and loose connective tissue.
Arteries leave the heart and become increasingly thin as they branch. These vessels have the main function of carrying blood and, consequently, nutrients and oxygen from the heart to the tissues.
Arteries carry blood under high pressure, so strong, elastic walls are needed. As previously mentioned, blood vessels, in general, have three tunics, however, in larger arteries, the tunica media is much thicker and rich in elastic fibers when compared to other vessels.
Veins result from the convergence of capillaries and become larger as they get closer to the heart. Unlike arteries, the tunica media of veins has fewer muscles and elastic fibers, and blood pressure in these vessels is low.
The main task of the veins is to carry blood from the body to the heart so that it can be pumped back into the body . Because some veins carry blood against gravity and blood pressure is low, these structures have some valves. These valves, formed by folds of the tunica intima, ensure the correct flow of blood towards the heart.
Capillaries are very thin blood vessels, lacking the tunica media and adventitia, which form a complex network of vessels. Because they have a thin wall, with only a few layers of cells, they become an ideal place for gas exchange to occur.
Curiosity: Some time ago the terms venous and arterial blood were used to indicate the blood that flowed in the veins and arteries, respectively. However, venous blood, which is rich in carbon dioxide, also circulates in arteries, more precisely in the pulmonary artery. The same occurs with arterial blood, rich in oxygen, which circulates in the pulmonary veins. Therefore, it is inappropriate to say that in the veins there is only venous blood and that in the arteries there is only arterial blood.