Blood vessels

In humans and other vertebrates, there are three types of blood vessels: arteries, veins and capillaries. Arteries and veins are distinguished from each other by the direction of blood flow, not by the nature of the blood (aerated or unaerated) they contain. Arteries carry blood from the heart to the tissues of the body; veins return it from the tissues to the heart.

Capillaries are microscopic vessels that are found in tissues and connect arteries to veins. Only through the walls of capillaries can the exchange of nutrients, gases and metabolic products between blood and tissues occur. Capillaries have extremely thin walls, consisting of a single layer of cells - endothelium, which serves as a continuation of the endothelial lining of arteries on one side and veins on the other.

Some capillaries are so small that red blood cells must bend when passing through them. The walls of arteries and veins are too thick for diffusion to occur through them; they consist of three clearly distinguishable layers: an outer connective tissue membrane, a middle layer of smooth muscle cells and an inner membrane formed from endothelium and connective tissue.

The outer lining contains fibrous tissue that gives the artery its strength and ability to withstand internal pressure, but also allows it to expand and contract with each heartbeat. The smooth muscles of the middle layer, contracting or relaxing, reduce or increase the lumen (cavity) of the artery and thereby regulate the amount of blood flowing to this organ.

In addition to the endothelial lining, the inner lining of most arteries contains a strong internal elastic membrane, which gives the walls additional strength. The walls of the arteries are supplied with two systems of nerves; impulses conducted by one system cause smooth muscle to contract, and impulses conducted by another system cause it to relax.

The largest artery, the aorta, near the heart has a diameter of about 2.5 cm, and its wall is about 3 mm thick. The walls of the veins are much weaker and thinner than the arterial walls, but consist of the same three layers. The outer connective tissue membrane contains fewer elastic fibers, and the middle muscle layer is thinner than the corresponding layers in the walls of the arteries; Most veins do not have an internal elastic membrane. Veins, unlike arteries, are in many places equipped with valves that prevent the reverse flow of blood.

The cells of the body are surrounded by a fluid that bathes them, called tissue fluid, and are not in direct contact with the blood. In order to reach the cells, substances must diffuse from the blood through the capillary wall and through the space filled with tissue fluid. An adult human has approximately 1015 cells, and the volume of fluid washing them is only 14 liters. To supply the same number of single-celled protozoa living in the sea with the gases and nutrients they need, I06 liters of sea water would be required.

Efficient devices (lungs, liver, intestines and kidneys), constantly replenishing the supply of oxygen and nutrients in body fluids and removing waste products, allow our body to live, despite the fact that it contains relatively little water. The small thickness of the capillaries means that each drop of blood passing through the capillary network acquires a large surface area through which diffusion can occur. It is estimated that one cubic centimeter of blood comes into contact with the surface of the capillaries, which is about 7000 cm2.

The number of capillaries throughout the body is almost impossible to count. In tissues with intense metabolism, such as muscles, capillaries are located very close to each other: the distance between adjacent capillaries is approximately twice their diameter. One researcher estimates that the number of capillaries in muscle tissue is approximately 240,000 per second.