The capillary is ... Blood capillaries. Capillary functions

Any living organism can not exist and develop without oxygen and nutrients. Oxygen, falling into the lungs from the external environment, is transmitted throughout the body by the circulatory system, which has a rather complex structure. Circulation of blood is provided by hollow tubes - arteries, arterioles, precapillaries, capillaries, postcapillaries, veins, venules and arteriolo-venous anastomoses. Carbon dioxide and other waste products of metabolism are removed from the body also with the help of these vessels. The more they are removed from the heart, the stronger their branching into smaller ones.

Capillaries: definition of the concept

If the artery and vein, carrying blood from the heart and to him, respectively, are large vessels, then the capillary is a very thin blood tube, with a diameter of only 5-10 microns. And since the veins and arteries, being only the delivery of nutrients to the cells, do not participate in the gas exchange between them and the blood, this function is assigned to the capillaries. Their first descriptions belong to the Italian scientist M. Malpighi, who in 1661 gave them the definition of a link between the arterial and venous vessels. Before him, Harvey predicted their existence.

Structure and dimensions of capillaries

These small vessels have approximate equal diameters in different organs. The larger ones reach a lumen of up to 30 μm, and the narrowest ones reach 5 μm. It is easy to see that the wide blood capillaries on the sections in the width in the tube lumen are lined with several layers of endothelial cells, whereas the lumen of the smallest is formed by a layer of only one or two cells. Such thin vessels are located in muscles having a striated structure, and since their diameter is smaller than in erythrocytes, the latter, when passing through a narrow blood channel, undergo significant deformation.

A capillary is a tube so thin that its wall, consisting of individual cells of the endothelium, which are in close contact with each other, does not have a muscle layer and therefore is incapable of contraction. The capillary network usually contains only 25% of the blood that can be contained in it. But changes in these volumes can be achieved with the inclusion of the mechanism of self-regulation, when the smooth muscle cells are relaxed.

Capillary bed, venules, arterioles

The blood flow is directed towards the heart through large vessels that are veins. Capillaries transmit blood to the veins via venules - the smallest collective components. They are formed in the special places of the capillary connection, which are called the capillary bed, and merge into the veins.

Functioning as a unit, the capillary bed regulates the local blood supply, while the requirements of the tissues in the necessary nutrients are met. A vessel carrying blood to the heart is defined as an artery. The capillary receives blood from the artery through the arteriolus - a smaller vessel than it is.

Arterioles in the circulatory system precede the capillaries. At the branches of the capillary arterioles in the walls of the vessels there are rings of muscle cells, which are clearly expressed and perform the function of sphincters. They regulate the processes of blood supply to the capillary network. Normally, only a small part of these sphincters, called precapillary ones, are open. Therefore, blood can flow at this time not through all available channels.

A characteristic feature of the circulation in the place of the capillary bed is that spontaneously periodically there are cycles of relaxation and contraction of the smooth muscle tissues that surround precapillary and arterioles. This allows you to create an intermittent, intermittent flow of blood through the capillary network.

Functions of the capillary endothelium

The endothelium of the capillary has sufficient permeability for the exchange between the tissues of the body and the blood of various kinds of substances. Therefore, what capillaries do is the transfer of nutrients and metabolic products.

Water and substances dissolved in it, normally pass easily through the walls of the vessel in both directions. But while the proteins and blood cells remain inside the capillaries. Products formed during the life process also pass through the blood barrier to transfer them to the places of excretion from the body. Thus, the capillary is a component of the integral part of all tissues of the body, which forms an extensive network of vessels interconnected, having close contact with cellular structures. Their main function is to supply all systems with substances necessary to ensure normal life activity and removal of waste materials.

Sometimes the size of the molecules can be too large for diffusion through endothelial cells. In this case, either the processes of capture-endocytosis or fusion-exocytosis-are used to transfer them. In inflammatory processes in the body, what capillaries do is part of the mechanism of the immune response. At the surface of the endothelium, receptor molecules arise that hold up immune cells and help them move to foci of infection or other damage in the extravascular space.

Each capillary is a constituent part of a huge network that provides blood supply to all organs. In this case, the larger the organism, the larger the capillary network. And the higher the activity of cells in metabolic processes, the greater the number of small vessels needed to meet the needs for various substances.

Movement of blood through the capillary network

Blood circulates in the circulatory system not only because pressure arises in the arteries due to active rhythmic reduction of the arterial walls, but also due to active narrowing and widening of the capillary ones. Blood capillaries perform a relatively slow flow of blood, the speed of which is not more than 0.5 mm per second. This is proved by numerous observations of this process. At the same time, narrowing and widening of these small vessels can reach up to 70% of the diameter of their lumen. Physiologists associate this ability with the peculiarity of the functioning of adventitial elements that accompany blood vessels and are defined as special capillary cells, capable of contracting.

It is also assumed that the endothelial walls of the capillaries themselves have a certain elasticity and possible contractility, and can change the lumen size. Some physiologists point out that they have seen short-term contractions of endothelial cells in places where there are no adventitial cells. Pathological conditions, such as severe burns or shock, can cause the capillary expansion to be 3 times higher than normal. Here, as a rule, there is a significant decrease in the velocity of blood flow, which allows it to accumulate in the capillary bed at the sites of damage. Compression of the capillaries also leads to a decrease in the rate of circulation in them.

Three kinds of capillaries

Continuous capillaries are those in which the intercellular connections are very dense. This allows diffusion to small ions and molecules.

Another type of capillary is fenestrated. Their walls are provided with lumens for the diffusion of larger molecules or their compounds. Such capillaries are located in the endocrine glands, intestines and other organs, where an intensive exchange of substances between tissues and blood is carried out.

Sinusoidal - such capillaries, the walls of which are different in structure and greater variability of internal lumens. They are present in those organs where the above-described, more typical species are absent.

Vascular problems

Arteries, veins, capillaries - all of them are insufficiently protected from environmental influences and are often damaged. The most delicate blood vessels of the body are especially vulnerable. Capillaries should be very small in order to let only the liquid component of blood into the cells, and not the necessary and more dense to separate. Therefore, these vessels have the thinnest, loose endothelial walls, through which the processes of diffusion of substances take place. It is the fact that they consist of a small number of cellular layers, and makes them fragile.

Capillaries do not have, like the veins and arteries, a protective layer. Therefore, they do not have protection from both external influences and damage by those substances that they carry along with blood. With any damage or disease, these vessels suffer in the first place. If a situation arises where the capillaries have burst and become damaged, they cease to perform their basic function of transferring nutrients. In this case, the cell that did not receive them from the vessel with the destroyed wall slows down its work and dies. And if the supply of blood is disturbed in the whole organ or in the system of organs, mass death of cells begins in them because of the deficiency of substances necessary for their vital activity. So in the body begin to develop diseases, one of the beginnings of which is the damage to the capillaries.

Look in the mirror

Very often, looking at your reflection in the mirror, you can see on the face of a small thread - red capillaries, which were not there before. Many are frightened, accepting their appearance for the symptoms of dangerous diseases. According to statistics, 80% of the total population finds such changes when expanded capillaries become visible through the skin. First of all, this indicates that the normal functioning of the vessels is disturbed. And although the expansion of capillaries does not bring any particular harm to health, it can worsen the appearance. Vascular meshes on the face - couperose - are a manifestation of the disease, quite harmless by its stage, but serve as signals about malfunctions in the body.

Mechanisms of pathology

First, the vessel is enlarged and enlarged so that it begins to shine through the skin and becomes visible. Most often this phenomenon can be observed on the face or on the skin of the hands and feet. Then the connective tissue of the skin is thinned, and the vessels under them are raised, become tuberous and become visible even more. The danger here is that the walls of the capillaries themselves become thinner and weaker, and this can lead to their rupture. And if the capillaries have burst, then it is necessary to take measures not only to eliminate cosmetic defects, but also to identify and treat pathologies that have caused damage to blood vessels.

Causes of pathologies of capillaries

Violations of the capillary circulation can be caused by a variety of factors. First of all, high blood pressure and age-related changes in blood vessels should be included. Their destruction in this case is the cause of aging of the whole organism. Various inflammations of the skin, abuse of sun baths, severe hypothermia lead to a violation of the integrity of the capillary walls.

The intake of certain hormonal drugs that have a relaxing effect on blood vessels causes their expansion and damage. In this case, large areas can be affected and complications may develop. Similar pathologies of capillaries can occur with hormonal failures of the body, for example, during pregnancy, abortion or after childbirth. Diseases of the liver, disorders of blood coagulability or venous outflow cause destruction of capillaries. An important role in this matter is played by hereditary predisposition.

Extended capillaries in a child

It is believed that problems with thin blood vessels can disturb only adults. But it also happens that the expanded capillaries also appear on the child's face. The reasons may be hormonal changes, heredity or weather conditions, negatively affecting the child's tender skin. Usually such problems go away as the child grows up. But in order to determine the risks of more serious pathologies, parents should receive a consultation from a dermatologist, who will decide the need for treatment or establish the timing of this phenomenon.