Any living organism cannot exist and develop without oxygen and nutrients. Oxygen, entering the lungs from the external environment, is carried throughout the body by the circulatory system, which has a rather complex structure. Blood circulation is provided by the hollow tubes - arteries, arterioles, precapillaries, capillaries, postcapillaries, veins, venules and arteriovenous anastomoses. Carbon dioxide and other waste metabolic products are also removed from the body using these vessels. The more they are removed from the heart, the stronger their branching into smaller ones.
Capillaries: definition of a concept
If the artery and vein, respectively carrying blood from the heart and to it, are large vessels, then the capillary is a very thin blood tube, with a diameter of only 5-10 microns. And since veins and arteries, being only the way of delivering nutrients to the cells, do not participate in the processes of gas exchange between them and the blood, this function is assigned to the capillaries. Their first descriptions belong to the Italian scientist M. Malpigi, who in 1661 gave them the definition of the link between arterial and venous vessels. Before him, W. Harvey predicted their existence.
The structure and size of capillaries
These small vessels have approximately equal diameters in different organs. The larger of them reach a clearance of up to 30 microns, and the narrowest - from 5 microns. It is easy to verify that the wide blood capillaries in the sections across the lumen of the tube are lined with several layers of endothelial cells, while the lumen of the smallest is formed by a layer of only one or two cells. Such thin vessels are located in muscles with a striated structure, and since their diameter is smaller than that of red blood cells, the latter experience significant deformation when passing through a narrow bloodstream.
A capillary is such a thin tube that its wall, consisting of individual endothelial cells that are in close contact with each other, does not have a muscle layer and therefore is not able to contract. The capillary network usually contains only 25% of the volume of blood that can fit in it. But changes in these volumes can be achieved with the inclusion of the mechanism of self-regulation, when smooth muscle cells are relaxed.
Capillary bed, venules, arterioles
Blood flow is directed to the heart through large vessels, which are veins. Capillaries transmit blood to veins through venules - the smallest collective components. They are formed in special places where capillaries join, called the capillary bed, and merge into veins.
Functioning as a whole, the capillary bed regulates local blood supply, while meeting the needs of tissues for the necessary nutrients. A vessel that carries blood to the heart is defined as an artery. A capillary receives blood from an artery through an arteriole - a vessel smaller than it is.
Arterioles in the circulatory system precede capillaries. At the points of branching from the capillary arterioles, in the walls of the vessels are rings of muscle cells, which are clearly expressed and perform the function of sphincters. They regulate the processes of blood flow into the capillary network. Normally, only a small part of these sphincters, called precapillary, is discovered. Therefore, blood may not flow at this time through all available channels.
A characteristic feature of blood circulation at the site of the capillary bed is that here spontaneously periodically there are cycles of relaxation and contraction of smooth muscle tissue that surround the precapillaries and arterioles. This allows you to create an intermittent, intermittent flow of blood through the network of capillaries.
Functions of Capillary Endothelium
The capillary endothelium has sufficient permeability to exchange various types of substances between the tissues of the body and the blood. Therefore, what the capillaries do is the transfer of nutrients and metabolic products.
Water and substances dissolved in it normally easily pass through the walls of the vessel in both directions. But at the same time, proteins and blood cells remain inside the capillaries. Products formed during the course of life also pass through the blood barrier to transfer them to places of excretion from the body. Thus, the capillary is a component of the integral part of all body tissues, forming an extensive network of vessels, interconnected, having close contact with cellular structures. Their main function is to supply all systems with the substances necessary to ensure normal functioning, and the disposal of waste substances.
Sometimes the size of the molecules may be too large for diffusion through endothelial cells. In this case, either capture - endocytosis or fusion - exocytosis processes are used to transfer them. In inflammatory processes in the body, what the capillaries do is part of the immune response mechanism. At the same time, receptor molecules appear on the surface of the endothelium that trap immune cells and help them move to foci of infection or other damage in the extravascular space.
Each capillary is an integral part of a huge network that provides blood supply to all organs. Moreover, the larger the body, the wider the capillary network. And the higher the activity of cells in metabolic processes, the greater the number of small vessels required in order to meet the needs for various substances.
The movement of blood through the capillary network
Blood circulates in the circulatory system, not only because pressure is created in the arteries due to the active rhythmic contraction of the arterial walls, but also due to the active narrowing and expansion of the capillary. Blood capillaries carry out 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, the narrowing and expansion of these small vessels can reach up to 70% of the diameter of their lumen. Physiologists attribute this ability to the functioning of adventitious elements that accompany blood vessels and are defined as special capillary cells that can contract.
It is also assumed that the endothelial walls of the capillaries themselves have a certain elasticity and possible contractility, and can change the size of the lumen. Some physiologists point out that they have seen short-term contractions of endothelial cells in places where there are no adventitious cells. Pathological conditions, such as a severe burn or shock, can cause the expansion of capillaries 3 times higher than normal. Here, as a rule, a significant decrease in the speed of blood flow occurs, which allows it to accumulate in the capillary bed at the sites of damage. Compression of capillaries also leads to a decrease in blood circulation in them.
Three types of capillaries
Continuous capillaries are those in which the intercellular connections are very dense. This allows diffusion of small ions and molecules.
Another type of capillary is fenestrated. Their walls are provided with gaps for 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 differ in structure and greater variability of the internal gaps. They are found in those organs where the above, more typical species are absent.
Vascular problems
Arteries, veins, capillaries - all of them are not sufficiently protected from environmental influences and are often damaged. Particularly vulnerable are the thinnest blood vessels in the body. The capillaries must be very small in order to allow only the liquid component of the blood to pass into the cells, and not to separate the necessary and denser one. Therefore, these vessels have the thinnest, loose endothelial walls through which the processes of diffusion of substances occur. The fact that they consist of a small number of cell layers makes them fragile.
Capillaries do not have, like veins and arteries, a protective layer. Therefore, they do not have protection both from external influences and from damage by those substances that they carry with blood. With any damage or disease, these vessels suffer first. If a situation arises when the capillaries burst and are damaged, they cease to fulfill their primary function of transferring nutrients. At the same time, a cell that did not receive them from a vessel with a destroyed wall slows down its work and dies. And if the blood supply is disrupted in the entire organ or in the organ system, mass cell death begins in them due to a deficiency of substances necessary for their vital functions. Thus, diseases begin to develop in the body, one of the principles of which is damage to capillaries.
Look in the mirror
Very often, looking at your reflection in the mirror, you can see small strings on the face - red capillaries, which were not there before. Many are scared, taking their appearance for the symptoms of dangerous diseases. According to statistics, 80% of the entire population find such changes when dilated capillaries become visible through the skin. First of all, this indicates that the normal functioning of blood vessels is impaired. Although the expansion of capillaries in itself does not bring much harm to health, it can worsen the appearance. Vascular nets on the face - rosacea - are a manifestation of the disease, its rather harmless stage, but serve as signals about malfunctions in the body.
Pathology mechanisms
First, the vessel dilates and enlarges so much 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 hands and feet. Then, the connective tissue of the skin is thinned, and the vessels below them rise, acquire tuberosity and become even more visible. 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 burst, then it is necessary to take measures not only to eliminate cosmetic defects, but also to identify and treat pathologies that caused vascular damage.
Causes of capillary pathologies
Disturbances of capillary circulation can be caused by a variety of factors. First of all, this should include high blood pressure and age-related changes in blood vessels. Their destruction in this case is the cause of aging of the whole organism. Various inflammation of the skin, the abuse of sunbathing, severe hypothermia lead to a violation of the integrity of the capillary walls.
Taking certain hormonal drugs that have a relaxing effect on the blood vessels causes their expansion and damage. In this case, large areas can be affected and complications develop. Similar capillary pathologies can occur with hormonal malfunctions of the body, for example, during pregnancy, abortion or after childbirth. Liver diseases, clotting disorders, or venous outflows cause capillary destruction. An important role in this matter is played by hereditary predisposition.
Dilated capillaries in a child
It is believed that problems with thin blood vessels can only bother adults. But it also happens that dilated capillaries appear on the children's face. The reasons may be hormonal changes, heredity or weather conditions that adversely affect children's delicate skin. Typically, such problems go away on their own as the child grows older. But in order to determine the risks of more serious pathologies, parents should get the advice of a dermatologist, who will decide the need for treatment or establish the temporality of this phenomenon.