Blood capillaries are responsible for metabolic exchanges between blood and interstitial fluid (a fluid that surrounds the cells). These small vessels have extremely thin walls that allow the continuous passage, in both directions, of gases, nutrients and metabolites. In order for these exchanges to take place it is important that the blood stream travels them at low speed and that its pressure, which is not excessive, remains within rather narrow ranges.
The fundamental characteristics of the capillaries are therefore the reduced diameter (from 5-10 µm, sufficient for the passage of red blood cells one at a time in single file, up to 30 µm), the thinness of the walls, the low hydrostatic pressure (35-40 mm Hg at the arterial end - 15-20 at the venous end) and the reduced speed of the blood flow passing through them (1 mm / second).
The capillary walls, unlike the venous and arterial ones, do not consist of three concentric tunas, but of a single layer of flattened endothelial cells resting on a basement membrane; the capillary wall is therefore devoid of muscle, elastic and fibrous fibers. This morphological peculiarity is intended to facilitate the exchange of substances with the interstitial liquid. On the other hand, many capillaries are associated with cells, called pericytes, which regulate the permeability of the endothelium, opposing these passages; the greater the number of pericytes and the smaller the capillary permeability. Not surprisingly, therefore, pericytes are particularly abundant in the central nervous system, where they contribute to the formation of the blood-brain barrier.
Three types of capillaries can be identified in the human circulatory system:
Continuous capillaries : they are so called because their cells form a wall without important spaces and interruptions. Although the endothelial cells are joined by tight junctions, there are still small spaces that give the capillary a certain permeability to water and solutes, but poor to proteins. Continuous capillaries are found mainly in the central and peripheral nervous system, in muscle tissue, in the lungs and in the skin; they are the most common. |
Fenestrated or discontinuous capillaries : they have pores in their walls of 80-100 nm, which in reality are not completely lost but subtended by a thin diaphragm (a plasma plate probably used to control the interchange between capillary and interstitium). They are abundant in the endocrine glands, in the pancreas, in the renal glomerulus (where the pores have no diaphragm) and in the intestine, where the windows increase the exchange capacity of endothelial cells. |
Sinusoidal capillaries : they are the most permeable of the three, because their very large endothelial wall has few junctions and large intercellular spaces. The endothelium and the basement membrane are discontinuous and this facilitates exchanges between blood and tissue. They are found in the liver, spleen, bone marrow, lymphoid organs and in some endocrine glands, where high permeability to proteins and large molecules is required. |
Approximately 2 billion capillaries are found in the human body, which together cover a length of approximately 80, 000 km and an exchange area of approximately 6300 m2 (the equivalent of two football fields).
The capillaries are divided into an arterial portion, which carries nutrient-rich blood and oxygen, and a venous portion, which collects the wastewater from the previous one (charged with carbon dioxide and waste substances in the meantime).
At the tissue level the capillaries tend to form intertwined nets called "capillary beds", while the flow that passes through them is called microcirculation. At this level the terminal arteriole continues with a metarteriole, a sort of channel for direct passage to the post capillary venule. In turn, from each metarteriola branch the so-called true capillaries, which intertwine to form the aforementioned capillary bed (for each bed, in relation to the sprayed organ, there are about ten to one hundred real capillaries).
At the point of origin of the true capillaries there is a ring of smooth muscle fibers, the "precapillary sphincter", which surrounds it. This sphincter acts like a valve, regulating the flow of blood in the microcirculatory bed; consequently, when the precapillary sphincters are contracted the flow is realized exclusively through the main vessel metarteriola duct; vice versa, when the sphincters are relaxed the blood flows into the capillaries and the tissue is abundantly perfused. Obviously, these are boundary conditions, since in most cases there will be an open capillary quota and a closed part. Therefore, the true capillary can be closed or open, while the metarteriole, being a preferential vessel, is always open (since it lacks sufficient musculature to act as a sphincter). As such, the metarteriole can bypass capillaries and direct blood directly into the venous circulation; this channel also allows the passage of white blood cells from the arterial to the venous circle (otherwise prevented by the reduced capillary caliber).
The amount of blood that enters a capillary bed is subject to an intrinsic control, linked to the stretching of the vessel, and to local stimuli (biochemical signals, such as the partial pressure of oxygen, carbon dioxide and the presence of vasodilator-vasoconstrictor signals ). Depending on the conditions, the bed is bypassed or completely perfused.
The capillary bed often takes on different shapes and characteristics from one organ to another, with differences in the number of channels, in the thickness of the meshes and in the permeability of the wall; particularly developed are the capillary networks of nerve centers, glands and pulmonary alveoli. The capillary density of a given tissue is in fact directly proportional to the metabolic activity of its cells, which leads to a greater demand for blood.
