These are the smallest blood vessels of the circulatory system in which both arteries and veins Of cylindrical shape and microscopic dimensions.
What are capillaries?
Both the venous system (large, medium and small caliber veins and venules) and the arterial system (large, medium and small caliber arteries and arterioles) branch out into ever smaller blood vessels, ending up in the capillaries: these are distributed to start from the arterioles , branching between the tissue cells, and then collect in larger vessels (called venules) which in turn pour their content into the veins, by means of which the blood returns to the heart .
From a functional point of view, the capillaries therefore represent the most important part of the vascular system : while in all the other blood vessels (arteries and arterioles, veins and venules) the blood is only passing through, in the capillaries – instead – all those metabolic exchanges occur and respirators between the blood and the intercellular liquid that guarantee the tissues the proper supply of oxygen and nutrients (through the capillaries deriving from the arteries) and, at the same time, the removal of the waste substances (through the capillaries deriving from the veins) . The function of the capillaries is feasible thanks to their particular structural characteristics: unlike the arteries and veins, in fact, their walls consist of asingle layer of flat endothelial cells , thanks to which they are very permeable.
The diameter of the capillaries varies according to the organ inside which they are located: the thinnest ones, with a diameter of 5-8 micrometers (symbol “µm”, equal to one thousandth of a millimeter) are located inside the lung, of the retina and striated muscles; the larger ones (12-20 µm, but can even reach 25-30 µm) are located at the level of the glands and bone marrow.
In addition to the networks formed by capillaries arranged between arteries and veins, there are also networks of capillaries positioned between two arteries or between two veins: in the first case we speak of “mirable arterial networks” and in the second of “mirable venous networks” .
What are capillaries for?
Their task is to ensure metabolic and respiratory exchanges between the blood and the intercellular liquid , so that all tissues are guaranteed the supply of both oxygen and nutrients and, at the same time, the removal of waste substances.