anatomy cardiovascular system capillaries in this lesson we will take a look at the smallest vessels in the human body capillaries capillary derives from the fact that these vessels are really extremely thin with a thickness much less than that of a hair if we want to be literal the capillaries are however the largest circulatory network among all those present in the human body as their surface is really enormous the definition of capillary is the following the capillaries are thin channels interposed between the terminal arteries and the roots of the venules therefore act as exchange vessels they serve so that the tissue can recharge with oxygen to exceed the carbon dioxide in the blood for this their extremely thin wall the capillaries have a thickness that varies between 5 and 20 microns which means that they are practically narrower than the largest dimension of a red blood cell in some bridges in fact red blood cells proceed in single file inside the capillaries the structure of a capillary is very simple it is a vessel composed of an internal endothelial layer whose cells as we have already said in the past have numerous functions a basal membrane and outside a series of cells derived from macrophages cache however here they carry out contractile activity which are called for the sites the pericytes by contracting and releasing they decide how much blood can pass into the capillary bed in the organism there are various types of capillaries the continuous capillaries are those just described for structure and are found in all those places where there is a need for a certain barrier between blood and tissue such as the central nervous system or the gonads the discontinuous capillaries instead are those capillaries that are found in those places that require greater filtration such as for example the kidneys or the gastrointestinal system or the lungs finally the sinusoid type capillaries are very particular capillaries they are extremely wide as regards their diameter they are discontinuous in the sense that the endothelial cells are missing in some points and on the internal edge there are cells derived from macrophages called Kapfer cells that carry out numerous functions this type of capillary the sinusoid is found only and exclusively in the liver and we will talk about it better in the description of this organ