Microcirculation structure62

Capillary vessels are very fine structures. Their walls are made of one layer of cells and a basal membrane. This is where nutrient and waste exchanges between tissues and blood take place. In humans there are about 10 billion capillaries with an exchange surface area of between 500 and 700 m2. Thus anywhere in the body it is rare for a cell to be more than 20 or 30 micrometers far from a capillary.

Generally nutrient arteries divide 6 to 8 times on entering the organs until the diameter becomes small enough for branches to be called arterioles (under 20^m in diameter). The arterioles then divide 2 to 5 times, decreasing to diameters of between 5 and 9 ^m, where they end by transferring blood to the capillaries.

The basic structure of microcirculation is the microcirculatory unit where blood enters the capillary network from an arteriole and leaves by a venule. Blood from the arteriole flows through a series of metarterioles (also called terminal arterioles) whose structure is halfway between that of arterioles and that of capillaries. After leaving the metarterioles, blood enters the capillary network which is made of some wide capillary vessels (called preferential channels) and other smaller ones that are called real capillaries. When blood leaves the capillary network it flows into the venules and returns to the general blood flow.

In their walls, arterioles have a thick muscle layer and thus they can undergo extensive changes in diameter. Metarterioles (terminal arterioles) have no continuous muscle layer but smooth muscle fibres surrounding the vessel at certain points. At the point where capillaries begin from the metarterioles, there is a somewhat thicker mass of smooth muscle fibres surrounding the beginning of the capillary. These muscle fibres are called pre-capillary sphincters. Recent work has shown that they are not organized in a complete ring but rather in a larger mass of cells. Either way, these smooth muscle fibres act like sphincters in that they can open or close the capillaries and their rhythmic activity generates vasomotion. Venules are considerably larger in diameter than arterioles, and their muscle fibre layer is much thinner. Nevertheless, because the pressure in the venous area is much less than in the artery area, venules are also able to contract considerably.

Microcirculation can vary in architecture in the different organs but the general structure is the same. Most of all, metarterioles and pre-capillary sphincters are in very close contact with the tissues they supply. Thus local conditions in the tissues such as nutrient, metabolic end-product and hydrogen ion concentrations can exert direct effects on these structures to control local blood flow in each of microcirculation units.

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