Microcirculation is the flow of blood in the system of smaller vessels of the body whose diameters are 100 ^m or less. This includes arterioles, metarterioles, capillaries, and venules. The microcirculation is involved chiefly in the exchange of gases, fluids, nutrients, and metabolic waste products.
Capillaries are thin tubular structures whose walls are one cell layer thick. They consist of highly permeable endothelial cells. In the peripheral circulation there are about 10 billion capillaries with an average length of about 1 mm. The estimated surface area of all the capillaries in the entire system is about 500 m2 and the total volume of blood contained in those capillaries is about 500 mL. Capillaries are so plentiful that it is rare that any cell in the body is more than 20 ^m away from a capillary.
Consider the structure of blood vessels in terms of branching "generations." If the aorta branches or bifurcates into two, those branches may be considered the second generation. Arteries branch six or eight times before they become small enough to be arterioles. Arterioles then branch another two to five times, for a total of eight to thirteen generations of branches within arteries and arterioles. Finally, the arterioles branch into metarterioles and capillaries that are 5 to 9 micrometers (^m; also called microns) in diameter.
In the last arteriole generation, blood flows from the arterioles into the metarterioles, which are sometimes called terminal arterioles. From there the blood flows into the capillaries.
There are two types of capillaries. One type is the relatively larger preferential channel and the second type is the relatively smaller true capillary. At the location where each true capillary begins from a metar-teriole, there is smooth muscle fiber surrounding the capillary known as the precapillary sphincter. The precapillary sphincter can open and close the entrance to a true capillary, completely shutting off blood flow through this capillary.
After leaving the capillaries most blood returns to venules and then eventually back into the veins. However, about 10 percent of the fluid leaving the capillaries enters the lymphatic capillaries and returns to the blood through the lymphatic system.
The wall of a capillary is one cell layer thick and consists of endothelial cells surrounded on the outside by a basement membrane. The wall thickness of a capillary is about 0.5 ^m. The inside diameter is approximately 4 to 9 ^m, which is barely large enough for an erythrocyte, or red blood cell, to squeeze through. In fact, erythrocytes, which are approximately 8 ^m in diameter, must often fold in order to squeeze through the capillary.
Capillary walls have intercellular clefts, which are thin slits between adjacent endothelial cells. The width of these slits is only about 6 to 7 nanometers (nm) (6 to 7 X 10 9 m) on the average. Water molecules and most water soluble ions diffuse easily through these pores.
The pores in capillaries are different sizes for different organs. For example, intercellular clefts in the brain are known as "tight junctions." These pores allow only extremely small molecules like water, oxygen, and carbon dioxide to pass through the capillary wall. In the liver, the opposite is true. The intercellular clefts in the liver are wide open and almost anything that is dissolved in the plasma goes through these pores.
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