The microcirculation of each organ is organized to serve that organ’s specific needs. In general, each nutrient artery entering an organ branches six to eight times before the arteries become small enough to be called arterioles, which generally have internal diameters of only 10 to 15 micrometers. Then the arterioles themselves branch two to five times, reaching diameters of 5 to 9 micrometers at their ends where they supply blood to the capillaries.

The arterioles are highly muscular, and their diameters can change manyfold. The metarterioles (the terminal arterioles) do not have a continuous muscular coat, but smooth muscle fibers encircle the vessel at intermittent points, as shown in Figure1.

Fig1. Components of the microcirculation. 

At the point where each true capillary originates from a metarteriole, a smooth muscle fiber usually encircles the capillary. This structure is called the precapillary sphincter. This sphincter can open and close the entrance to the capillary.

The venules are larger than the arterioles and have a much weaker muscular coat. Yet the pressure in the venules is much less than that in the arterioles, so the venules can still contract considerably despite the weak muscle.

This typical arrangement of the capillary bed is not found in all parts of the body, although a similar arrangement may serve the same purposes. Most important, the metarterioles and the precapillary sphincters are in close contact with the tissues they serve. Therefore, the local conditions of the tissues—the concentrations of nutrients, end products of metabolism, hydrogen ions, and so forth—can cause direct effects on the vessels to control local blood flow in each small tissue area.

Structure of the Capillary Wall. Figure 2 shows the ultramicroscopic structure of typical endothelial cells in the capillary wall as found in most organs of the body, especially in muscles and connective tissue. Note that the wall is composed of a unicellular layer of endothelial cells and is surrounded by a thin basement membrane on the outside of the capillary. The total thickness of the capillary wall is only about 0.5 micrometer. The internal diameter of the capillary is 4 to 9 micrometers, barely large enough for red blood cells and other blood cells to squeeze through.

Fig2. Structure of the capillary wall. Note especially the inter cellular cleft at the junction between adjacent endothelial cells; it is  believed that most water-soluble substances diffuse through the  capillary membrane along the clefts. Small membrane invaginations,  called caveolae, are believed to play a role in transporting macromolecules across the cell membrane. Caveolae contain caveolins, which  are proteins that interact with cholesterol and polymerize to form  the caveolae. 

“Pores” in the Capillary Membrane. Figure 2 shows two small passageways connecting the interior of the capillary with the exterior. One of these passageways is an intercellular cleft, which is the thin-slit, curving channel that lies at the top of the figure between adjacent endothelial cells. Each cleft is interrupted periodically by short ridges of protein attachments that hold the endothelial cells together, but between these ridges, fluid can percolate freely through the cleft. The cleft normally has a uniform spacing with a width of about 6 to 7 nanometers (60 to 70 angstroms), which is slightly smaller than the diameter of an albumin protein molecule.

Because the intercellular clefts are located only at the edges of the endothelial cells, they usually represent no more than 1/1000 of the total surface area of the capillary wall. Nevertheless, the rate of thermal motion of water molecules, as well as most water-soluble ions and small solutes, is so rapid that all of these substances diffuse with ease between the interior and exterior of the capillaries through these “slit-pores,” the intercellular clefts.

Present in the endothelial cells are many minute plasmalemmal vesicles, also called caveolae (small caves). These plasmalemmal vesicles form from oligomers of proteins called caveolins that are associated with molecules of cholesterol and sphingolipids. Although the precise functions of caveolae are still unclear, they are believed to play a role in endocytosis (the process by which the cell engulfs material from outside the cell) and trans cytosis of macromolecules across the interior of the endothelial cells. The caveolae at the surface of the cell appear to imbibe small packets of plasma or extracellular fluid that contain plasma proteins. These vesicles can then move slowly through the endothelial cell. Some of these vesicles may coalesce to form vesicular channels all the way through the endothelial cell, which is demonstrated in Figure 2.

Special Types of “Pores” Occur in the Capillaries of Certain Organs. The “pores” in the capillaries of some organs have special characteristics to meet the peculiar needs of the organs. Some of these characteristics are as follows:

 1. In the brain, the junctions between the capillary endothelial cells are mainly “tight” junctions that allow only extremely small molecules such as water, oxygen, and carbon dioxide to pass into or out of the brain tissues.

2. In the liver, the opposite is true. The clefts between the capillary endothelial cells are wide open so that almost all dissolved substances of the plasma, including the plasma proteins, can pass from the blood into the liver tissues.

3. The pores of the gastrointestinal capillary mem branes are midway in size between those of the muscles and those of the liver.

4. In the glomerular capillaries of the kidney, numerous small oval windows called fenestrae penetrate all the way through the middle of the endothelial cells so that tremendous amounts of small molecular and ionic substances (but not the large molecules of the plasma proteins) can filter through the glomeruli without having to pass through the clefts between the endothelial cells.

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