Microcirculation is the flow of blood in the system of smaller vessels of the body whose diameters are 100 ^m or less. Virtually all cells in the body are spatially close to a microvessel. There are tens of thousands of microvessels per gram of tissue in the human body. These include arte-rioles, metarterioles, capillaries, and venules. The microcirculation is involved chiefly in the exchange of water, gases, hormones, nutrients, and metabolic waste products between blood and cells. A second major function of the microcirculation is regulation of vascular resistance.

Normally, all microvessels other than capillaries are partially constricted by contraction of smooth muscle cells. If that is so, then why are the capillaries not constricted in diameter? It is because capillaries do not have smooth muscle cells in the wall. Precapillary sphincters can close off some capillaries, but capillaries cannot change their diameters.

If all of your smooth muscle cells relaxed instantaneously, you would faint immediately. Such relaxation would allow a large increase in volume of blood in the cardiovascular system, dropping blood pressure precipitously. In skeletal muscle and other tissues, a large number of capillaries can remain closed for long periods due to the contraction of the precapillary sphincter. This mechanism is a part of the regulation of vascular resistance.

These capillaries provide a reserve flow capacity and can open quickly in response to local conditions, such as a fall in the partial pressure of oxygen, when additional flow is required.

2.11.1 Capillary structure

Capillaries are thin tubular structures whose walls are one cell layer thick. They consist of highly permeable endothelial cells. Figure 2.29 shows the various layers of a capillary. 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

Figure 2.29 Structure of a human capillary. (From Tanner and Rhodes, 2003, p. 264.)

Endothelial vesicle 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 branches, 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 ^m 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. The first type is the relatively larger preferential channel; the second type is the relatively smaller true capillary. At the location where each true capillary begins from a metarteriole, there is smooth muscle fiber, known as the precapillary sphincter, surrounding the capillary. 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.

Arteriolar wall structure. Arterioles are tubes of endothelial cells surrounded by a basement membrane, a single or double layer of smooth muscle cells, and a thin outer layer of connective tissue. In most organs, arteriolar smooth muscle cells operate at about half their maximum length.

2.11.2 Capillary wall structure

A capillary is a tube of endothelial cells surrounded by a basement membrane. The wall of a capillary is one cell layer thick and consists of endothelial cells surrounded on the outside by a basement membrane. Pericytes (Rouget cells) are connective tissue and may be a primitive form of vascular smooth muscle cells, and add structural integrity to the capillary. Precapillary sphincters are small helical twists of smooth muscle around the smallest arterioles (metarterioles). These sphincters are very responsive to the local or intrinsic effects like the concentration of oxygen or carbon dioxide.

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 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 of 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 also goes through these pores.

Because a single capillary has such a small diameter, each capillary taken alone has a very high hydraulic resistance. However, when you add together many capillaries in parallel, the capillaries account for only about 15 percent of the total resistance in each organ.

2.11.3 Pressure control in the microvasculature

Arterioles regulate vascular pressures and microvascular resistance. A constant "conflict" exists between preserving arterial pressure by increasing arteriolar resistance and allowing all regions to receive sufficient perfusion to provide oxygen to the tissue. By decreasing resistance in small vessels, more blood flows and oxygen perfusion increases. By decreasing resistance in arterioles and capillaries, however, system blood pressure falls.

The normal pressure in an arteriole is typically 30 to 70 mmHg, while the pressure in a venule is 10 to 16 mmHg. Large arteries are simply conduits to allow blood transfer between locations. These large vessels have very low resistance and do not play a significant role in pressure regulation. Small arteries (0.5 to 1 mm in diameter) control 30 to 40 percent of total vascular resistance, and arterioles (<500 ^m in diameter) combined with those small arteries make up 70 to 80 percent of total vascular resistance. Nearly all of the rest of the resistance (20 to 30 percent) comes from capillaries and venules.

Constriction of smooth muscle cells maintains the resistance in these resistance vessels. Release of norepinephrine from the sympathetic nervous system contributes to smooth muscle constriction. When fully relaxed, the diameter of the vessel surrounded by smooth muscle can nearly double. If the vessel diameter doubles, the resistance over a fixed length of vessel increases by 24 = 16 times.

Arterioles possess significant smooth muscle cells and can dilate 60 to 100 percent from their resting diameter. Arterioles can also constrict 40 to 50 percent from their resting diameter for long periods.

Only approximately 10 percent of muscle capillaries are open at rest. Therefore, muscle is capable of increasing its rate of bulk flow tenfold as the need for oxygen increases. Since humans are not capable of increasing their cardiac output tenfold (the human heart can achieve a maximal fivefold increase), shutting down other capillary beds is a necessary pressure control mechanism. That is, you can increase the blood flow to some muscle tenfold so long as not all muscles in the body try to do that simultaneously.

2.11.4 Diffusion in capillaries

Lipid-soluble molecules like O2 and CO2 easily pass through the lipid components of endothelial cell membranes. Water-soluble molecules like glucose must diffuse through water-filled pathways in the capillary wall between adjacent endothelial cells. The water-filled pathways are known as pores. In humans, adjacent endothelial cells are held together by tight junctions, which have occasional gaps.

Capillaries of the brain and spinal cord allow only the smallest water-soluble molecules to pass through. Pores are partially filled with submicron fibers (fiber matrix) that act as a filter.

Most pores allow passage of 3- to 6-nm molecules. This pore size allows the passage of water, inorganic ions, glucose, and amino acids. Pores exclude passage of large molecules like serum albumin and globular proteins. A limited number of large pores exist. It is possible that these larger pores are actually defects in the cell wall. Large pores allow passage of virtually all large molecules. Because of these large pores and their associated leakage, nearly all serum albumin molecules leak out of the cardiovascular system each day.

2.11.5 Venules

Venules are endothelial tubes that are usually surrounded by a mono-layer of vascular smooth muscle cells. Smooth muscle cells in venules are much smaller in diameter, but longer than those of arterioles.

The smallest venules are more permeable than capillaries. A significant amount of the diffusion that occurs in the microcirculation, therefore, occurs in the venules.

Tight junctions are more frequent and have bigger pores. Therefore, it is probable that much of the exchange of large water-soluble molecules occurs as blood passes through small venules.

The venous system acts like a huge blood reservoir, and at rest, approximately two-thirds of blood is located in the venous system. By changing venule diameter, volume of blood in tissue can change up to 20 mL/kg. For a 70-kg (154 lb) person, the volume of blood ready for circulation can change by nearly 1.5 L.

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Essentials of Human Physiology

Essentials of Human Physiology

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