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Guyton Physiology : Guyton and Hall Textbook of Medical Physiology CHAPTER 16 UNIT The Microcirculation and Lymphatic System: Capillary Fluid Exchange, Interstitial I V Fluid, and Lymph Flow The most purposeful func- the capillary. This is called the precapillary sphincter. This tion of the circulation occurs sphincter can open and close the entrance to the capillary. in the microcirculation: This The venules are larger than the arterioles and have a is transport of nutrients to much weaker muscular coat. Yet the pressure in the venules the tissues and removal of is much less than that in the arterioles, so the venules can cell excreta. The small arte- still contract considerably despite the weak muscle. rioles control blood flow to This typical arrangement of the capillary bed is not each tissue, and local conditions in the tissues in turn con- found in all parts of the body, although a similar arrange- trol the diameters of the arterioles. Thus, each tissue, in ment may serve the same purposes. Most important, the most instances, controls its own blood flow in relation to its metarterioles and the precapillary sphincters are in close individual needs, a subject that is discussed in Chapter 17. contact with the tissues they serve. Therefore, the local The walls of the capillaries are extremely thin, con- conditions of the tissues—the concentrations of nutri- structed of single-layer, highly permeable endothelial ents, end products of metabolism, hydrogen ions, and so cells. Therefore, water, cell nutrients, and cell excreta can forth—can cause direct effects on the vessels to control all interchange quickly and easily between the tissues and local blood flow in each small tissue area. the circulating blood. The peripheral circulation of the whole body has Structure of the Capillary Wall. Figure 16-2 shows about 10 billion capillaries with a total surface area esti- the ultramicroscopic structure of typical endothelial cells mated to be 500 to 700 square meters (about one-eighth in the capillary wall as found in most organs of the body, the surface area of a football field). Indeed, it is rare that especially in muscles and connective tissue. Note that the any single functional cell of the body is more than 20 to wall is composed of a unicellular layer of endothelial cells 30 micrometers away from a capillary. 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 Structure of the Microcirculation the capillary is 4 to 9 micrometers, barely large enough for and Capillary System red blood cells and other blood cells to squeeze through. The microcirculation of each organ is organized specifi- “Pores” in the Capillary Membrane. Figure 16-2 cally to serve that organ’s needs. In general, each nutrient shows two small passageways connecting the interior of artery entering an organ branches six to eight times before the capillary with the exterior. One of these is an inter- the arteries become small enough to be called arterioles, cellular cleft, which is the thin-slit, curving channel that which generally have internal diameters of only 10 to 15 lies at the bottom of the figure between adjacent endothe- micrometers. Then the arterioles themselves branch two lial cells. Each cleft is interrupted periodically by short to five times, reaching diameters of 5 to 9 micrometers at ridges of protein attachments that hold the endothelial their ends where they supply blood to the capillaries. cells together, but between these ridges fluid can perco- The arterioles are highly muscular, and their diameters late freely through the cleft. The cleft normally has a uni- can change manyfold. The metarterioles (the terminal form spacing with a width of about 6 to 7 nanometers (60 arterioles) do not have a continuous muscular coat, but to 70 angstroms), slightly smaller than the diameter of an smooth muscle fibers encircle the vessel at intermittent albumin protein molecule. points, as shown in Figure 16-1 by the black dots on the Because the intercellular clefts are located only at the sides of the metarteriole. edges of the endothelial cells, they usually represent no At the point where each true capillary originates from more than 1/1000 of the total surface area of the capillary a metarteriole, a smooth muscle fiber usually encircles wall. Nevertheless, the rate of thermal motion of water 177 Unit IV The Circulation cell appear to imbibe small packets of plasma or extra- cellular 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 demon- strated in Figure 16-2. 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 Figure 16-1 Structure of the mesenteric capillary bed. (Redrawn from Zweifach BW: Factors Regulating Blood Pressure. New York: peculiar needs of the organs. Some of these characteris- Josiah Macy, Jr., Foundation, 1950.) tics are as follows: 1. In the brain, the junctions between the capillary endothelial cells are mainly “tight” junctions that allow molecules, as well as most water-soluble ions and small sol- only extremely small molecules such as water, oxygen, utes, is so rapid that all of these diffuse with ease between and carbon dioxide to pass into or out of the brain the interior and exterior of the capillaries through these tissues. “slit-pores,” the intercellular clefts. 2. In the liver, the opposite is true. The clefts between the Present in the endothelial cells are many minute plas- capillary endothelial cells are wide open so that almost malemmal vesicles, also called caveolae (small caves). all dissolved substances of the plasma, including the These form from oligomers of proteins called caveolins plasma proteins, can pass from the blood into the liver that are associated with molecules of cholesterol and tissues. sphingolipids. Although the precise functions of cave- 3. olae are still unclear, they are believed to play a role in The pores of the gastrointestinal capillary membranes endocytosis (the process by which the cell engulfs material are midway between those of the muscles and those of from outside the cell) and transcytosis of macromolecules the liver. across endothelial cells. The caveolae at the surface of the 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 Intercellular cleft tremendous amounts of very small molecular and ionic Basement membrane substances (but not the large molecules of the plasma proteins) can filter through the glomeruli without hav- ing to pass through the clefts between the endothelial Caveolae cells. (Plasmalemmal vesicles) Vesicular Endothelial channel?? cell Flow of Blood in the Capillaries—Vasomotion Caveolin Blood usually does not flow continuously through the Phospholipid capillaries. Instead, it flows intermittently, turning on and off every few seconds or minutes. The cause of this inter- Sphingolipid mittency is the phenomenon called vasomotion, which Cholesterol means intermittent contraction of the metarterioles and precapillary sphincters (and sometimes even the very small arterioles as well). Regulation of Vasomotion. The most important factor found thus far to affect the degree of opening and closing of the metarterioles and precapillary sphincters is Figure 16-2 Structure of the capillary wall. Note especially the the concentration of oxygen in the tissues. When the rate intercellular cleft at the junction between adjacent endothelial of oxygen usage by the tissue is great so that tissue oxy- cells; it is believed that most water-soluble substances diffuse gen concentration decreases below normal, the intermit- through the capillary membrane along the clefts. Small mem- brane invaginations, called caveolae, are believed to play a role in tent periods of capillary blood flow occur more often, and transporting macromolecules across the cell membrane. Caveolae the duration of each period of flow lasts longer, thereby contain caveolins, proteins which interact with cholesterol and allowing the capillary blood to carry increased quanti- polymerize to form the caveolae. ties of oxygen (as well as other nutrients) to the tissues. 178 Chapter 16 The Microcirculation and Lymphatic System: Capillary Fluid Exchange, Interstitial Fluid, and Lymph Flow This effect, along with multiple other factors that control Lipid-Soluble Substances Can Diffuse Directly local tissue blood flow, is discussed in Chapter 17. Through the Cell Membranes of the Capillary Endothelium. If a substance is lipid soluble, it can dif- Average Function of the Capillary System fuse directly through the cell membranes of the capillary UNIT Despite the fact that blood flow through each capillary is without having to go through the pores. Such substances intermittent, so many capillaries are present in the tissues include oxygen and carbon dioxide. Because these sub- that their overall function becomes averaged. That is, there stances can permeate all areas of the capillary membrane, is an average rate of blood flow through each tissue capil- their rates of transport through the capillary membrane are lary bed, an average capillary pressure within the capillar- many times faster than the rates for lipid-insoluble sub- I ies, and an average rate of transfer of substances between stances, such as sodium ions and glucose that can go only V the blood of the capillaries and the surrounding interstitial through the pores. fluid. In the remainder of this chapter, we are concerned Water-Soluble, Non-Lipid-Soluble Substances with these averages, although one must remember that the Diffuse Through Intercellular “Pores” in the Capillary average functions are, in reality, the functions of literally Membrane. billions of individual capillaries, each operating intermit- Many substances needed by the tissues are tently in response to local conditions in the tissues.
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