Capillaries

Capillaries (fig. 20.4) are the "business end" of the circulatory system. All the rest of the system exists to serve them, because capillaries are almost the only point in the circulatory system where materials are exchanged between the blood and tissue fluid. Capillaries are ideally suited to their role. They consist only of endothelium and a basement membrane. Capillaries have walls as thin as 0.2 to 0.4 ^m. They average about 5 ^m in diameter at the prox-

2vasa = vessels + vasorum = of the vessels

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Chapter 20 The Circulatory System: Blood Vessels and Circulation 751

Sem Precapillary Sphincter

Capillaries

Figure 20.4 Vascular Cast of Blood Vessels in Human Skeletal Muscle. This was prepared by injecting the vessels with a polymer, digesting away all tissue to leave a replica of the vessels, and photographing the cast through the SEM. From R. G. Kessel and R. H. Kardon, Tissues and Organs: A Text-Atlas of Scanning Electron Microscopy (W. H. Freeman & Co., 1979).

Sphincters open

Capillaries

Figure 20.4 Vascular Cast of Blood Vessels in Human Skeletal Muscle. This was prepared by injecting the vessels with a polymer, digesting away all tissue to leave a replica of the vessels, and photographing the cast through the SEM. From R. G. Kessel and R. H. Kardon, Tissues and Organs: A Text-Atlas of Scanning Electron Microscopy (W. H. Freeman & Co., 1979).

imal (arterial) end, widen to about 9 ^m in diameter at the distal (venous) end, and often branch along the way. (Recall that an erythrocyte is about 7 ^m in diameter.) The number of capillaries has been estimated at a billion and their total surface area at 6,300 m2. But a more important point is that scarcely any cell in the body is more than 60 to 80 ^m away from the nearest capillary. There are a few exceptions. Capillaries are scarce in tendons and ligaments and absent from cartilage, epithelia, and the cornea and lens of the eye.

Capillary Beds

Capillaries are organized in groups called capillary beds— usually 10 to 100 capillaries supplied by a single metarte-riole (fig. 20.5). The metarteriole continues through the bed as a thoroughfare channel leading directly to a venule. Capillaries arise from the proximal end of the metarteriole and lead into its distal end or directly into the venule.

There is a precapillary sphincter at the entrance to each capillary. When the sphincters are open, the capillaries are well perfused with blood and they engage in exchanges

Precapillary sphincters

Metarteriole

Sphincters open

Precapillary sphincters

Metarteriole

Thoroughfare Channel

Thoroughfare channel

Venule

Sphincters closed

Thoroughfare channel

Venule

Sphincters closed

Regulation Capillary Bed Sphincters

Venule

Figure 20.5 Control of Perfusion of a Capillary Bed.

(a) Precapillary sphincters dilated and capillaries well perfused.

(b) Precapillary sphincters closed, with blood bypassing the capillaries.

Venule

Figure 20.5 Control of Perfusion of a Capillary Bed.

(a) Precapillary sphincters dilated and capillaries well perfused.

(b) Precapillary sphincters closed, with blood bypassing the capillaries.

with the tissue fluid. When the sphincters are closed, blood bypasses the capillaries, flows through the thoroughfare channel to a venule, and does not engage in significant fluid exchange. There is not enough blood in the body to fill the entire vascular system at once; consequently, about three-quarters of the body's capillaries are closed at any given time. The shifting of blood flow from one capillary bed to another is discussed later in the chapter.

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752 Part Four Regulation and Maintenance

Types of Capillaries

Two types of capillaries are distinguished by the ease with which they allow substances to pass through their walls and by structural differences that account for their greater or lesser permeability:

1. Continuous capillaries occur in most tissues, such as skeletal muscle. Their endothelial cells, held together by tight junctions, form an uninterrupted tube. The cells usually have narrow intercellular clefts about 4 nm wide between them. Small solutes, such as glucose, can pass through these clefts, but plasma proteins, other large molecules, and formed elements are held back. The continuous capillaries of the brain lack intercellular clefts and have more complete tight junctions that form the blood-brain barrier discussed in chapter 14.

2. Fenestrated capillaries have endothelial cells that are riddled with holes called fenestrations4 (filtration pores) (fig. 20.6). Fenestrations are about 20 to 100 nm in diameter and are usually covered by a thin mucoprotein diaphragm. They allow for the rapid passage of small molecules but still retain proteins and larger particles in the bloodstream. Fenestrated capillaries are important in organs that engage in rapid absorption or filtration—the kidneys, endocrine glands, small intestine, and choroid plexuses of the brain, for example.

Sinusoids are irregular blood-filled spaces in the liver, bone marrow, spleen, and some other organs. They are twisted, tortuous passageways that conform to the shape of the surrounding tissue. Some of them are continuous capillaries with very thin walls; others are fenes-trated capillaries with extraordinarily large pores that allow the blood plasma to come into direct contact with the perivascular cells. Even proteins and blood cells can pass through these pores; this is how albumin, clotting factors, and other proteins synthesized by the liver enter the blood and how newly formed blood cells enter the circulation from the bone marrow and lymphatic organs.

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Responses

  • Khadija
    When capillary sphincters are closed blood goes through?
    8 years ago
  • calliope
    Why are there no capillary beds in the cornea?
    2 years ago

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