Capillaries

The arterial system branches extensively (table 13.10) to deliver blood to over 40 billion capillaries in the body. As evidence of the extensiveness of these branchings, consider the fact that scarcely any cell in the body is more than 60 to 80 | m away from any capillary. The tiny capillaries provide a total surface area of 1,000 square miles for exchanges between blood and tissue fluid.

The amount of blood flowing through a particular capillary bed depends primarily on the resistance to blood flow in the small arteries and arterioles that supply blood to that capillary bed. Vasoconstriction in these vessels thus decreases blood flow to the capillary bed, whereas vasodilation increases blood flow.

The relatively high resistance in the small arteries and arterioles in resting skeletal muscles, for example, reduces capillary blood flow to only about 5% to 10% of its maximum capacity. In some organs (such as the intestine), blood flow may also be regulated by circular muscle bands called precapillary sphincters at the origin of the capillaries (fig. 13.26).

Unlike the vessels of the arterial and venous systems, the walls of capillaries are composed of just one cell layer—a simple squamous epithelium, or endothelium (fig. 13.27). The absence of smooth muscle and connective tissue layers permits a more rapid exchange of materials between the blood and the tissues.

Types of Capillaries

Different organs have different types of capillaries, distinguished by significant differences in structure. In terms of their endothe-lial lining, these capillary types include those that are continuous, those that are fenestrated, and those that are discontinuous.

Continuous capillaries are those in which adjacent en-dothelial cells are closely joined together. These are found in muscles, lungs, adipose tissue, and in the central nervous system. The lack of intercellular channels in continuous capillaries in the CNS contributes to the blood-brain barrier (chapter 7). Continuous capillaries in other organs have narrow intercellular channels (from 40 to 45 A in width) that permit the passage of molecules other than protein between the capillary blood and tissue fluid (fig. 13.27).

Examination of endothelial cells with an electron microscope has revealed the presence of pinocytotic vesicles (fig. 13.27), which suggests that the intracellular transport of material may occur across the capillary walls. This type of transport appears to be the only mechanism of capillary exchange available within the central nervous system and may account, in part, for the selective nature of the blood-brain barrier.

Heart and Circulation 393

Table 1 3.10 Characteristics of the Vascular Supply to the Mesenteries in a Dog

Total

Cross-Sectional

Table 1 3.10 Characteristics of the Vascular Supply to the Mesenteries in a Dog

Total

Cross-Sectional

Kind of Vessels

Diameter (mm)

Number

Area (cm2)

Length (cm)

Total Volume (cm3)

Aorta

10

1

0.8

40

30

Large arteries

3

40

3.0

20

60

Main artery branches

1

600

5.0

10

50

Terminal branches

.06

1,800

5.0

1

25

Arterioles

0.02

40,000,000

125

0.2

25

Capillaries

0.008

1,200,000,000

600

0.1

60

Venules

0.03

80,000,000

570

0.2

110

Terminal veins

1.5

1,800

30

1

30

Main venous branches

2.4

600

27

10

270

Large veins

6.0

40

11

20

220

Vena cava

12.5

1

1.2

40

_50

Note: The pattern of vascular supply is similar in dogs and humans.

Source: Animal Physiology, 4th ed. by Gordon et al., © 1982. Adapted by permission of Prentice-Hall, Inc., Upper Saddle River, NJ.

Note: The pattern of vascular supply is similar in dogs and humans.

Source: Animal Physiology, 4th ed. by Gordon et al., © 1982. Adapted by permission of Prentice-Hall, Inc., Upper Saddle River, NJ.

■ Figure 13.27 An electron micrograph of a capillary in the heart. Notice the thin intercellular channel (middle left) and the capillary wall, composed of only one cell layer. Arrows show some of the many pinocytotic vesicles.

Fenestrated capillaries occur in the kidneys, endocrine glands, and intestines. These capillaries are characterized by wide intercellular pores (800 to 1,000 A) that are covered by a layer of mucoprotein, which serves as a basement membrane over the capillary endothelium. This mucoprotein layer restricts the passage of certain molecules (particularly proteins) that might otherwise be able to pass through the large capillary pores. Discontinuous cap illaries are found in the bone marrow, liver, and spleen. The distance between endothelial cells is so great that these capillaries look like little cavities (sinusoids) in the organ.

Angiogenesis refers to the formation of new blood vessels from preexisting vessels, which are usually venules. Since all living cells must be within 100 |lm of a capillary, angiogenesis is required during tissue growth. Angiogenesis is thus involved in the pathogenesis of neoplasms (tumors), and of the blindness caused by neovascularization of the retina in diabetic retinopathy and age-related macular degeneration (the most common cause of blindness). The treatment of these diseases may therefore be improved by inhibiting angiogene-sis. Treatment for ischemic heart disease, on the other hand, may be improved by promoting angiogenesis in the coronary circulation. These therapies may manipulate paracrine regulators known to promote angiogenesis, including vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF).

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