Ganong's Review of Medical Physiology, 23rd Edition

538 SECTION VICardiovascular Physiology

as 3000 nm. They therefore permit the passage of large mole-
cules, including plasma proteins, which is important for hepatic
function (see Chapter 29). The permeabilities of capillaries in
various parts of the body, expressed in terms of their hydraulic
conductivity, are summarized in Table 32–10.
Capillaries and postcapillary venules have pericytes around
their endothelial cells (Figure 32–19). These cells have long
processes that wrap around the vessels. They are contractile
and release a wide variety of vasoactive agents. They also syn-
thesize and release constituents of the basement membrane
and extracellular matrix. One of their physiologic functions
appears to be regulation of flow through the junctions
between endothelial cells, particularly in the presence of
inflammation. They are closely related to the mesangial cells
in the renal glomeruli (see Chapter 38).

LYMPHATICS

The lymphatics serve to collect plasma and its constituents

that have exuded from the capillaries into the interstitial space.

They drain from the body tissues via a system of vessels that

coalesce and eventually enter the right and left subclavian

veins at their junctions with the respective internal jugular

veins. The lymph vessels contain valves and regularly traverse

lymph nodes along their course. The ultrastructure of the

small lymph vessels differs from that of the capillaries in sev-

eral details: No fenestrations are visible in the lymphatic endo-

thelium; very little if any basal lamina is present under the

endothelium; and the junctions between endothelial cells are

open, with no tight intercellular connections.

ARTERIOVENOUS ANASTOMOSES

In the fingers, palms, and ear lobes, short channels connect ar-

terioles to venules, bypassing the capillaries. These arteriove-

nous (A-V) anastomoses, or shunts, have thick, muscular

walls and are abundantly innervated, presumably by vasocon-

strictor nerve fibers.

VENULES & VEINS

The walls of the venules are only slightly thicker than those of

the capillaries. The walls of the veins are also thin and easily

distended. They contain relatively little smooth muscle, but

considerable venoconstriction is produced by activity in the

noradrenergic nerves to the veins and by circulating vasocon-

strictors such as endothelins. Variations in venous tone are

important in circulatory adjustments.

The intima of the limb veins is folded at intervals to form

venous valves that prevent retrograde flow. The way these

valves function was first demonstrated by William Harvey in

the 17th century. No valves are present in the very small veins,

the great veins, or the veins from the brain and viscera.

FIGURE 32–19 Cross-sections of capillaries. Left: Type of capillary found in muscle. Right: Fenestrated type of capillary. (Reproduced with
permission from Fawcett DW: Bloom and Fawcett, Textbook of Histology, 11th ed. Saunders, 1986.)

Vesicles

Basal

lamina

Pericyte

Pericyte

Fenestrations

or pores

Interdigitated

junction

TABLE 32–10 Hydraulic conductivity of
capillaries in various parts of the body.

Organ Conductivitya

Type of

Endothelium

Brain (excluding circumventricular

organs)

3

Skin 100 Continuous

Skeletal muscle 250

Lung 340

Heart 860

Gastrointestinal tract (intestinal

mucosa)

13,000

Fenestrated

Glomerulus in kidney 15,000

aUnits of conductivity are 10–13 cm (^3) s–1 dyne–1.
Data courtesy of JN Diana.