Ganong's Review of Medical Physiology, 23rd Edition

570
SECTION VI
Cardiovascular Physiology

drains through the ophthalmic and pterygoid venous plex-
uses, through emissary veins to the scalp, and down the sys-
tem of paravertebral veins in the spinal canal.
The cerebral vessels have a number of unique anatomic fea-
tures. In the choroid plexuses, there are gaps between the
endothelial cells of the capillary wall, but the choroid epithe-
lial cells that separate them from the cerebrospinal fluid (CSF)
are connected to one another by tight junctions. The capillar-
ies in the brain substance resemble nonfenestrated capillaries
in muscle (see Chapter 32), but there are tight junctions
between the endothelial cells that limit the passage of sub-
stances through the junctions. In addition, there are relatively
few vesicles in the endothelial cytoplasm, and presumably lit-
tle vesicular transport. However, multiple transport systems
are present in the capillary cells. The brain capillaries are sur-
rounded by the endfeet of astrocytes (Figure 34–1). These
endfeet are closely applied to the basal lamina of the capillar-
ies, but they do not cover the entire capillary wall, and gaps of
about 20 nm occur between endfeet (Figure 34–2). However,
the endfeet induce the tight junctions in the capillaries (see
Chapter 32). The protoplasm of astrocytes is also found
around synapses, where it appears to isolate the synapses in
the brain from one another.

INNERVATION

Three systems of nerves innervate the cerebral blood vessels.
Postganglionic sympathetic neurons have their cell bodies in
the superior cervical ganglia, and their endings contain nor-
epinephrine. Many also contain neuropeptide Y. Cholinergic
neurons that probably originate in the sphenopalatine ganglia

also innervate the cerebral vessels, and the postganglionic cho-

linergic neurons on the blood vessels contain acetylcholine.

Many also contain vasoactive intestinal peptide (VIP) and

peptide histidyl methionine (PHM-27) (see Chapter 7). These

nerves end primarily on large arteries. Sensory nerves are

found on more distal arteries. They have their cell bodies in

the trigeminal ganglia and contain substance P, neurokinin

A, and calcitonin gene-related peptide (CGRP). Substance

P, CGRP, VIP, and PHM-27 cause vasodilation, whereas

TABLE 34–1
Resting blood flow and O
2
consumption of various organs in a 63-kg adult man
with a mean arterial blood pressure of 90 mm Hg and an O
2
consumption of 250 mL/min.

Blood Flow Arteriovenous

Oxygen

Difference

(mL/L)

Oxygen

Consumption

Resistance

(R units)

a

Percentage of Total

Region

Mass

(kg) mL/min

mL/100

g/min mL/min

mL/100

g/min Absolute

per

kg

Cardiac

Output

Oxygen

Consumption

Liver 2.6 1500 57.7 34 51 2.0 3.6 9.4 27.8 20.4

Kidneys 0.3 1260 420.0 14 18 6.0 4.3 1.3 23.3 7.2

Brain 1.4 750 54.0 62 46 3.3 7.2 10.1 13.9 18.4

Skin 3.6 462 12.8 25 12 0.3 11.7 42.1 8.6 4.8

Skeletal

muscle

31.0 840 2.7 60 50 0.2 6.4 198.4 15.6 20.0

Heart muscle 0.3 250 84.0 114 29 9.7 21.4 6.4 4.7 11.6

Rest of body 23.8 336 1.4 129 44 0.2 16.1 383.2 6.2 17.6

Whole body 63.0 5400 8.6 46 250 0.4 1.0 63.0 100.0 100.0

a
R units are pressure (mm Hg) divided by blood flow (mL/s).
Reproduced with permission from Bard P (editor):
Medical Physiology,
11th ed. Mosby, 1961.

FIGURE 34–1

Relation of fibrous astrocyte (3) to a capillary

(2) and neuron (4) in the brain.

The endfeet of the astrocyte process-

es form a discontinuous membrane around the capillary (1). Astrocyte

processes also envelop the neuron.

(Adapted from Krstic RV:

Die Gewebe des

Menschen und der Säugetiere.

Springer, 1978.)

2

3

1

4