Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 34

Circulation Through Special Regions 575

This can be applied clinically using inhaled nitrous oxide
(N
2
O)
(Kety method).
The average cerebral blood flow in
young adults is 54 mL/100 g/min. The average adult brain
weighs about 1400 g, so the flow for the whole brain is about
756 mL/min. Note that the Kety method provides an average
value for perfused areas of brain because it gives no informa-
tion about regional differences in blood flow. It also can only
measure flow to perfused parts of the brain. If the blood flow
to a portion of the brain is occluded, the measured flow does
not change because the nonperfused area does not take up
any N
2
O.
In spite of the marked local fluctuations in brain blood flow
with neural activity, the cerebral circulation is regulated in
such a way that total blood flow remains relatively constant.
The factors involved in regulating the flow are summarized in
Figure 34–8.

ROLE OF INTRACRANIAL PRESSURE

In adults, the brain, spinal cord, and spinal fluid are encased,
along with the cerebral vessels, in a rigid bony enclosure. The
cranial cavity normally contains a brain weighing approximate-
ly 1400 g, 75 mL of blood, and 75 mL of spinal fluid. Because
brain tissue and spinal fluid are essentially incompressible, the
volume of blood, spinal fluid, and brain in the cranium at any
time must be relatively constant
(Monro–Kellie doctrine).
More importantly, the cerebral vessels are compressed whenev-
er the intracranial pressure rises. Any change in venous pres-
sure promptly causes a similar change in intracranial pressure.
Thus, a rise in venous pressure decreases cerebral blood flow
both by decreasing the effective perfusion pressure and by com-
pressing the cerebral vessels. This relationship helps to compen-
sate for changes in arterial blood pressure at the level of the
head. For example, if the body is accelerated upward (positive
g
), blood moves toward the feet and arterial pressure at the level
of the head decreases. However, venous pressure also falls and
intracranial pressure falls, so that the pressure on the vessels de-

creases and blood flow is much less severely compromised than

it would otherwise be. Conversely, during acceleration down-

ward, force acting toward the head (negative

g

) increases arterial

pressure at head level, but intracranial pressure also rises, so that

the vessels are supported and do not rupture. The cerebral ves-

sels are protected during the straining associated with defeca-

tion or delivery in the same way.

AUTOREGULATION

As seen in other vascular beds, autoregulation is prominent in

the brain (Figure 34–9). This process, by which the flow to

many tissues is maintained at relatively constant levels despite

variations in perfusion pressure, is discussed in Chapter 32. In

the brain, autoregulation maintains a normal cerebral blood

flow at arterial pressures of 65 to 140 mm Hg.

ROLE OF VASOMOTOR

& SENSORY NERVES

The innervation of large cerebral blood vessels by postgangli-

onic sympathetic and parasympathetic nerves and the addi-

tional distal innervation by sensory nerves have been described

above. The nerves may also modulate tone indirectly, via the

release of paracine substances from astrocytes. The precise role

of these nerves, however, remains a matter of debate. It has

been argued that noradrenergic discharge occurs when the

blood pressure is markedly elevated. This reduces the resultant

passive increase in blood flow and helps protect the blood–

brain barrier from the disruption that could otherwise occur

(see above). Thus, vasomotor discharges affect autoregulation.

With sympathetic stimulation, the constant-flow, or plateau,

part of the pressure-flow curve is extended to the right (Figure

34–9); that is, greater increases in pressure can occur without

an increase in flow. On the other hand, the vasodilator hydral-

azine and the angiotensin-converting enzyme (ACE) inhibitor

captopril reduce the length of the plateau. Finally, neurovascu-

lar coupling may adjust local perfusion in response to changes

in brain activity (see below).

FIGURE 34–8
Diagrammatic summary of the factors
affecting overall cerebral blood flow.

Brain, spinal

cord, and

spinal fluid

Vertebral

column

Intracranial Cranium

pressure

Local con-

striction and

dilation of

cerebral

arterioles

Mean arterial pressure

at brain level

Viscosity of blood

Mean venous pressure

at brain level

FIGURE 34–9

Autoregulation of cerebral blood flow (CBF)

during steady-state conditions.

The blue line shows the alteration

produced by sympathetic stimulation during autoregulation.

100

50

70 140

Arterial pressure (mm Hg)

CBF