576
SECTION VI
Cardiovascular Physiology
BLOOD FLOW IN VARIOUS
PARTS OF THE BRAIN
A major advance in recent decades has been the development
of techniques for monitoring regional blood flow in living,
conscious humans. Among the most valuable methods are
positron emission tomography (PET)
and related techniques
in which a short-lived radioisotope is used to label a com-
pound and the compound is injected. The arrival and clear-
ance of the tracer are monitored by scintillation detectors
placed over the head. Because blood flow is tightly coupled to
brain metabolism, local uptake of 2-deoxyglucose is also a
good index of blood flow (see below and Chapter 1). If the 2-
deoxyglucose is labeled with a short-half-life positron emitter
such as
18
F,
11
O, or
15
O, its concentration in any part of the
brain can be monitored.
Another valuable technique involves magnetic resonance
imaging (MRI). MRI is based on detecting resonant signals
from different tissues in a magnetic field.
Functional mag-
netic resonance imaging (fMRI)
measures the amount of
blood in a tissue area. When neurons become active, their
increased discharge alters the local field potential. A still
unsettled mechanism triggers an increase in local blood flow
and oxygen. The increase in oxygenated blood is detected by
fMRI. PET scanning can be used to measure not only blood
flow but the concentration of molecules, such as dopamine, in
various regions of the living brain. On the other hand, fMRI
does not involve the use of radioactivity. Consequently, it can
be used at frequent intervals to measure changes in regional
blood flow in a single individual.
In resting humans, the average blood flow in gray matter is
69 mL/100 g/min compared with 28 mL/100 g/min in white
matter. A striking feature of cerebral function is the marked
variation in local blood flow with changes in brain activity. An
example is shown in Figure 34–10. In subjects who are awake
but at rest, blood flow is greatest in the premotor and frontal
regions. This is the part of the brain that is believed to be con-
cerned with decoding and analyzing afferent input and with
intellectual activity. During voluntary clenching of the right
hand, flow is increased in the hand area of the left motor cortex
and the corresponding sensory areas in the postcentral gyrus.
Especially when the movements being performed are sequen-
tial, the flow is also increased in the supplementary motor area.
When subjects talk, there is a bilateral increase in blood flow in
the face, tongue, and mouth-sensory and motor areas and the
upper premotor cortex in the categorical (usually the left)
hemisphere. When the speech is stereotyped, Broca’s and Wer-
nicke’s areas do not show increased flow, but when the speech is
creative—that is, when it involves ideas—flow increases in both
these areas. Reading produces widespread increases in blood
flow. Problem solving, reasoning, and motor ideation without
movement produce increases in selected areas of the premotor
and frontal cortex. In anticipation of a cognitive task, many of
the brain areas that will be activated during the task are acti-
vated beforehand, as if the brain produces an internal model of
the expected task. In right-handed individuals, blood flow to
the left hemisphere is greater when a verbal task is being per-
formed and blood flow to the right hemisphere is greater when
a spatial task is being performed (see Clinical Box 34–2).BRAIN METABOLISM & OXYGEN
REQUIREMENTS
UPTAKE & RELEASE OF
SUBSTANCES BY THE BRAINIf the cerebral blood flow is known, it is possible to calculate
the consumption or production by the brain of O
2
, CO
2
, glu-
cose, or any other substance present in the bloodstream by
multiplying the cerebral blood flow by the difference between
the concentration of the substance in arterial blood and its
concentration in cerebral venous blood (Table 34–3). When
calculated in this fashion, a negative value indicates that the
brain is producing the substance.OXYGEN CONSUMPTION
O
2
consumption by the human brain (
cerebral metabolic rate
for O
2
,
CMRO
2
) averages approximately 20% of the total
body resting O
2
consumption (Table 34–1). The brain isFIGURE 34–10
Activity in the human brain at five different horizontal levels while a subject generates a verb that is appropriate
for each noun presented by an examiner.
This mental task activates the frontal cortex (slices 1–4), anterior cingulate gyrus (slice 1), and poster-
ior temporal lobe (slice 3) on the left side and the cerebellum (slices 4 and 5) on the right side. Light purple, moderate activation; dark purple,
marked activation.
(Based on PET scans in Posner MI, Raichle ME:
Images of Mind.
Scientific American Library, 1994.)