CHAPTER 34
Circulation Through Special Regions 577extremely sensitive to hypoxia, and occlusion of its blood sup-
ply produces unconsciousness in a period as short as 10 s. The
vegetative structures in the brain stem are more resistant to
hypoxia than the cerebral cortex, and patients may recover
from accidents such as cardiac arrest and other conditions
causing fairly prolonged hypoxia with normal vegetative func-
tions but severe, permanent intellectual deficiencies. The basal
ganglia use O
2
at a very high rate, and symptoms of Parkinson
disease as well as intellectual deficits can be produced by
chronic hypoxia. The thalamus and the inferior colliculus are
also very susceptible to hypoxic damage (see Clinical Box 34–3).ENERGY SOURCES
Glucose is the major ultimate source of energy for the brain;
under normal conditions, 90% of the energy needed to main-
tain ion gradients across cell membranes and transmit electri-
cal impulses comes from this source. Glucose enters the brain
via GLUT 1 in cerebral capillaries (see above). Other trans-
porters then distribute it to neurons and glial cells.
Glucose is taken up from the blood in large amounts, and
the RQ (respiratory quotient; see Chapter 21) of cerebral tis-
sue is 0.95–0.99 in normal individuals. Importantly, insulin is
not required for most cerebral cells to utilize glucose. In gen-
eral, glucose utilization at rest parallels blood flow and O
2
consumption. This does not mean that the total source of
energy is always glucose. During prolonged starvation, appre-
ciable utilization of other substances occurs. Indeed, evidence
indicates that as much as 30% of the glucose taken up under
normal conditions is converted to amino acids, lipids, and
proteins, and that substances other than glucose are metabo-
lized for energy during convulsions. Some utilization of
amino acids from the circulation may also take place even
though the amino acid arteriovenous difference across the
brain is normally minute.
The consequences of hypoglycemia in terms of neural func-
tion are discussed in Chapter 21.GLUTAMATE & AMMONIA REMOVAL
The brain’s uptake of glutamate is approximately balanced by
its output of glutamine. Glutamate entering the brain takes
up ammonia and leaves as glutamine. The glutamate–
glutamine conversion in the brain—the opposite of the reac-
tion in the kidney that produces some of the ammonia enter-
ing the tubules—serves as a detoxifying mechanism to keep
the brain free of ammonia. Ammonia is very toxic to nerve
cells, and ammonia intoxication is believed to be a major
cause of the bizarre neurologic symptoms in hepatic coma
(see Chapter 29).CORONARY CIRCULATION
ANATOMIC CONSIDERATIONS
The two coronary arteries that supply the myocardium arise
from the sinuses behind two of the cusps of the aortic valve at
the root of the aorta (Figure 34–11). Eddy currents keep the
valves away from the orifices of the arteries, and they are
patent throughout the cardiac cycle. Most of the venous blood
returns to the heart through the coronary sinus and anteriorCLINICAL BOX 34–2
Changes in Cerebral Blood Flow in Disease
Several disease states are now known to be associated with
localized or general changes in cerebral blood flow, as re-
vealed by PET scanning and fMRI techniques. For example,
epileptic foci are hyperemic during seizures, whereas flow
is reduced in other parts of the brain. Between seizures,
flow is sometimes reduced in the foci that generate the sei-
zures. Parietooccipital flow is decreased in patients with
symptoms of agnosia (see Chapter 14). In Alzheimer dis-
ease, the earliest change is decreased metabolism and
blood flow in the superior parietal cortex, with later spread
to the temporal and finally the frontal cortex. The pre- and
postcentral gyri, basal ganglia, thalamus, brain stem, and
cerebellum are relatively spared. In Huntington disease,
blood flow is reduced bilaterally in the caudate nucleus,
and this alteration in flow occurs early in the disease. In
manic depressives (but interestingly, not in patients with
unipolar depression), there is a general decrease in cortical
blood flow when the patients are depressed. In schizophre-
nia, some evidence suggests decreased blood flow in the
frontal lobes, temporal lobes, and basal ganglia. Finally,
during the aura in patients with migraine, a bilateral de-
crease in blood flow starts in the occipital cortex and
spreads anteriorly to the temporal and parietal lobes.TABLE 34–3
Utilization and production
of substances by the adult human brain in vivo.
SubstanceUptake (+) or Output (–)
per 100 g of Brain/min Total/min
Substances utilized
Oxygen +3.5 mL +49 mL
Glucose +5.5 mg +77 mg
Glutamate +0.4 mg +5.6 mg
Substances produced
Carbon dioxide –3.5 mL –49 mL
Glutamine –0.6 mL –8.4 mgSubstances not used or produced in the fed state:
lactate, pyruvate, total
ketones, and
α
-ketoglutarate.