Ganong's Review of Medical Physiology, 23rd Edition

578 SECTION VI Cardiovascular Physiology
CLINICAL BOX 34–3
Stroke
When the blood supply to a part of the brain is interrupted,
ischemia damages or kills the cells in the area, producing
the signs and symptoms of a stroke. There are two general
types of strokes: hemorrhagic and ischemic. Hemorrhagic
stroke occurs when a cerebral artery or arteriole ruptures,
sometimes but not always at the site of a small aneurysm.
Ischemic stroke occurs when flow in a vessel is compromised
by atherosclerotic plaques on which thrombi form.
Thrombi may also be produced elsewhere (eg, in the atria
in patients with atrial fibrillation) and pass to the brain as
emboli where they then lodge and interrupt flow. In the
past, little could be done to modify the course of a stroke
and its consequences. However, it has now become clear
that in the penumbra, the area surrounding the most severe
brain damage, ischemia reduces glutamate uptake by
astrocytes, and the increase in local glutamate causes excitotoxic
damage and death to neurons (see Chapter 7). In
experimental animals, and perhaps in humans, drugs that
prevent this excitotoxic damage significantly reduce the effects
of strokes. In addition, clot-lysing drugs such as tissuetype
plasminogen activator (t-PA) (see Chapter 32) are of
benefit in ischemic strokes. Both antiexcitotoxic treatment
and t-PA must be given early in the course of a stroke to be
of maximum benefit, and this is why stroke has become a
condition in which rapid diagnosis and treatment have become
important. In addition, of course, it is important to
determine if a stroke is thrombotic or hemorrhagic, since
clot lysis is contraindicated in the latter.
cardiac veins (Figure 34–12), which drain into the right atrium.
In addition, there are other vessels that empty directly
into the heart chambers. These include arteriosinusoidal vessels,
sinusoidal capillary-like vessels that connect arterioles to
the chambers; thebesian veins that connect capillaries to the
chambers; and a few arterioluminal vessels that are small arteries
draining directly into the chambers. A few anastomoses
occur between the coronary arterioles and extracardiac arterioles,
especially around the mouths of the great veins. Anastomoses
between coronary arterioles in humans only pass
particles less than 40 μm in diameter, but evidence indicates
that these channels enlarge and increase in number in patients
with coronary artery disease.
PRESSURE GRADIENTS & FLOW
IN THE CORONARY VESSELS
The heart is a muscle that, like skeletal muscle, compresses its
blood vessels when it contracts. The pressure inside the left
ventricle is slightly higher than in the aorta during systole
Marginal
branch
Right coronary
artery
Left coronary
artery
Circumflex branch
Anterior
descending
branch
Septal
branches
Marginal
branch
FIGURE 34–11 Coronary arteries and their principal
branches in humans. (Reproduced with permission from Ross G: The
cardiovascular system. In: Essentials of Human Physiology. Ross G [editor]. Copyright ©
1978 by Year Book Medical Publishers.)
(Table 34–4). Consequently, flow occurs in the arteries supplying
the subendocardial portion of the left ventricle only
during diastole, although the force is sufficiently dissipated in
the more superficial portions of the left ventricular myocardium
to permit some flow in this region throughout the cardiac
cycle. Because diastole is shorter when the heart rate is high,
left ventricular coronary flow is reduced during tachycardia.
On the other hand, the pressure differential between the aorta
and the right ventricle, and the differential between the aorta
and the atria, are somewhat greater during systole than during
diastole. Consequently, coronary flow in those parts of the
heart is not appreciably reduced during systole. Flow in the
right and left coronary arteries is shown in Figure 34–13. Because
no blood flow occurs during systole in the subendocardial
portion of the left ventricle, this region is prone to
ischemic damage and is the most common site of myocardial
infarction. Blood flow to the left ventricle is decreased in
Extracoronary
arteries
Arterioles
Posterior descending branch
Arteriosinusoidal
vessels
Coronary
arteries
Arterioles
Capillaries
Veins
Coronary sinus or
anterior cardiac
veins
Heart chambers
Arterioluminal
vessels
Thebesian
veins
FIGURE 34–12 Diagram of the coronary circulation.