Ganong's Review of Medical Physiology, 23rd Edition

578
SECTION VI
Cardiovascular Physiology

cardiac veins (Figure 34–12), which drain into the right atri-
um. In addition, there are other vessels that empty directly
into the heart chambers. These include
arteriosinusoidal ves-
sels,
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 ar-
teries draining directly into the chambers. A few anastomoses
occur between the coronary arterioles and extracardiac arteri-
oles, especially around the mouths of the great veins. Anasto-
moses between coronary arterioles in humans only pass
particles less than 40
μ
m 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

(Table 34–4). Consequently, flow occurs in the arteries sup-

plying 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 myocardi-

um 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. Be-

cause no blood flow occurs during systole in the subendocar-

dial 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

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 compro-

mised 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 se-

vere brain damage, ischemia reduces glutamate uptake by

astrocytes, and the increase in local glutamate causes exci-

totoxic damage and death to neurons (see Chapter 7). In

experimental animals, and perhaps in humans, drugs that

prevent this excitotoxic damage significantly reduce the ef-

fects of strokes. In addition, clot-lysing drugs such as tissue-

type 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 be-

come 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.

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.)

FIGURE 34–12 Diagram of the coronary circulation.

Marginal

branch

Right coronary

artery

Left coronary

artery

Circumflex branch

Anterior

descending

branch

Septal

branches

Marginal

branch

Posterior descending branch

Extracoronary

arteries

Coronary

arteries

Arterioles

Capillaries

Arterioles

Arteriosinusoidal

vessels

Heart chambers

Thebesian

Coronary sinus or veins

anterior cardiac

veins

Arterioluminal

Veins vessels