The coronary arterioles contain both alpha and beta adrenergic receptors, which promote vasoconstriction and vasodilation, respectively. Norepinephrine released by sympathetic nerve fibers stimulates alpha-adrenergic receptors to raise vascular resistance at rest. Epinephrine released by the adrenal medulla can stimulate the beta-adrenergic receptors to produce vasodilation when the sympa-thoadrenal system is activated during the fight-or-flight reaction.
Most of the vasodilation that occurs during exercise, however, is due to intrinsic metabolic control mechanisms. The intrinsic mechanisms occur as follows: (1) as the metabolism of the myocardium increases, there are local accumulations of carbon dioxide, K+, and adenosine in the tissue, together with depletion of oxygen; (2) these localized changes act directly on the vascular smooth muscle to cause relaxation and vasodilation.
Under abnormal conditions blood flow to the myocardium may be inadequate, resulting in myocardial ischemia (chapter 13). The inadequate flow may be due to blockage by atheromas and/or blood clots or to muscular spasm of a coronary artery (fig. 14.17). Occlusion of a coronary artery can be visualized by inserting a catheter (plastic tube) into a brachial or femoral artery all the way to the opening of the coronary arteries in the aorta and then injecting a radiographic contrast material. The picture thus obtained is called an angiogram.
In a technique called balloon angioplasty, an inflatable balloon is used to open the coronary arteries. However, restenosis (recurrence of narrowing) often occurs. For this reason, a cylindrical support called a stent may be inserted to help keep the artery open. If the occlusion is sufficiently great, a coronary bypass may be performed. In this procedure, a length of blood vessel, usually taken from the saphenous vein in the leg, is sutured to the aorta and to the coronary artery at a location beyond the site of the occlusion (fig. 14.18).
Clinical Investigation Clues
Remember that Charlie was very weak when he was found.
■ How did his dehydration affect his cardiac output?
■ How would this effect cause him to be weak?
■ Figure 14.17 Angiograms of the left coronary artery of a heart patient. These angiograms were taken (a) when the patient's ECG was normal and (b) when the ECG showed evidence of myocardial ischemia. Notice that a coronary artery spasm (see arrow in [b]) appears to accompany the ischemia.
■ Figure 14.18 A diagram of coronary artery bypass surgery. Segments of the saphenous vein of the patient are commonly used as coronary bypass vessels.
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