What Are The Coronary Arteries?

There is one organ that is deprived of any flow of blood during systole - the heart itself. When the heart beats, it shortens to half of the volume it is when relaxed. This compresses the blood vessels within its walls, so that the flow of blood within them, and therefore of blood and glucose, stops momentarily.

Instead, these blood vessels, the coronary arteries, so named because they encircle the upper surface of the heart like a crown, have to fill with blood when the heart is relaxing, in diastole. They originate from openings just at the base of the aorta, so that as the valves close against the backward flow of blood, it is diverted into them. The relaxing myocardium virtually sucks its blood supply deep into itself.

There are two main coronary arteries, one mainly supplying the right, and one the left, sides of the heart. They quickly branch out and dive deep within the heart muscle, so that every part of the heart has its share of oxygen and glucose. In the young and healthy, the coronary arteries are wide, and smooth-lined, so that the blood flows without friction or disturbance through them.

There is even plenty of extra capacity when extra flow is needed. The step-up in energy, for example, when we move from rest to a 100 metre sprint requires shifting the heart rate from 70 to, say, more than 200 beats per minute. This may be combined with a short and sharp rise in blood pressure, perhaps doubling it. At times like these the heart may use up ten to twenty times the amounts of oxygen and glucose used when resting - and must still be able to recover, without tiring, in the very much shorter time between beats.

Athletic training helps the heart to do this very efficiently. The more we use our muscles, the more efficient they become, and the heart muscle is no exception - provided we keep our coronary vessels in a healthy state, too. The coronary arteries are unique in that they widen dramatically in response to the workload they are asked to face. Their walls are marvellously elastic, so that they can take the extra flow of blood through them. There are even extra, unused, channels, called 'collaterals' that will open up to take extra blood if necessary.

Good athletes in training have resting heart rates of 60 or even less, and at top speed may only double or treble them. An unfit, overweight person who runs for a bus may have a pulse racing well above 200 a minute, that will take 20 minutes or more to return to the resting rate between 80 and 90. Add another half as much again to the rise in heart rate and recovery time for a heavy smoker.