Angina starts when the supply of oxygen and glucose does not keep up with demand. If something impedes the easy access of oxygen and glucose to the myocardium, and the heart needs to continue beating, then it will try to find its energy source from other substances, like fats, and it will try to burn them up without using oxygen. Most people, when they were children, have felt the results of this anaerobic energy process in the stitch felt in the side during running.
The pain of a stitch is caused by the accumulation of lactic acid in muscles in the side and back that have been overused. (Fats do not burn all the way down to carbon dioxide, but only get as far as lactic acid, a more complex substance, that is more difficult to remove from the tissues.)
The pain in angina has the same root. Lactic acid also builds up in a heart trying to beat without a good enough oxygen supply: the pain of angina can be similar to that of a stitch. The difference between the two is that you can tolerate a stitch, because back muscles are not as important, and eventually recover with rest. The heart muscle needs a much faster resupply of oxygen if it is to survive.
This supply of oxygen comes from the coronary arteries—so called because they form a crown around the top of the heart, passing their branches over the heart surface to feed the muscles that form the walls of its four chambers. In the normal heart, the three main coronary arteries and their branches are wide, strong, elastic tubes, which can expand enormously to cope with any extra flow of blood needed when the demand rises.
Naturally, this demand for a flow of blood varies hugely. When you’re asleep or at complete rest, the heart rate falls to sixty beats per minute or below, and the blood pressure falls accordingly. At such times, the-heart’s demand for oxygen and glucose is low. At the opposite end of the demand spectrum are times of extreme exercise. Sprinting, whether it is to catch a train or bus or to win an Olympic gold medal, can increase the heart’s need for oxygen by twenty times or more. And if the heart is beating at or above a rate of 180 per minute, the time for recovery between beats shortens to a tenth of a second. The myocardium must be very efficient, and awash with oxygen and glucose, in order to cope with that.
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