An athlete’s muscles need a lot of oxygen when they’re playing, and the heart has to pump more blood to meet the oxygen demand, and it has to beat faster. The rapid heart rate also increases the heart muscles’ oxygen need, and the heart’s blood vessels have to dilate to allow more blood to flow through. If the heart’s oxygen demand is not met, the cardiac muscles cannot pump blood to the whole body, including the brain, and the brain stops functioning, and the player collapses. If the player does not get immediate medical attention, the heart muscle dies, and...