Heart rate, or the rate at which the muscles in the heart contract, is controlled by the sinoatrial node, a group of cells in the cardiac muscle of the right atrium. Heart rate decreases when you are asleep and increases when you are awake. Some
people require an artificial pacemaker to regulate their heart rate if it is too slow. Which hypothesis explains how the body might be affected by a dangerously slow heart rate?
A. The body might have trouble falling asleep of feeling rested after a full night of slee
B. Cells in the body might not receive enough oxygen, which could cause the person to faint.
C. The adrenal gland might not produce enough hormones to prepare the body for a quick reaction
D. Breathing might become shallow, which could cause the person to retain too much carbon dioxide.