A)Heart rate is controlled by a pacemaker—an area within the heart that initiates contractions of the heart muscles. If left alone, the pacemaker would set an unchanging beat. When you exercise, your heart needs to pump more blood through your body. Why is this necessary?(1 point) Responses Blood contains too much carbon dioxide and too little oxygen. Blood contains too much carbon dioxide and too little oxygen. Blood contains too much oxygen and too little carbon dioxide.