During exercise, the ventricles increase in size and a stronger contraction is used to force the blood to the heart and throughout the body. Cardiac output is increased. Temperature has a direct effect on heart rate, and one of the ways in which mammals regulate their internal temperature is by controlling peripheral blood circulation. If left alone, the pacemaker of the heart would send out regular impulses causing the heart to beat at a constant rate. However, there are special chemical sensitive sensory receptors located in some of the major arteries of the body.
Carbon dioxide is the waste product produced during strenuous activity such as mountain climbing.
The high levels of CO2 stimulate these special receptors to send a signal to the brain. The brain, in turn, sends a signal to the pacemaker to increase the heart rate. The increased heart rate sends more blood to the lungs to get rid of the excess carbon dioxide. The effects of oxygen concentration on the heart and blood vessels are rapid. Oxygen deficiency in the cardiac tissue causes dilation of the coronary capillaries, thereby increasing blood flow and oxygen supply.