Under such conditions, a single drop of blood may pass through the heart several times in one minute. At rest, most of your muscles are inactive and your heart rate will be accordingly slow. When standing, muscles in the lower body, as well as the trunk and head, must work and the heart will beat even faster. If you run or exercise, the heart will automatically adjust to speed up in response to the increased activity. 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.