As the level of activity increases the muscles start to work harder. The heart rate rises to supply extra oxygen and glucose to muscle cells to fuel this increased respiration and remove the extra carbon dioxide it produces. Sensory nerves monitor the levels of these gases in the blood and signal to the brain so that it can adjust the heart rate.

An average person can maintain their pulse at around 130 beats/minute to cope with activity for short periods of time. Being overweight, or doing very little exercise, means that the body is less able to cope with the demands of physical activity.

Elite marathon runners may have resting heart rates as low as 40-50 beats/minute because training has made their cardiovascular system extremely efficient. During a race they can run for long periods of time without their pulse rising much over 110 beats/minute.