Exercise increases the demand for oxygen by the muscles and other organs. When the heart is beating faster, as happens when running or swimming, the brain and muscles increase their demand for oxygen in order to perform cellular respiration to make energy. As a result of this increased demand for oxygen, there is also an elevated supply of carbon dioxide that must be removed from the body. Respiratory rate is a factor of the intensity of a person's workout and the duration of that workout. Respiration rate can increase as much as 20 times during very heavy exercise. This results in hyperpnea, an increased depth and rate of breathing in response to higher metabolic demands of the body. Hyperpnea does not alter concentrations of oxygen or carbon dioxide in the blood because it increases the depth and rate of breathing only enough to meet the respiratory demands of tissue cells.
When a person starts exercising, their breathing initially increases rapidly and continues to increase until the body reaches a steady state. This is often called "getting a second wind." When exercise stops, breathing rate drops off quickly, but not in great amounts. As the body sits at rest, the breathing rate gradually returns to normal. It is believed that several neural factors produce the changes in breathing during exercise, one of which is psychological stimuli, like preparing mentally to exercise. This is called a feed-forward mechanism, an anticipatory response that initiates the feedback loops involved with the action. Motor activation of skeletal muscles and input reaching the respiratory centers from sensors that respond to movement also play a role in increasing the respiration rate.
Another factor that can change respiration rate is living in an area of high altitude. Most people live in areas that are below 8,000 ft (2,400 m). These areas do not have great changes between atmospheric pressure and the pressure within the lungs. In order to breathe, air pressure within the lungs must be less than that of the atmospheric pressure. At higher altitude, air pressure is lower, making it more difficult to breathe. In addition, less oxygen is available at higher altitudes. People who are accustomed to living in these areas are able to travel to higher altitude areas for short periods of time without any major breathing problems. However, if someone who lives at sea level (0 ft/0 m) travels to higher altitudes without allowing acclimatization (time for the human body to adapt to lower levels of oxygen) to take place, they may experience headaches, shortness of breath, nausea, and dizziness. These symptoms occur because the person's lungs are not accustomed to dealing with air that has a different oxygen and carbon dioxide content. This sometimes happens when people go skiing in the mountains.
If a person moves from lower altitudes to a higher altitude area for the long term, their body will gradually adjust to the changes. As the partial pressure of oxygen in the arteries decreases, chemoreceptors become more sensitive to changes in carbon dioxide levels. The large decrease of oxygen at high altitudes fires these receptors almost instantly. Breathing increases to equalize the gases within the arteries. Over time, the body reduces its overall carbon dioxide levels to less than 40 mm Hg. Lower than normal hemoglobin saturation occurs at higher altitudes because there is less oxygen in the air. Additionally, 2,3-bisphosphoglycerate (BPG) concentrations increase at higher altitudes, resulting in less oxygen being taken up by hemoglobin. With all these physiological changes happening, the kidneys produce a hormone called erythropoietin to increase the rate of hematopoiesis, or red blood cell formation. Over time, the person will acclimate, or adapt, to the changes in oxygen levels in the atmosphere.