The rate and depth of human respiration is controlled by many different factors. Some are under conscious control; others aren't. In fact, respiration is unique among organ system functions in that while it’s generally under the control of the autonomic nervous system--which regulates respiration without us needing to think about it--we can also exert control over our breathing. In addition to chemicals released by the nervous system that communicate information about respirations, various other chemical factors affect the rate of respiration.
Oxygen
For a long time, there was debate in the scientific community as to whether the body had the ability to sense blood oxygen concentration and regulate respiration accordingly. It seems intuitive that oxygen concentration should affect respiration, because a major purpose of the lungs is to saturate the blood with oxygen. Still, the body doesn't seem as sensitive to blood oxygen as it does to concentrations of other chemicals. According to Dr. Gary Thibodeau in his text “Anatomy and Physiology,” oxygen concentration of the blood is monitored by small sensory organs called aortic bodies, and low oxygen concentration causes respiration rate to increase. There is no respiratory response to high oxygen concentrations.
Carbon Dioxide
Carbon dioxide is one of the chemicals that most significantly affect respiration rate. Parts of the brain and the aortic bodies jointly sense carbon dioxide, and either lower or higher than normal concentrations of the compound elicit a response. Too much carbon dioxide increases respiration rate, because each breath pulls in oxygen and exhales carbon dioxide. Too little carbon dioxide causes apnea, or a temporary cessation of breathing, Dr. Thibodeau says.
Acid
Acidity provides another mechanism for tracking carbon dioxide concentration in the blood, because carbon dioxide combines with water to form an acid. Parts of the brain are very pH sensitive. They’re meant to sense acid from carbon dioxide but are incapable of distinguishing between acids, so any acid in the bloodstream causes respiration rate to increase. On the other hand, chemicals that make the blood basic--the opposite of acidic--are interpreted to mean that respirations should stop temporarily, says Dr. Lauralee Sherwood in her book, “Human Physiology.”
Epinephrine
Dr. Sherwood notes that the "fight or flight" hormone, epinephrine, affects the body in many different ways. Each is designed to prepare a person for action. Heart rate increases, blood is diverted from the stomach toward the muscles, and respirations increase under the influence of this hormone. For this reason, anything that induces an adrenaline response, such as becoming angry or finding out about a surprise inspection at work, causes breathing to become rapid.
Acetylcholine
The neurotransmitter of the “rest and digest” branch of the nervous system, acetylcholine acts to constrict airways and slow respiration. Because bringing air into the lungs is drying and because oxygen is capable of damaging cells in excess, it’s best for general health if an animal doesn’t breathe any more than necessary. When evasive action isn’t required and the person isn’t exercising, acetylcholine reduces respiration rate because cells require less oxygen under such conditions.
References
- “Anatomy and Physiology”; Gary Thibodeau, Ph.D.; 2007
- “Human Physiology”; Lauralee Sherwood, Ph.D.; 2004
