Respiration is defined as the act of breathing in and breathing out. Inspiration, followed by expiration is one single breath. When the body inhales, oxygen is carried to the lungs and when the breath is exhaled, carbon dioxide – a gaseous product of the body's metabolism, is expelled. While this breath may be taken in and exhaled by the respiratory muscles, the order to do so, actually originates in the brain.
Respiration Control and the Brain
The respiratory center is located in the brainstem, in a specific part of the stem called the medulla oblongata. Located on the brainstem, above the medulla oblongata is the pons. The pons serves several functions, one of which sends stimuli to the medulla to control the rate and depth of respiration via the apneustic and pneumotaxic centers.
The medulla oblongata contains the dorsal respiratory group (DRG) and the ventral respiratory group (VRG). The DRG controls the process of inspiration and the VRG controls the process of expiration, both send stimuli to the muscles that control breathing to tell them when to inhale or exhale.
The stimuli sent by the DRG and VRG groups are part of the autonomic nervous system (ANS). The ANS works involuntarily, meaning there is no conscious control over it. While the respiratory groups involuntarily send stimuli to urge the body to take a breath, the muscles that control the act of respiration, are entirely voluntary, so the body can, if it so chooses, make the decision to ignore it.
The Brain and Respiratory Muscle Battle
While a person can make the conscious choice not to breathe, a breath cannot be held in forever. Certainly the time taken between breaths can be lengthened, as is evidenced by breath-hold divers, but in the end the brain, if healthy and intact, will always find a way to force the body to take a breath.
The brain is the one organ most sensitive to oxygen deprivation, requiring ten times more oxygen than any other organ. Try to deprive it and it will seize control. When a breath is held and carbon dioxide levels begin to rise in the body, a reflex is triggered by the cerebral cortex that overrides the medulla and causes the diaphragm and the rib muscles to spasm. This spasm is extremely painful and difficult to ignore, resulting in a person gulping or gasping for air.
If a person manages to endure the spasm, the brain has a second back–up plan, unconsciousness. Without awareness, a person is unable to consciously control the breathing process and so it reverts back to the autonomic nervous system. Breathing is quickly restored.
While the process of unconsciousness has yet to be fully determined, it is known that the reticular activating system (RAS) located in the brainstem, controls the ability of alertness, sleep and attention span. Oxygen deprivation to the brain can affect the RAS system, causing a person to become unconscious. Could this be the brain in survival mode?
Study Shows Brain May Have a Third Back–Up
In May, 2001, Professor Mark Mintun, a professor of radiology and psychiatry at Washington University School of Medicine in St. Louis, discovered that the brain actually holds a reserve of oxygen to assist it in coping with grueling situations.
Professor Mintun says his findings challenge, “the previously accepted idea that blood flow increases occur during tasks such as reading to raise oxygen levels in the brain.” Rather, Professor Mintun’s studies showed that the amount of oxygen entering the body has no correlation to cerebral blood flow, thus, “The brain appears to have some sort of built-in insurance policy,” Professor Mintun says, because, “even when partially deprived of oxygen, it can still take care of itself.”
The brain certainly is not infallible; it is subject to other body systems. Requiring 750–1000ml of blood flow per minute and 20% of the body’s oxygen intake, it is clearly a slave to the circulatory, cardiac and respiratory systems. If Professor Mintun’s studies hold true however, and the brain does hold oxygen in reserve, the brain may have in its arsenal, a third back–up plan.
Sources:
Professor Mintun, Mark A. M.D et al “Blood flow and oxygen delivery to human brain during functional activity: Theoretical modeling and experimental data.” National Academy of Sciences Journal May, 2001
Ullrich, Dr. Peter F. Jr., MD “Cervical Nerves” spine-health.com October, 2009
Currie, David "Regulation of Respiration" East Tennessee State University Jan, 2009
Elizabeth Batt - Elizabeth Batt is a former large animal nurse, certified NREMT, lover of equines and conservationist.
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