The brain is the primary controller of respiratory rate. It receives input from sensors that detect oxygen and carbon dioxide levels in the blood. Blood pH also influences respiratory rate. Activity level and the presence of drugs or alcohol are important factors that also can affect respiratory rate.
Brainstem Rhythmicity Center
Breathing usually takes place without your conscious awareness. The rhythmicity center in the brainstem controls this function. Within this center are I nerve cells that control inspiration and E nerve cells that control exhalation. The I and E cells alternate to coordinate the rhythmic pattern of inhalation and exhalation. Conditions that damage the rhythmicity center, such as a brainstem stroke, are often catastrophic, causing respiratory arrest--cessation of breathing.
Cerebral Cortical Input
You can voluntarily control your breathing. The conscious control of breathing is under the direction of an area of the brain known as the cerebral cortex, which controls all voluntary muscle movement.
Blood Carbon Dioxide Level
The amount of carbon dioxide in the blood is a primary influence on respiratory rate. As your activity level increases, your cells--especially muscles cells--produce increased amounts of carbon dioxide. The rhythmicity center in the brainstem detects increased carbon dioxide and increases the respiratory rate to eliminate the excess. The lungs release carbon dioxide into the air during exhalation. In a review article published in "The Journal of Physiology," Dr. Jerome Dempsey and his colleagues note that blood carbon dioxide level is a critical factor in the control of breathing during sleep.
Blood Oxygen Level
Blood oxygen content is a secondary influence on respiratory rate. Normally, the blood oxygen level is 80 to 100 millimeters of Hg. Respiratory rate is stimulated if it drops below 50. A blood oxygen level below 50 is extremely low, which is why this respiratory control is of secondary importance compared with other controls.
Blood pH
Sensors called the aortic and carotid bodies detect changes in blood pH. The lungs and kidneys collaboratively control blood pH. An abnormally low blood pH increases the respiratory rate. Rapid breathing releases increased amounts of carbon dioxide, a major determinant of blood pH. If excessive carbon dioxide is expelled through rapid breathing--such as occurs if you hyperventilate--you feel dizzy and may faint. Breathing into a paper bag alleviates this feeling because you rebreathe carbon dioxide, bringing the blood level back to normal. How quickly you become dizzy with hyperventilation demonstrates the body's exquisite sensitivity to sudden changes in blood pH and carbon dioxide content.
Drugs and Alcohol
Certain prescription drugs, such as narcotic pain relievers and sedatives, and the illicit drug heroin, can depress the activity of the respiratory rhythmicity center in the brainstem. High levels of alcohol have the same effect. In the article, "Alcohol and the Nervous System" published in the "Journal of Neurology, Neurosurgery, and Psychiatry," Drs. McIntosh and Chick note that acute alcohol poisoning can require hospitalization and respiratory support. Drug and alcohol-related depression of respiratory rate can cause respiratory arrest, the most common cause of death in alcohol poisoning and narcotic drug overdose.
References
- University of Virginia School of Medicine: Ventilation
- McGraw-Hill Higher Education: Respiratory Physiology
- Eastern Kentucky University: Human Physiology--Respiration
- GlobalRPh.com: Arterial Blood Gases
- Journal of Physiology: The Ventilatory Responsiveness to CO2 below Eupnoea as a Determinant of Ventilatory Stability in Sleep
