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 -- a reflection of its relative acidity or alkalinity -- also influences respiratory rate. Activity level and the presence of drugs or alcohol are other important factors that affect respiratory rate.
Brainstem Rhythmicity Center
Breathing usually takes place outside of your conscious awareness. The rhythmicity center in the brainstem controls this function. Within this center are so-called I nerve cells that control inspiration and E nerve cells that control exhalation. The I and E nerve 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 -- the cessation of breathing.
Cerebral Cortical Input
As anyone who has ever blown out a candle or practiced yoga knows, 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. Strokes in certain areas of the cerebral cortex and conditions that depress a person's level of consciousness can interfere with the voluntary control of respiration.
Blood Carbon Dioxide
The amount of carbon dioxide in the blood exerts a strong 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. The blood carbon dioxide level is a critical factor in the control of breathing during sleep.
Blood oxygen content exerts a secondary influence on respiratory rate. Normally, the blood oxygen level is 80 to 100 mmHg. 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 to other mechanisms of respiratory rate regulation.
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 might feel dizzy or 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 street drug heroin can depress the activity of the respiratory rhythmicity center in the brainstem. A high blood alcohol level has the same effect. People with acute alcohol poisoning frequently must be hospitalized to mechanically support their breathing. 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.