Respiration is controlled by three factors that sense the need for the body's increased or decreased requirement for oxygen. These are the concentration of oxygen (O2), carbon dioxide (CO2), and hydrogen (H+) ion concentration in the blood.
Throughout the body chemoreceptors sense the concentration of oxygen, carbon, and carbon dioxide and then send a message to the central chemoreceptors located in the medulla near the respiratory center of the brain and through cere-brospinal fluid to respond to changes.
When an increase in carbon dioxide is detected and there is an increase in hydrogen ions, the message goes out to increase ventilation. Hydrogen ions are measured using the pH scale. The pH of normal blood is between 7.35 and 7.45. A pH lower than 7.35 means the blood is acidic and a pH higher than 7.45 means pH is alkaline. The chemoreceptors respond to an increase in CO2 and a decrease in pH by increasing ventilation. If the CO2 level remains elevated, the stimulus to increase ventilation is lost.
There are chemoreceptors located in carotid arteries and aortic arteries that monitor changes in oxygen pressure (PO2) levels in the arteries. These are called peripheral chemoreceptors. Once the oxygen pressure falls below <60 mmHg, the peripheral chemoreceptors send a message to the respiratory center in the medulla to increase ventilation.
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