Respiration is regulated by the levels of CO2, O2 and H+ ions in the blood. For example-
1. Carbon Dioxide: Chemoreceptor cells located in the aortic body, carotid body and medulla are stimulated by blood CO2 levels and stimulate respiration. An increase in blood CO2 pressure increases respiration rate and a decrease in CO2 levels decreases respiration rate.
2. Oxygen: When there is a lack or excess of O2 in the blood, the chemoreceptor cells of the aortic body and carotid body are stimulated and stimulate the respiratory center. When the blood lacks O2, the stimulus goes from the respiratory center to the lungs and increases the respiratory rate. Again, if there is an excess of O2 in the blood, stimulation depresses the respiratory center and respiratory rate decreases.
3. Hydrogen Ion: When the level of H+ ion in the blood increases, the respiration rate increases. Again, when the level of H+ ions in the blood decreases, the respiration rate decreases.