Action of carbon dioxide receptor cells
The respiratory center is sensitive to increased carbon dioxide levels and increased acidity of the blood. It is peculiar that even though metabolism requires oxygen yet the respiratory centre is not very sensitive to decreased oxygen levels. However, chemo-receptors located in carotid bodies and aortic arches near the heart respond to decreased Po2 levels by increasing the depth and frequency of breathing. Figure depicts the control process graphically.
Figure: Action of carbon dioxide receptor cells
In contrast aquatic animals that breathe through gills are very sensitive to decreased oxygen levels. respiratory movement in many specie increases if oxygen levels in surrounding medium fall. In amphibia the respiration in tadpoles is regulated by oxygen levels and in adult frogs the mechanism is controlled by carbon dioxide levels.