Because an animal’s need for O₂ varies with activity level, animals adjust their respiratory rate to meet their cells’ changing demand for O₂. Respiration is a unique physiological process in that it is controlled by both the voluntary and involuntary components of the nervous system (Chapter 35). In sleep, breathing is maintained at a resting rate by the involuntary part of the nervous system. Indeed, in most circumstances, breathing is controlled unconsciously.

The regulation of blood O₂ levels is a key example of homeostasis, like core body temperature (Chapter 35) and blood-glucose levels (Chapter 38). Recall that homeostasis often depends on sensors that monitor the levels of the chemical being regulated. In the case of breathing, the sensors are chemoreceptors located within the brainstem and in sensory structures called the carotid and aortic bodies that are located in the neck and near the heart (Fig. 39.10). The carotid bodies sense O₂ and proton (H+) concentrations of the blood going to the brain, and the aortic bodies monitor their levels in blood moving to the body. In contrast, chemoreceptors in the brainstem sense CO₂ and H+ concentrations. The most important factor in the control of breathing is the amount of CO₂ in the blood. If the concentration of CO₂ in the blood is too high, chemoreceptors in the brainstem stimulate motor neurons that activate the respiratory muscles to contract more strongly or more frequently. Stronger or faster breathing rids the blood of excess CO₂ and increases the supply of O₂ to the body.

Breathing can also be controlled voluntarily. It is a simple matter to choose to hold your breath. Holding the breath makes it possible for humans and marine mammals to dive under water; it is also critical to the production of speech, song, and sound for communication. Sound is produced by voluntarily adjusting the magnitude and rate of airflow over the vocal cords of mammals, the syrinx of songbirds, or the glottal folds of calling amphibians, such as some toads and frogs.