The rate and depth of breathing is regulated primarily by carbon dioxide levels. Poor ventilation leads to increases in blood carbon dioxide and decreased blood pH. Chemoreceptors in the aortic arch and carotid arteries monitor blood pH and carbon dioxide levels. Chemoreceptors also respond to very low levels of oxygen in the blood.
Signals from the chemoreceptors go to respiratory centers in the brainstem that increase the rate and depth of breathing (ventilation). Signals are sent through the phrenic nerve to the diaphragm. Stretch receptors in the walls of the lungs limit how much the thorax can expand.
The pons (in the brainstem) receives input from the medulla oblongata. This brain center coordinates smooth inspiration and expiration. Higher brain centers can voluntarily over-ride the automatic rhythm of breathing when we are conscious.
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