Despite the involuntary nature of most respiratory reflexes, it is quite obvious that there are also considerable voluntary control of breathing through respiratory muscle movements. Voluntary control is accomplished by descending pathways from the cerebral cortex to the motor neurons of the respiratory muscles. This voluntary control of respiration cannot be maintained when the involuntary stimuli, such as an elevated PCO2 or H+ concentration, become intense. An example is the inability to hold one’s breath for very long. The opposite of breath-holding—deliberate hyperventilation —lowers alveolar and arterial PCO2 and increases PO2.
Unfortunately, swimmers sometimes voluntarily hyperventilate immediately before underwater swimming to be able to hold their breath longer. We say "unfortunately" because the low PCO2 may still permit breath-holding at a time when the exertion is lowering the arterial PO2 to levels that can cause unconsciousness and lead to drowning. Besides the obvious forms of voluntary control, respiration must also be controlled during such complex actions as speaking, singing, and swallowing.