Breathing is an involuntary process that is controlled by the medulla and pons of the brain stem. The frequency of normal, involuntary breathing is controlled by three groups of neurons or brain stem centers: the medullary respiratory center, the apneustic center, and the pneumotaxic center.owever, if alveolar Po2 is 60 mm Hg, then arterial blood will have a Po2 of 60 mm Hg, in which case the hypoxemia is severe enough to stimulate peripheral chemoreceptors in the carotid and aortic bodies. In turn, the chemoreceptors instruct the medullary inspiratory center to increase the breathing rate.