When muscles contract during exercise, they consume large amounts of O2 and produce large amounts of CO2. During vigorous exercise, O2 consumption and pulmonary ventilation both increase dramatically. At the onset of exercise, an abrupt increase in pulmonary ventilation is followed by a more gradual increase. With moderate exercise, the increase is due mostly to an increase in the depth of ventilation rather than to increased breathing rate. When exercise is more strenuous, the frequency of breathing also increases.
The abrupt increase in ventilation at the start of exercise is due to neural changes that send excitatory impulses to the inspiratory area in the medulla oblongata. These changes include (1) anticipation of the activity, which stimulates the limbic system; (2) sensory impulses from proprioceptors in muscles, tendons, and joints; and (3) motor impulses from the primary motor cortex (precentral gyrus). The more gradual increase in ventilation during moderate exercise is due to chemical and physical changes in the bloodstream, including (1) slightly decreased pCO2, due to increased O2 consumption; (2) slightly increased pCO2, due to increased CO2 production by contracting muscle fibers; and (3) increased temperature, due to liberation of more heat as more O2 is utilized. During strenuous exercise, HCO3- buffers H+ released by lactic acid in a reaction that liberates CO2, which further increases pCO2.