Figure 1 shows how ventilation increases in proportion to oxygen consumption at different levels of steady-state exercise (e.g., on a bicycle). Above the lactate threshold, ventilation increases more than CO2 production, so arterial PCO2 decreases and arterial PO2 increases; hence, the respiratory system is able to meet the metabolic demands of the muscles at all levels of exercise, at least in normal humans. However, the mechanical and

VO2 and VCO2, mL/min, Multiples of resting level

FIGURE 1 Respiratory effects of progressively increasing levels of exercise. Alveolar ventilation increases in proportion to CO2 production until very high levels of exercise are reached (after the break in the curves), which maintains arterial blood gases at normal levels. However, at very high levels of exercise, lactic acid production decreases pH. This acidosis and possibly other factors provide an excessive stimulus for ventilation so PaCO2 decreases and PaCO2 increases.

muscular work of breathing and the efficiency of pulmonary gas exchange can limit gas exchange during disease.

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