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The subject then ( Figure 3–7B) breathes in all 500 mL of the gas.

Initially ( Figure 3–7A), there is no test gas in the subject’s airways or lungs. A subject breathes in from a balloon filled with 500 mL of a test gas such as helium (mixed with oxygen) that is not taken up by or liberated from the pulmonary capillary blood. The anatomic dead space is illustrated in Figure 3–7. During exercise, the FRC may be lower than the relaxation volume because of active contraction of the expiratory muscles. Either or both of these may occur in babies, who have higher FRCs than would be predicted from the great inward elastic recoil of their lungs and the small outward recoil of their chest walls. The FRC may be greater than the relaxation volume if the next inspiration occurs before the relaxation volume is reached, either because of high breathing rates or high resistance to expiratory airflow in the larynx or peripheral airways, or because of active contraction of the inspiratory muscles at end expiration. Thus, the lung volume at which the inward elastic recoil of the lungs is equal and opposite to the outward elastic recoil of the chest wall is sometimes referred to as the relaxation volume of the respiratory system. However, the respiratory muscles may have significant tone at the FRC, and in certain circumstances, the FRC may be greater than or even less than the lung volume of the totally relaxed respiratory system.