Respiratory time constant defines the rate at which the lung deflates during passive exhalation. A true measurement of static compliance requires installing a known volume of gas into the lung and the measuring airway pressure at equilibrium in the absence of respiratory muscle activity. In the normal infant, dynamic compliance should be equal to static compliance. Lung compliance changes with volume history, meaning that it decreases with fixed tidal volumes and increased after deep breaths that recruit air spaces that may have been poorly ventilated or atelectatic. The periodic sigh in spontaneous breathing is associated with an increase in lung compliance and in oxygenation.
Dynamic compliance decreases as respiratory rate increases.
The primary function of the circulation is to pick up oxygen from the lungs and deliver it to the tissues, and then pick carbon dioxide from the tissues and deliver it to the lungs. The exchange of oxygen and carbon dioxide with the blood occurs within the alveolar volume of the lungs. The alveolar volume acts as a large sink from which oxygen is continuously extracted by the blood and to which carbon dioxide is continuously added. This mechanism for acquiring oxygen from the atmosphere and excreting carbon dioxide into the atmosphere is the alveolar ventilation. Since the conducting airways do not participate in gas exchange, they constitute the anatomic dead space.