Which statement best defines physiologic dead space?

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Dead space is the portion of each breath that does not participate in gas exchange. Physiologic dead space includes both the air in the conducting airways (anatomic dead space) and the air in alveoli that are ventilated but not perfused (alveolar dead space). Therefore, physiologic dead space is the sum of anatomic and alveolar dead spaces. This differs from total lung volume, which is a measure of how much air the lungs can hold, not how much air is unused for gas exchange. In healthy lungs, alveolar dead space is small, but it can increase with conditions that reduce pulmonary perfusion, such as a pulmonary embolism.