Physiological dead space is best described as which of the following?

• A. From the nares to the bronchioles and includes the volume of air in the alveoli that cannot participate in gas exchange
• B. From the alveolar sacs to the alveoli that do not participate in gas exchange
• C. From the nares to the bronchioles and includes the volume of air in the alveoli that participates in gas exchange
• D. From the trachea to the bronchioles

Answer: (A)

Physiological dead space is the portion of the tidal volume that does not participate in gas exchange. It includes two components: anatomic dead space, which is the air in the conducting airways from the nose down to the end of the bronchioles, and alveolar dead space, which is air in ventilated but poorly perfused alveoli that cannot exchange gases. The best description is the one that covers the conducting airways up to the bronchioles and also notes the alveolar air that cannot participate in gas exchange. This captures both parts of dead space. The other options either describe only alveolar dead space, or only the conducting airways without acknowledging the alveolar portion, or imply that alveoli present gas exchange when they do not.