Dead space represents the volume of ventilated air that does not participate in gas exchange. The two types of dead space are anatomical dead space and physiologic dead space. Anatomical dead space is represented by the volume of air that fills the conducting zone of respiration made up by the nose, trachea, and bronchi. This volume is considered to be 30% of normal tidal volume (500 mL); therefore, the value of anatomic dead space is 150 mL. Physiologic or total dead space is equal to anatomic plus alveolar dead space which is the volume of air in the respiratory zone that does not take part in gas exchange. The respiratory zone is comprised of respiratory bronchioles, alveolar duct, alveolar sac, and alveoli. In a healthy adult, alveolar dead space can be considered negligible. Therefore, physiologic dead space is equivalent to anatomical. One can see an increase in the value of physiologic dead space in lung disease states where the diffusion membrane of alveoli does not function properly or when there are ventilation/perfusion mismatch defects.