Dead space in lungs refers to the volume of air that is inhaled but does not participate in gas exchange because it either remains in the conducting airways or reaches alveoli that are not perfused or poorly perfused). There are two types of dead space:

• Anatomical dead space: This is 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 anatomical dead space is 150 mL).

• Physiological dead space: This is equal to anatomical dead space 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, physiological dead space is equivalent to anatomical dead space).

An increase in the value of physiological dead space can be seen in lung disease states where the diffusion membrane of alveoli does not function properly or when there are ventilation/perfusion mismatch defects. Clinically, disease states and environmental factors, such as smoking, all play a major role in the increase of dead space. Increases in dead space can be seen in lung disease states including emphysema, pneumonia, and acute respiratory distress syndrome (ARDS) ).