The respiratory system is responsible for the intake of oxygen and the expulsion of carbon dioxide from the body. Respiratory volumes describe the volume of air in the lungs at different phases of the respiratory cycle. Tidal volume is the air breathed in and out during normal, quiet breathing. Inspiratory reserve volume is the air that can be forcefully inspired beyond the tidal volume. In contrast, expiratory reserve volume refers to the air that can be expelled from the lungs after a normal expiration. Residual volume, which keeps the alveoli open and prevents lung collapse, is the air in the lungs after a forceful exhale.
Respiratory Capacities
Respiratory capacities, such as inspiratory, functional residual, vital, and total lung capacities, are measured to diagnose respiratory disorders. The inspiratory capacity (IC) is the maximum amount of air that can be inhaled after a normal exhalation, calculated by adding the tidal volume (TV) and inspiratory reserve volume (IRV). The functional residual capacity (FRC) represents the air in the lungs after a quiet expiration. It is the combined residual volume (RV) and expiratory reserve volume (ERV). Vital capacity (VC) is the maximum amount of air that a person can exhale after taking the deepest breath possible and is the sum of TV, IRV, and ERV. Finally, the total lung capacity (TLC) is the sum of all lung volumes and provides crucial information about a person's respiratory status. In obstructive pulmonary diseases, TLC, FRC, and RV may increase because the lungs hyperinflate, whereas in restrictive diseases, VC, TLC, FRC, and RV decline because lung expansion is limited.
Minute Ventilation
Minute ventilation, or the total volume of air inspired and expired each minute, is calculated by multiplying tidal volume by the respiratory rate. In a healthy adult at rest, minute ventilation is typically around 6000-8000 mL/min. Lower-than-normal minute ventilation may indicate a problem with the lungs or respiratory system. Note that not all inhaled air reaches the respiratory zone of the respiratory system, as some of it remains in the conducting airways, known as the anatomic dead space.
Alveolar Ventilation
Alveolar ventilation is the volume of air per minute that reaches the respiratory zone and participates in external respiration. This measure is typically around 4200 mL/min in a healthy adult at rest. It is calculated by multiplying the respiratory rate by the difference between the tidal volume and the volume of air remaining in the anatomic dead space. Alveolar ventilation provides a more accurate assessment of gas exchange in the lungs than minute ventilation alone. Together, minute and alveolar ventilation are essential measures of pulmonary function that can help diagnose respiratory disorders and track treatment progress.
