Lung capacities

The lung capacities are the sum of different lung volumes at different points of the respiratory cycle and also give information about a person’s health. These can be calculated using lung volumes measured with a spirometer.

  • Inspiratory capacity (IC): it is the total amount of air that can be inspired. It corresponds to the sum of the TV and the IRV. IC = TV + IRV

  • Vital Capacity (VC): it is the total amount of air that can be expired after a deep inspiration. It is the sum of the IRV, the TV, and the ERV. VC = IRV + TV + ERV

  • Functional Residual Capacity (FRC): The amount of air that remains in the lungs after a normal expiration. It is the sum of the ERV and the RV. FRC = ERV + RV

  • Total Lung Capacity (TLC): It is the sum of all lung volumes. TLC = IRV + TV + ERV + RV

Age and general health factors such as asthma are factors affecting loss of elasticity of the lung tissue, weakness of the respiratory muscles, and reduction of the ribs, which influence the changes in lung capacities. Additionally, pregnancy affects the positioning of the diaphragm, which limits the expandability of the lungs.

This is an image of a pair of lungs. The lungs are divided into four parts colored differently to indicate the different lung volumes. From top to bottom: the first part, which is the biggest, is labeled as 3000 milliliters and it corresponds to the inspiratory reserve volume. The second part is labeled with 500 milliliters and it refers to the tidal volume. The third part is labeled as 1000 milliliters, which would be the expiratory reserve volume. And, finally, the last part is labeled as 1200 milliliters and it refers to the residual volume. A line imitating a respirometry analysis result is also shown inside the lungs. Bellow the lungs, small colored rectangles are the legend to match the colors in the lungs with the volumes they refer to.

Figure 1. Schematic illustration of a set of lungs and their volumes and capacities.