General

Lung volumes and Capacities:- Brief Explanation

Lung volumes refer to the amount of air that is inhaled or exhaled or left in the lungs after forced respiration. Respiration takes place only in the alveoli and the alveolar duct. So, there is no gaseous exchange in the respiratory tract such as the mouth and trachea. This respiratory passage is the anatomical dead space (about 150 ml). Alveolar dead space refers to the alveolar duct where gaseous exchange does not take place. Alveolar dead space is negligible in healthy people.

Lung Volumes

  • Tidal Volume (TV):- The total amount of air that passes in and out of the lungs during each breathing cycle (about 500 ml at rest).
  • Inspiratory Residual Volume (IRV):- The extra volume of air that can be inhaled during maximal inspiration.
  • Expiratory Residual Volume (ERV):- The extra volume of air that can be exhaled during maximal expiration.
  • Residual Volume (RV):- The amount of air that remains in the lungs after maximal expiration. If the lungs are empty then the tissues of the lungs will adhere to each other.  So, the lungs will collapse. Thus, it helps to prevent the lungs from collapsing. Besides, the direct measurement of residual volume is not possible.

Lung Capacities

  • Inspiratory Capacity (IC):- It is the sum of tidal volume and inspiratory reserve volume. In other words, the total volume of air that can be inspired during maximal inspiration is the inspiratory capacity.
  • Functional Residual Capacity (FRC):- It is the sum of expiratory residual volume and the residual volume. In other words, the total amount of air that remains in the lungs after a quiet expiration is the functional residual capacity.
  • Vital Capacity (VC):- Maximum volume of air that passes in and out of the lungs during force breathing.

Mathematically,

VC= TV + IRV + ERV

  • Total Lung Capacity (TLC):- Maximum amount of air, lungs can hold in a single breathing cycle is the total lung capacity. It is normally around 6 liters.

Mathematically,

TLC = VC + RV

      = (ERV + IC) + RV

      = ERV + TV + IRV + RV

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