Module 4: Physiology of the Respiratory System - Lung Volumes.

LUNG VOLUMES: Introducing the spirometer:

The illustration on the right shows a simple water-sealed rolling drum spirometer. The drum rotates at a constant speed and a pen records the drum’s excursion during quiet, tidal breathing; deep inspiration and a full exhalation.

Note the following:

• At full inspiration the volume of air in the lungs is the total lung capacity.
• The total amount of air that can be exhaled from the lungs is the vital capacity.
• The amount of air left in the lungs at the end of exhalation is the residual volumes.
• This figure illustrates the subdivisions of lung volumes and compares them to recordings made on the rolling drum spirometer during quiet tidal breathing, deep inspiration and full exhalation.

In breathing the respiratory muscles must overcome the elastic recoil of the lung (which tends to collapse the lung) and the elastic recoil of the rib cage and chest wall (which tends to spring outwards). At full exhalation (Residual Volume) the dominant elastic force is the recoil of the chest wall. At full inspiration (Total Lung Capacity) the dominant force is the elastic recoil of the lungs. FRC or Functional Residual Capacity occurs when the elastic recoil forces of the chest and lungs are equal but in opposite directions.