Pulmonary ventilation can be studied by recording the volume movement of air into and out of the lungs, a method called spirometry. A typical basic spirometer is shown in Figure 1. It consists of a drum inverted over a chamber of water, with the drum counterbalanced by a weight. In the drum is a breathing gas, usually air or oxygen; a tube connects the mouth with the gas chamber. When one breathes into and out of the chamber, the drum rises and falls, and an appropriate recording is made on a moving sheet of paper.
Fig1. A spirometer.
Figure 2 shows a spirogram indicating changes in lung volume under different conditions of breathing. For ease in describing the events of pulmonary ventilation, the air in the lungs has been subdivided in this diagram into four volumes and four capacities, which are the average for a young adult man. Table 1 summarizes the average pulmonary volumes and capacities.
Fig2. A diagram showing respiratory excursions during normal breathing and during maximal inspiration and maximal expiration.
Table1. Average Pulmonary Volumes and Capacities for a Healthy, Young Adult Man
Pulmonary Volumes
To the left in Figure 2 are listed four pulmonary lung volumes that, when added together, equal the maximum volume to which the lungs can be expanded. The significance of each of these volumes is the following:
1. The tidal volume is the volume of air inspired or expired with each normal breath; it amounts to about 500 milliliters in the average adult male.
2. The inspiratory reserve volume is the extra volume of air that can be inspired over and above the normal tidal volume when the person inspires with full force; it is usually equal to about 3000 milliliters.
3. The expiratory reserve volume is the maximum extra volume of air that can be expired by forceful expiration after the end of a normal tidal expiration; this volume normally amounts to about 1100 milliliters.
4. The residual volume is the volume of air remaining in the lungs after the most forceful expiration; this volume averages about 1200 milliliters.
Pulmonary Capacities
In describing events in the pulmonary cycle, it is sometimes desirable to consider two or more of the volumes together. Such combinations are called pulmonary capacities. To the right in Figure 2 are listed the important pulmonary capacities, which can be described as follows:
1. The inspiratory capacity equals the tidal volume plus the inspiratory reserve volume. This capacity is the amount of air (about 3500 milliliters) a person can breathe in, beginning at the normal expiratory level and distending the lungs to the maximum amount.
2. The functional residual capacity equals the expiratory reserve volume plus the residual volume. This capacity is the amount of air that remains in the lungs at the end of normal expiration (about 2300 milliliters).
3. The vital capacity equals the inspiratory reserve volume plus the tidal volume plus the expiratory reserve volume. This capacity is the maximum amount of air a person can expel from the lungs after first filling the lungs to their maximum extent and then expiring to the maximum extent (about 4600 milliliters).
4. The total lung capacity is the maximum volume to which the lungs can be expanded with the greatest possible effort (about 5800 milliliters); it is equal to the vital capacity plus the residual volume.
All pulmonary volumes and capacities are usually about 20 to 25 percent less in women than in men, and they are greater in large and athletic people than in small and asthenic people.
