Lung volumes and capacities

The main job of the lungs is gas exchange, pulling oxygen into the body and getting rid of carbon dioxide.

Normally, during an inhale - the diaphragm contracts to pull downward, and chest muscles contract to pull open the chest to suck in air like a vacuum cleaner, and then during an exhale - the muscles relax, allowing the lungs to spring back to their normal size pushing that air out.

Now, we can use a spirometer to measure the volume of air that moves in and out of the lungs with each breath using an instrument called a spirometer; the test is called spirometry.

At this point there are more sophisticated electronic spirometers, but a classic example is having an air chamber submerged in water that the person can breathe into.

As they take air in, the chamber moves down into the water, which moves a pencil that traces as it moves, then when they breathe out, the chamber moves up and the pen moves down.

So, if this is a healthy adult woman, as she breathes the spirometer makes a wave-like tracing on the paper.

The plot you end up with therefore has volume of air on the vertical axis, and the horizontal axis shows time.

During normal, quiet breathing the volume of air moving in and out with each breath is represented by the height of the wave and it’s called the tidal volume; it’s typically around 0.5 L or 500 ml.

After a few cycles, we might ask the woman to inhale the maximum volume of air that she can, and then exhale the maximum volume of air that she can. The volume of air that she maximally inhales above the tidal volume is known as the inspiratory reserve volume, and it’s typically around 3 liters. This is sort of a like a massive backup capacity that you don’t typically use, but might need to in a specific situation like if you’re going for a dive in the ocean.

Similarly, the expiratory reserve volume is the volume of air that she maximally exhales below the tidal volume, and it’s typically around 1.2 liters.

Now, even after she attempts to exhale all the air from the lungs, it turns out that some air still remains in the lungs and this is known as the residual volume, and it’s typically around 1.2 liters as well.