How To Calculate Expiratory Reserve

Your breath and your heartbeat are more synonymous with life in the moment than any other physical processes you enjoy; alone, these are the stuff of countless poems, meditative practices and other human indulgences. Your lungs are essentially inflatable reservoirs tasked with drawing air into the body, extracting oxygen from it while offloading carbon dioxide, and continuing this work uninterrupted for decades on end.

Medical types have ways of measuring how well your lungs are performing, both in relation to others who are the same age and sex, and compared to other tests you have have taken along the way. One of the quantities measured by such a ​pulmonary function test (PFT)​ is ​expiratory reserve volume (ERV)​, which is the amount of air you could theoretically blow out after you have already exhaled in a normal (non-forced) way, as you've presumably been doing while reading this article. A variety of factors can affect ERV and other measures of pulmonary health.

PFTs are administered when health professionals suspect an underlying respiratory disorder, or are aware of one and are tracking its progress and the effectiveness of any treatments being given the taker of the tests.

The following parameters describing the function of the lungs are most easily understood with the help of a diagram.

​Volumes​: The total capacity of your lungs is, aptly, called the total lung capacity (TLC). This is the sum of the residual volume (RV), which is a small amount air you can't expel no matter how hard you try; the ERV, which as stated above is the amount of air you could push out after already exhaling normally; the tidal volume (TV), or the amount of air you draw in and out with a normal breath; and the inspiratory reserve volume (IRV), which is the counterpart of ERV at the "fuller" end, or the amount if air you could take in even after inhaling normally.

​Flow rates:​ When you take a PFT, you are asked to do things such as breathe as hard as you can for a short period of time or blow out as much air as you can over a span of five to ten seconds or however long it takes you to become as "deflated" as possible. The results provide information about the structural integrity of your lungs, which are made not of muscle but of tubes and specialized, highly vascular (i.e., well supplied with blood) tissue.

FEV₁, for example, measures the amount of air you can blow out in the first second of all-out exhaling. A number if similar parameters exist, such as peak expiratory flow rate (PEFR), which is the maximal speed that you can force air from your lungs in units of volume per time (e.g., liters per second).

Putting all of the above together, you can see that

\(TLC = RV + ERV + TV + IRV\)

A different form of this, which emphasizes the residual volume formula, is

\(RV = TLC − IRV − TV − ERV\)

Likewise, the expiratory reserve volume is given by:

\(ERV = TLC − IRV − RV − TV\)

Your ERV will be higher when you are standing than when you are sitting. This makes intuitive sense, since people and other animals are generally upright when moving and need to have access to as much air as possible while exercising at any intensity.

Contrary to popular belief, lung capacity is a genetic trait and cannot be increased via exercise in generally healthy persons. Instead, it is a function of your age, sex and height, and to some extent your ethnic background. A list of normal values for age and sex can be found in the Resources.