Lung Volumes in Respiratory Physiology

Respiratory physiology uses both lung volumes and capacities to asses lung function. The main difference between a volume and capacity is that a capacity basically involves the sum of more than one volume. The major lung volumes and capacities include total lung capacity, vital capacity, forced vital capacity, tidal volume, residual volume, expiratory reserve volume, inspiratory reserve volume, functional residual capacity, inspiratory capacity, anatomical dead space, and physiologic dead space. We will discuss the meanings of the volumes involved in respiratory physiology.

Tidal volume is the volume of air that is breathed in and out (inspired and expired) during normal breathing. Thus the name "tidal" (since its like the waves of a tide when graphed). Inspiratory reserve volume (IRV) is the volume of air that can be inspired after the tidal volume. So, essentially this is the amount of air that you can breathe in on top of the tidal volume. Expiratory reserve volume (ERV)? is the volume of air that can be expired after breathing out a normal breath of air. Residual volume is air that will stay in the lungs even after you have breathed out all the air possible.

Besides these volumes, another important concept is that of dead space. This refers to air that is not involved in gas exchange (physiologic dead space) or air that stays in the tubes or bronchioles of the lungs (anatomic dead space).

These meaurements are an essential part of determining an individual lung function. The capacities and volumes along with additional data can be plotted on a graph and help diagnose problems such as COPD or Asthma. For example, someone with COPD may have a marked decrease in their forced expiratory volume (a classic way to diagnose COPD).