Respiratory system: Lung volume studies

A doctor is studying respiratory function in a group of male patients. The doctor begins by comparing the respiratory rates of the males with their inspiratory volumes. Participants are hooked up to a machine that measures both of these parameters. Inspiratory volume is measured as the amount of air that enters the lungs with each breath the person takes. Respiratory rate is measured as the number of breaths per minute.

Figure 1 Inspiratory volume vs. respiratory rate

The doctor then measures residual lung volume in the individual patients. Each patient is asked to breathe in air mixed with gas X (a gas that is insoluble in blood). Patients first expire as much air as possible; they then take their deepest breath of air mixed with gas X, and the inspiratory volume is measured. After holding their breath for five seconds, allowing for gas X to mix with the residual air, they breathe out. The doctor measures the partial pressure of gas X in the exhaled air. The results for three patients are shown in Table 1.

Table 1 Patient data

The physician understands that the dilution factor for gas X equals the total lung volume divided by the volume of inspired air.