Residual lung volume in a patient

When we breathe in, we increase the volume of our lungs. When we breathe out, we decrease the volume of our lungs. However, the volume of our lungs never goes to zero. There is always air in the lungs. The amount of air left in the lungs after you breath out is called “residual lung volume”.

A doctor wants to measure residual lung volume in a patient. In order to do this, she has a subject breathe in air with a gas X that is insoluble in blood. The subject takes a breath of air with gas X mixed into it. She measures how much air he breathes in. The subject then holds his breath for five seconds, allowing for gas X to mix with the residual air, and then breathes out. She measures the partial pressure of gas X in the air he breathes out. The results are shown below:

Partial pressure of gas X in inspired air: 18 mmHg

Inspirational volume: 700 mL

Partial pressure of gas X in expired air: 6 mmHg

The physician understood that the dilution factor would equal the total volume divided by the volume of inspired air.