Sentence:  In the lower center rack an 8 - channel recorder indicates the percentage of carbon dioxide and nitrogen from the upper and lower lobes of one lung , the total volume of inhalation per breath , the flow of <WSD>air</WSD> from both lobes , and the pressure of the two lobes with respect to each other . 
Meaning of the word marked with <WSD> tags:  a mixture of gases (especially oxygen) required for breathing; the stuff that the wind consists of