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 <WSD>volume</WSD> of inhalation per breath , the flow of air from both lobes , and the pressure of the two lobes with respect to each other . 
Meaning of the word marked with <WSD> tags:  the amount of 3-dimensional space occupied by an object