- It could be the tidal volume because it affects how a person inhales and exhales normally.
- It could be the residual volume and functional residual volume, because it increases its amount. Because it traps air.
- On the other hand it also affects the Forced vital capacity because it decreases the amount of air that can be forcibly exhaled immediately after one inhalation.
To learn obstructive/restrictive lung diseases, I find it easiest to think in extremes at first, with vivid descriptions of why. So in obstructive lung diseases, like COPD/emphysema:
Total volume increases because so many of the walls of the alveoli have been destroyed, they are like giant floppy bags instead of nice firm bubbles. There's more empty space, and total volume is therefore increased.
Reserve Volume and End Respiration Volume increase because these alveoli are large and stretched and floppy and can't push as much air out, even when you try. (Decreased elastic recoil.) They also flop shut often before the air is expelled - trapping air in.
Tidal volume may not change at all, but it takes longer to exhale than with normal lungs. That is why FEV1 and FVC are lower, for the same reason.