[portugal (OP)]

 - why would there be a difference between the predicted Vital capacity of lungs and calculated on by spirometry?
-why cant residual volume be deterimed by spirometry? how can this be measured?


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[abrooks051]

Disease states and lung damage can cause a difference between predicted and obtained results.
Residual volume, "the first breath, only leaves the lungs when the person expires and not completely even then. It is measured by body plethysmography. You are placed in a sealed box where the volume and pressure around and within your body is measured to measure residual volume. Hope this helps.

[alancalverd]

If the result always equalled the prediction, there would be no point in making measurements, taking exams, or holding elections. The difference between expectation and observation leads to further knowledge - in this case, a possible diagnosis.

If the difference is zero, the diagnosis is usually the physiological certainty known as "death", the academic or sporting  certainty known as "cheating", or the political certainty known as "totalitarianism".