What is the difference between vital capacity forced vital capacity?

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What is the difference between vital capacity forced vital capacity?

VC = IRV + TV + ERV; it exists whether person tries to demonstrate it or not (i.e. measurable, but not measured)

FVC = that which the person actively demonstrates (i.e. measured)

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That's just my assumption.
 
The only thing that I can think of, besides what Phloston has said, is generally FVC is a little less than true VC. This is because during forced expiration the enormous positive pressure inside the chest compresses the bronchioles a little, leading to less than absolute maximum expulsion of air.
 
The only thing that I can think of, besides what Phloston has said, is generally FVC is a little less than true VC. This is because during forced expiration the enormous positive pressure inside the chest compresses the bronchioles a little, leading to less than absolute maximum expulsion of air.

Are you sure about that? Although, yes, you'd be increasing intrathoracic and -abdominal pressure during FVC, I wouldn't think the pressures are great enough to induce early-dynamic small airway closure. I would think that's only in pathologic states.
 
Yeah I definitely remember my physio prof saying that, just double checked the class slides too. However, I think he might've just been trying to lay down a principle, and point out a theoretical difference. I doubt it would have much practical implications.

Kind of like using creatinine clinically to estimate GFR, though its bound to be a little over-estimate (compared to inulin).
 
so on the step exam can we consider them as the same value? 4.8 liters is the volume of VC is 4.8 liters also the volume of FVC?
 
so on the step exam can we consider them as the same value? 4.8 liters is the volume of VC is 4.8 liters also the volume of FVC?

I would assume FVC = VC, for all practical purposes.

Unless it was a really tricky question trying to specifically test that concept of excessive force used in forcefully expiring leading to some amount of air remaining in the lung. The only time I saw a question on this though was on my physio respiratory block exam, by my prof lol.
 
Are you sure about that? Although, yes, you'd be increasing intrathoracic and -abdominal pressure during FVC, I wouldn't think the pressures are great enough to induce early-dynamic small airway closure. I would think that's only in pathologic states.

That's right. FVC is significantly different from VC (or "SVC''--slow vital capacity--as it's sometimes called) only in pathological states. Very likely due to dynamic airway obstruction.

http://www.ncbi.nlm.nih.gov/pubmed/17890480
http://www.ncbi.nlm.nih.gov/pubmed/9669830
 
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