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### NS FL #2 Biology Q 3

Posted: Tue Jan 29, 2019 8:20 am
Hi! I was confused between choices C and D for this question. I completely understand Choice C---how 15 ml/kg is the residual volume i.e, minimum lung volume @ max intrapleural pressure. However, I was inclined to answer D because the questions asks "why" lung volume never drops below this so-called residual volume. To me it seemed like choice C was merely defining the residual volume pertaining to the question, but D was providing an explanation for/hitting at the "why" aspect of there being a minimum lung volume.

I read the explanation and was confused by what this meant: "This is true because if air was forcibly removed from the lungs, below a certain point, the lungs would collapse under intrapleural pressure. However, this will not occur spontaneously from exhaling with too much force. "

Would appreciate any clarification. Thanks!

### Re: NS FL #2 Biology Q 3

Posted: Tue Jan 29, 2019 9:09 am
Thanks for the question! You are right that D is a correct statement. And I think the way of approaching it with the "why" in mind makes good sense. The way I look at it is that D addresses why there is a residual volume at all, but not necessarily (or not specifically enough) at why that 15 ml/kg is reached.

In other words, C explains precisely why 15 ml/kg is the residual volume -- because, at max pressure, this is how much volume is left in the lungs. D explains what would happen if we ventured below that (and gives a historical/evolutionary mechanism for the existence of residual volume) but sort of assumes or accepts or takes it as 15 ml/kg. It's a subtle distinction, but does that make sense?

Keep up the hard work!

### Re: NS FL #2 Biology Q 3

Posted: Tue Jan 29, 2019 9:15 am
Ah! Tricky, but yes, I see it. If the question asked "why it never goes below a certain volume" as opposed to 15ml/kg then, D would probably be the way to go, correct?
Thanks so much!!

### Re: NS FL #2 Biology Q 3

Posted: Tue Jan 29, 2019 11:47 am
Potentially, yes! I think it'd be right especially if the question wanted an evolutionary reason or a population-level reason. As it is, it's more about the mechanism leading to the residual volume itself rather than the reasons underlying that residual volume... if that makes sense!

You're very welcome!