# TBR Chemistry CH 6 passage VI #40

This forum made possible through the generous support of SDN members, donors, and sponsors. Thank you.

#### happily123

##### Full Member
7+ Year Member
Can someone please explain this question to me?

Given in the passage: "there is the tidal volume, which is the volume of gas inspired or expired in a normal breath."

For lungs with a tidal volume of 400ml and a total volume following normal expiration of 1200ml. How does internal pressure change to cause inspiration?

A. it increases 33%
B. it increases 25%
C. it decreases 25%
D. it decreases 33%

the answer is C

Members don't see this ad.

Pressure change is a linear relationship.

We know that to inhale, the internal pressure must be lower than the atmosphere - so that rules out A and B.

If the tidal volume is 400 cc and the volume before inhalation (residual volume) is 1200 cc, when we inhale - the total volume goes to 1600 cc.

So we are exacting a change of 400 cc/1600 cc = 25%.

Last edited:
Pressure change is a linear relationship.

We know that to inhale, the internal pressure must be lower than the atmosphere - so that rules out A and B.

If the tidal volume is 400 cc and the volume before inhalation (residual volume) is 1200 cc, when we inhale - the total volume goes to 1600 cc.

So we are exacting a change of 400 cc/1200 cc = 25%.

isn't 400/1200 33%?

isn't 400/1200 33%?

Yes, but that is a mistype - sorry.

The line right before I wrote 1600 cc.

Members don't see this ad :)
Yes, but that is a mistype - sorry.

The line right before I wrote 1600 cc.

ohh ok thank you!

1 user
It may seem counterintuitive (or even arbitrary) to compare the 400 to 1600, instead of 1200. Here is an alternate explanation:

P1 x V1 = P2 x V2

You know the new volume V2 is (4/3 V1). Therefore the new pressure P2 must be (3/4 P1).

A drop in pressure from P1 to 3/4 P1 is a drop of 25%.

1 user