# FRC vs. RV : Respiratory system

#### SaintJude

I'm confused b/w definitions. Kaplan says:

Residual volume = certain volume of air that can never be removed from healthy lungs during normal breathing process

Then what is FRC & ERV ? I know FRC = ERV + RV...

TLC = ERV + VC
VC = TV + ERV + IRV

OP
S

#### SaintJude

The functional residual capacity is the air left in after a regular breath (tidal volume). Then the residual volume is the air that left in after a forceful breath out, a "vital capacity" breath. That's why the FRC includes the RV.

#### MedPR

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i'm confused b/w definitions. Kaplan says:

Residual volume = certain volume of air that can never be removed from healthy lungs during normal breathing process

then what is frc & erv ? I know frc = erv + rv...

Tlc = erv + vc
vc = tv + erv + irv
Your equations for TLC and VC aren't right. TLC=IRV+ERV+TV+RV and VC=IRV+TV+ERV so TLC=VC+RV.

The functional residual capacity is the air left in after a regular breath (tidal volume). Then the residual volume is the air that left in after a forceful breath out, a "vital capacity" breath. That's why the frc includes the rv.
TV=Tidal Volume=Volume inspired or expired in a quiet breath.

IRV=Inspiratory Reserve Volume=Additional amount that can be inspired forcefully after a TV inspiration.

ERV=Expiratory Reserve Volume=Amount that can be forcefully expired after a TV expiration.

RV=Residual Volume=Amount remaining after the most forceful expiration.

IC=Inspiratory Capacity=TV+IRV=Amount that can be inspired after an ERV expiration.

FRC=ERV+RV=Amount leftover after a normal expiration.

VC=Vital Capacity=IRV+TV+ERV=Max amount that can be expired after a max inspiration.

TLC=Total Lung Capacity=IRV+ERV+TV+RV

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#### MedPR

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Is this required for the MCAT? I didn't learn it until now in physio.

OP
S

It's in Kaplan

#### MedPR

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It's in Kaplan
Oh I've had a hard time with equations all throughout physio even though they're usually pretty simple. Keeping the terms straight is tough for me.

#### Morsetlis

##### I wish I were a dentist
7+ Year Member
Kaplan overteaches you. Why you care about lung volumes in General Bio I will never know.

FRC + ERV is not a useful volume or capacity. It duplicates the ERV, so you end up with 2ERV + RV. How on earth is that useful?

In terms of lungs, you only care about the concept that exhalation without stress is passive. The alveoli recoil, and the air is pushed out following a pressure gradient. When all muscles are relaxed, you are at FRC. The normal breathing uses TV. The maximum amount of breathing you can do to move the maximum amount of air in/out of your lungs (ventilation) is VC. Further than that is too complicated for the MCAT.

#### MedPR

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This would make for a difficult passage imo. So many terms to keep track of.

#### pm1

This would make for a difficult passage imo. So many terms to keep track of.
agreed.

#### Morsetlis

##### I wish I were a dentist
7+ Year Member
This would make for a difficult passage imo. So many terms to keep track of.
That's the whole point. You don't need to know it, but after being exposed to it they expect you to manipulate the information logically.