Lung volumes and capacities are one of those “easy points” on Step—if you can recall what combines with what under pressure. The fastest way to stop mixing them up is to anchor everything to a single picture in your head and one clean acronym that tells you the math.
The Mental Picture: “Stacked Blocks” in the Chest
Imagine your lungs as a vertical stack of 4 blocks (bottom → top):
- RV (Residual Volume): air you can’t blow out
- ERV (Expiratory Reserve Volume): extra you can blow out after a normal exhale
- TV (Tidal Volume): normal breath in/out
- IRV (Inspiratory Reserve Volume): extra you can inhale after a normal inhale
If you can picture the stack, the capacities become “which blocks are included.”
The Acronym Trick: TIRE-V (pronounced “tire-ve”)
Think: TIRE is what you move; V is what you can’t see leave.
Volumes = T I R E
- T = TV (Tidal Volume)
- I = IRV
- R = RV
- E = ERV
Capacities = combos built from the same letters
Use the “build rules” below.
One-Liner Definitions (Ultra-Quick Recall)
Volumes (single blocks)
- TV: air moved in a normal breath
- IRV: extra air inhaled after a normal inhalation
- ERV: extra air exhaled after a normal exhalation
- RV: air remaining after maximal exhalation (can’t be exhaled)
Capacities (stacked blocks)
- IC (Inspiratory Capacity): what you can inhale starting at end-expiration
- FRC (Functional Residual Capacity): what’s left after a normal exhale
- VC (Vital Capacity): what you can blow out after maximal inhale
- TLC (Total Lung Capacity): everything in the lungs after maximal inhale
The High-Yield Table (with the exact “add these blocks” rules)
| Term | What it includes | Formula |
|---|---|---|
| TV | normal breath | — |
| IRV | extra inhale after normal inhale | — |
| ERV | extra exhale after normal exhale | — |
| RV | leftover after forced exhale | — |
| IC | TV + IRV | IC = TV + IRV |
| FRC | ERV + RV | FRC = ERV + RV |
| VC | IRV + TV + ERV | VC = IRV + TV + ERV |
| TLC | VC + RV (all 4 blocks) | TLC = VC + RV = IRV + TV + ERV + RV |
Visual/Mnemonic Device: “FRC is the Floor”
If the block-stack is your lung, then:
- FRC sits at the “floor level” after a normal exhale
- And FRC = ERV + RV (the two bottom blocks)
That’s why FRC changes a lot with things that affect resting lung/chest wall mechanics (like emphysema or obesity).
The Single Most-Tested Fact: What Spirometry Can’t Measure
Spirometry cannot measure anything that includes RV
So it cannot directly measure:
- RV
- FRC (because it includes RV)
- TLC (because it includes RV)
Measured by spirometry: TV, IRV, ERV, IC, VC
Not measured by spirometry: RV, FRC, TLC
(Those require helium dilution or body plethysmography—Step 1 classic.)
USMLE High-Yield Clinical Patterns (Volumes/Capacities)
Obstructive disease (e.g., emphysema/COPD)
Air trapping → harder to exhale fully:
- ↑ RV
- ↑ FRC
- ↑ TLC (often increased)
- ↓ VC (often decreased due to trapping)
Quick reason: loss of elastic recoil + airway collapse → more air left behind.
Restrictive disease (e.g., pulmonary fibrosis)
Small, stiff lungs:
- ↓ TLC (hallmark)
- ↓ FRC
- ↓ RV (often)
- ↓ VC
Quick reason: reduced lung compliance → can’t expand to normal volumes.
Lightning Round: Exam-Style One-Liners
- “RV is the air you can’t get out.”
- “FRC is what’s left after a normal breath out.”
- “VC is the max you can move (out) between full in and full out.”
- “If it contains RV, spirometry won’t measure it.”
- “Obstructive traps air (↑RV/↑FRC). Restrictive shrinks total volume (↓TLC).”