Respiratory System
Each time you inhale and exhale, you’re moving air in and out of the lungs. But the lungs are never fully empty — nor do you use all their capacity at once. They’re designed with reserves — so you can climb stairs, shout, or run without gasping for life. Understanding lung volumes and capacities helps explain ventilation, gas exchange, and disease states like asthma, COPD, and restrictive lung disease.
🫁 Lung Volumes — The Basic Building Blocks
There are four primary volumes measured during normal and forced breathing.
| Volume | Definition | Normal Value (Adult male) | Notes / Clinical Importance |
|---|---|---|---|
| Tidal Volume (TV) | Volume of air inhaled or exhaled in one quiet breath | ~500 mL | Increases during exercise |
| Inspiratory Reserve Volume (IRV) | Extra air that can be inhaled after a normal inspiration | ~3000 mL | Used during deep breathing |
| Expiratory Reserve Volume (ERV) | Extra air that can be exhaled after a normal expiration | ~1200 mL | Reduced in obstructive disease |
| Residual Volume (RV) | Air left in lungs after maximum expiration | ~1200 mL | Cannot be expelled; prevents lung collapse |
Mnemonic: TIER — Tidal, Inspiratory reserve, Expiratory reserve, Residual.
Clinical tip: Residual volume ensures alveoli remain open for continuous gas exchange — even between breaths.
🫧 Lung Capacities — Combinations of Volumes
Capacities are simply sums of two or more volumes, describing the total potential of the lungs.
| Capacity | Formula | Definition | Normal Value |
|---|---|---|---|
| Inspiratory Capacity (IC) | TV + IRV | Maximum air that can be inhaled after a normal expiration | ~3500 mL |
| Functional Residual Capacity (FRC) | ERV + RV | Air remaining in lungs after normal expiration | ~2400 mL |
| Vital Capacity (VC) | IRV + TV + ERV | Maximum air exhaled after a maximal inspiration | ~4700 mL |
| Total Lung Capacity (TLC) | IRV + TV + ERV + RV | Total air in lungs after maximum inspiration | ~5900 mL |
High-yield distinction: VC = maximum usable lung volume. FRC = “resting” volume of the lungs (where recoil of lungs = expansion of chest wall). RV & FRC cannot be measured by simple spirometry.
⚙️ How Lung Volumes Are Measured
- Spirometry: Measures volumes that move in and out of the lungs (TV, IRV, ERV, VC). Cannot measure RV, FRC, or TLC directly because they contain non-expelled air.
- Helium Dilution Method: Person breathes from a closed system with known helium concentration. Measures FRC based on helium dilution — since helium doesn’t cross the alveolar membrane.
- Body Plethysmography: Based on Boyle’s Law (P₁V₁ = P₂V₂). Patient sits in airtight box; changes in pressure used to calculate lung volumes precisely.
Exam pearl: Plethysmography is the most accurate method for determining FRC and TLC.
🌡️ Dynamic Lung Volumes (Ventilatory Rates)
Beyond static volumes, we also measure how fast air moves — reflecting airway resistance and muscle strength.
| Parameter | Definition | Normal Value / Significance |
|---|---|---|
| Minute Ventilation (MV) | TV × Respiratory rate | ~6 L/min at rest |
| Alveolar Ventilation (VA) | (TV – Dead space) × RR | ~4.2 L/min (actual air reaching alveoli) |
| Dead Space (VD) | Air not participating in gas exchange (e.g., trachea, bronchi) | ~150 mL |
| Forced Vital Capacity (FVC) | Air forcibly exhaled after full inspiration | Used in PFTs |
| Forced Expiratory Volume (FEV₁) | Air exhaled in first second of FVC | ~80% of FVC in normal lungs |
FEV₁/FVC ratio: ↓ (<70%) → Obstructive disease (e.g., asthma, COPD). Normal or ↑ → Restrictive disease (e.g., fibrosis, scoliosis).
💨 Functional Residual Capacity (FRC) — The Balancing Point
At the end of quiet expiration, the lungs are not empty — they contain FRC. At this point:
- Elastic recoil of lungs (inward) = outward pull of chest wall.
- This equilibrium keeps alveoli open and gas exchange continuous.
Clinical tie: ↓ FRC → in obesity, supine position, anesthesia. ↑ FRC → in emphysema (loss of elastic recoil).
🫁 Alveolar Ventilation vs. Dead Space Ventilation
| Type | Description | Example |
|---|---|---|
| Anatomical dead space | Conducting airways (no alveoli) | Trachea, bronchi |
| Alveolar dead space | Alveoli ventilated but not perfused | Pulmonary embolism |
| Physiological dead space | Anatomical + alveolar | Increases in lung disease |
🧠 Clinical Correlations
| Condition | Effect on Lung Volumes | Explanation |
|---|---|---|
| Obstructive disease (COPD, asthma) | ↑ RV, ↑ FRC, ↓ VC | Air trapping due to airway collapse |
| Restrictive disease (fibrosis, scoliosis) | ↓ TLC, ↓ VC, ↓ RV | Reduced compliance → smaller lung expansion |
| Aging | ↑ RV, ↓ VC | Loss of elasticity |
| Emphysema | ↑ TLC, ↑ RV | Lungs overstretched, barrel chest appearance |
Exam favorite: In emphysema → lungs can fill, but can’t empty effectively → air trapping → ↑ RV and TLC.
🧩 Summary Table — Volumes and Capacities
| Parameter | Formula | Normal Value (mL) | Measured by Spirometry? |
|---|---|---|---|
| TV | — | 500 | ✅ |
| IRV | — | 3000 | ✅ |
| ERV | — | 1200 | ✅ |
| RV | — | 1200 | ❌ |
| IC | TV + IRV | 3500 | ✅ |
| FRC | ERV + RV | 2400 | ❌ |
| VC | IRV + TV + ERV | 4700 | ✅ |
| TLC | IRV + TV + ERV + RV | 5900 | ❌ |