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Human Physiology II · Systems Physiology

Lung Volumes and Capacities

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Study tools

In 30 seconds

Spirometry measures the air a person moves into and out of the lungs and decomposes it into four volumes — tidal volume, , , and — that combine into capacities such as vital capacity, total lung capacity, , and . Because part of every breath fills the conducting airways () rather than reaching alveoli, gas exchange depends on , not total . The separates obstructive disease (slow airflow out, low ratio) from restrictive disease (small or stiff lung, normal or high ratio).

Why this matters

Spirometry is a cornerstone of pulmonary assessment because the volume-versus-time trace encodes both lung size and how fast air can move. An obstructive pattern (low FEV1/FVC) points toward airflow limitation such as asthma or COPD; a restrictive pattern (reduced TLC with a normal or high ratio) suggests a small or stiff lung. Normal values and interpretive criteria vary by institution, jurisdiction, and population reference equations; these notes support education only and do not replace clinical instruction or supervision.

The college version

1. The four lung volumes

  • : air moved per normal breath (~500 mL).
  • Inspiratory reserve volume (IRV): extra air inhaled beyond a normal breath (~3000 mL).
  • Expiratory reserve volume (ERV): extra air exhaled beyond a normal breath (~1100 mL).
  • Residual volume (RV): air left after maximal expiration (~1200 mL). It is never exhaled, so simple spirometry cannot measure it.

2. The four lung capacities

Capacities are sums of two or more volumes:

  • Inspiratory capacity (IC) = TV + IRV — maximum air in from resting end-expiration.
  • Functional residual capacity (FRC) = ERV + RV — air left after a quiet expiration; the resting baseline that keeps alveoli open and buffers gas concentrations.
  • = TV + IRV + ERV — maximum air exhaled after a maximum inspiration.
  • = TV + IRV + ERV + RV — total air the lungs can hold.

3. Dead space and alveolar ventilation

Not all inhaled air reaches the alveoli. Anatomic dead space (~150 mL) is the conducting-zone volume (trachea to terminal bronchioles) that never participates in gas exchange. Alveolar dead space is the volume of alveoli that are ventilated but not perfused. Together they make physiologic dead space. The air that actually reaches perfused alveoli is alveolar ventilation. Minute ventilation is total air moved per minute (TV × rate), but only alveolar ventilation drives gas exchange — so rapid, shallow breathing can move lots of air while ventilating the alveoli poorly.

How it works

  1. During quiet breathing only the tidal volume (~500 mL) moves in and out.
  2. About 150 mL of it stays in the conducting airways (anatomic dead space).
  3. The remaining ~350 mL reaches the alveoli and exchanges gas each breath.
  4. Multiplying these per-breath amounts by rate gives minute and alveolar ventilation.
  5. A spirometer records volume against time during quiet and maximal maneuvers.
  6. A forced maximal expiration yields FEV1 and FVC; their ratio reveals the pattern of limitation.
  7. Exhaled volumes are measured directly; RV and the capacities containing it (FRC, TLC) need methods such as helium dilution or body plethysmography.

Common confusions

Do not confuseWithDifference
VolumeCapacityA volume is a single measurement; a capacity sums two or more volumes
Residual volumeFunctional residual capacityRV is the un-exhalable remainder; FRC = RV + ERV
Minute ventilationAlveolar ventilationMinute = total air moved; alveolar = the portion reaching perfused alveoli
Anatomic dead spaceAlveolar dead spaceAnatomic is conducting-zone volume; alveolar is ventilated-but-unperfused alveoli
Vital capacityTotal lung capacityVC excludes residual volume; TLC includes it
ObstructiveRestrictiveObstructive = slow airflow out (low FEV1/FVC); restrictive = small/stiff lung

Memory aids

"TV + IRV + ERV + RV = TLC." Capacities are made by adding neighbors: IC (TV+IRV), FRC (ERV+RV), VC (TV+IRV+ERV), TLC (everything). For the disease split, "Obstructive = can't get it Out" (low FEV1/FVC) versus "Restrictive = can't get it In" (small volumes, ratio preserved).

Quick review

Topic Recap

Spirometry breaks breathing into four volumes — TV, IRV, ERV, RV — which combine into four capacities (IC, FRC, VC, TLC). Only air reaching perfused alveoli does useful work, so alveolar ventilation (which subtracts dead space) matters more than minute ventilation. The FEV1/FVC ratio is the key index separating obstructive from restrictive disease.

Knowledge Check

  1. Which two volumes add to form functional residual capacity?
  2. Why can residual volume not be measured by ordinary spirometry?
  3. A person breathes 500 mL × 10/min with 200 mL dead space. What is their alveolar ventilation?
  4. In obstructive lung disease, does FEV1/FVC rise or fall, and why?
  5. Which single capacity is the sum of all four volumes?

Answers and Rationales

  1. Expiratory reserve volume + residual volume (FRC = ERV + RV).
  2. Because it remains after a maximal expiration and is never exhaled; it needs helium dilution or body plethysmography.
  3. (500 − 200) × 10 = 3,000 mL/min — dead space is subtracted from every breath before multiplying by rate.
  4. It falls, because narrowed airways slow forced expiration, so FEV1 drops far more than FVC.
  5. Total lung capacity (TLC = TV + IRV + ERV + RV).
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Think of your lungs as a jug with a fixed maximum size. Most of the time you pour a little air in and out with each breath (tidal volume), and some is always left at the bottom that never drains, even when you try (residual volume). On top of your ordinary pour you can deliberately add extra (inspiratory reserve) or pour out extra (expiratory reserve). The "capacities" are just named combinations of these amounts — like "the most I can move from empty to full" (vital capacity) or "everything the jug can hold" (total lung capacity).

Where it stops being exact: a jug is rigid and its leftover is just gravity's doing, but lung residual volume exists because the chest wall and closed airways physically stop the lungs from ever emptying. Real volumes vary with body size, age, sex, and fitness, and disease patterns show up in how fast you can blow the air out.

Simple Example

Breathe normally, then take the biggest breath you can: the extra air is your inspiratory reserve volume. Now blow out as hard and long as you can: the extra you force out is your expiratory reserve volume. What you still cannot exhale is residual volume.

Worked example

  1. Minute ventilation: VE = VT × f (VT = tidal volume, f = respiratory rate). Example: 500 mL × 12 breaths/min = 6,000 mL/min.
  2. Alveolar ventilation: VA = (VT - VD) × f (VD = dead-space volume). With VD = 150 mL: (500 - 150) × 12 = 4,200 mL/min. The same 6 L/min as 300 mL × 20 breaths/min gives only (300 - 150) × 20 = 3,000 mL/min — worse gas exchange because dead space is "paid" on every breath.
  3. Forced maneuvers. FEV1 is the volume exhaled in the first second of a forced expiration; FVC is the total forced vital capacity. Normally FEV1/FVC ≈ 0.75–0.80. In obstructive disease the airways are narrowed, so FEV1 falls out of proportion to FVC and the ratio drops. In restrictive disease the lung is small or stiff, so FEV1 and FVC shrink together and the ratio stays normal or rises.

Key takeaways

  • High yield: Tidal volume (~500 mL) is the workhorse; residual volume (~1200 mL) can never be exhaled.
  • High yield: Capacities are sums — IC = TV+IRV, FRC = ERV+RV, VC = TV+IRV+ERV, TLC = all four.
  • High yield: FRC is the resting balance point that keeps alveoli open between breaths.
  • High yield: Alveolar ventilation, not minute ventilation, determines gas exchange (dead space is paid every breath).
  • High yield: FEV1/FVC below normal = obstructive; normal or high = restrictive.

Keep learning

Ready to build on this? Continue to the next lesson.

Study tools & related lessonsYou’ll learn to · Key vocabulary · Related

You’ll learn to

  • Define the four primary lung volumes — tidal volume, inspiratory reserve volume, expiratory reserve volume, and residual volume — with representative adult values.
  • Identify the four lung capacities and explain which volumes combine to form each.
  • Distinguish anatomic from alveolar dead space and explain why alveolar ventilation, not minute ventilation, determines gas exchange.
  • Use FEV1 and the FEV1/FVC ratio to separate obstructive from restrictive patterns of lung disease.

Key vocabulary

Tidal volume (TV)
Air moved per normal breath (~500 mL)
Inspiratory reserve volume (IRV)
Extra air inhaled beyond a normal breath
Expiratory reserve volume (ERV)
Extra air exhaled beyond a normal breath
Residual volume (RV)
Air left after maximal expiration
Inspiratory capacity (IC)
TV + IRV
Functional residual capacity (FRC)
ERV + RV
Vital capacity (VC)
TV + IRV + ERV
Total lung capacity (TLC)
TV + IRV + ERV + RV
Anatomic dead space
Conducting-zone volume (~150 mL)
Alveolar dead space
Ventilated but unperfused alveoli
Minute ventilation
TV × respiratory rate
Alveolar ventilation
(TV − dead space) × rate
FEV1/FVC ratio
Fraction of forced VC exhaled in 1 second

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