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lung volumes subdivisions spirometry diagram TLC FRC RV

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Lung Volumes — Fishman's Pulmonary Diseases and Disorders

Definitions and Subdivisions

Lung volumes and capacities are the fundamental descriptors of respiratory function. The convention divides the total volume of gas in the lungs into volumes (non-overlapping compartments) and capacities (combinations of two or more volumes).

TABLE 31-1 Glossary for Static Lung Volumes and Capacities

Volumes (primary, non-overlapping):

Term	Symbol	Definition
Tidal volume	TV (Vt)	Volume inspired or expired per breath during quiet breathing
Inspiratory reserve volume	IRV	Maximum additional volume inspired from end-tidal inspiration
Expiratory reserve volume	ERV	Maximum volume expired from the resting end-expiratory level
Residual volume	RV	Volume remaining after maximal expiration

Capacities (sums of volumes):

Term	Symbol	Definition
Inspiratory capacity	IC	IRV + TV; max volume inspired from end-expiratory level
Vital capacity	VC	Max volume expired from TLC (= TLC − RV)
Functional residual capacity	FRC	RV + ERV; volume in lungs at resting end-expiratory position
Total lung capacity	TLC	All volumes combined; volume at end of maximal inspiration
Closing capacity	CC	Closing volume + RV (CC/TLC, %)
RV/TLC ratio	RV/TLC%	Expressed as a percentage

Determinants of Key Lung Volumes

Functional Residual Capacity (FRC)

FRC is the major reference point for all lung volume subdivisions. It is set by the balance of opposing recoil forces of the lungs (inward) and chest wall (outward) when the respiratory muscles are at rest. — Fishman's, Ch. 10

Total Lung Capacity (TLC)

TLC is determined by the balance between:

The force-generating capacity of the inspiratory muscles, and
The opposing elastic recoil of lungs + chest wall.

Weakness of inspiratory muscles or increased lung stiffness → ↓ TLC Loss of lung retractive force (e.g., emphysema) → ↑ TLC — Fishman's, Ch. 10

Residual Volume (RV)

RV is set by the balance between:

Expiratory muscle force + lung recoil (favoring volume decrease), and
Outward recoil of chest wall (favoring expansion).

In middle-aged and older individuals, small airway closure at low lung volumes and air trapping becomes an important determinant of RV. — Fishman's, Ch. 10

Measurement Methods

Spirometry

Spirometers measure volumes directly but cannot measure RV, FRC, or TLC — these require indirect techniques. Spirometry provides: TV, IRV, ERV, VC, IC, FVC, FEV₁, PEF, FEF₂₅–₇₅, MVV.

Technique: the patient breathes quietly to establish the resting end-expiratory level (FRC), then inspires maximally → slow, complete expiration. Attempts to maximize flow in obstructive disease paradoxically reduce volumes due to dynamic airway compression.

Indirect Measurement of FRC (and thus RV, TLC)

RV is determined indirectly: (1) Measure FRC by one of three methods → (2) Measure ERV spirometrically → (3) RV = FRC − ERV.

1. Closed-Circuit Helium Dilution

Based on the conservation of mass principle and insolubility of He in tissues.
System loaded with ~10% helium; patient breathes quietly until equilibration.
Formula: FRC = V_He × (C₁ − C₂) / C₂
Limitation: underestimates lung volume in patients with trapped gas (bullae, severe obstruction) — communicating gas only.

2. Open-Circuit Nitrogen Washout

Patient breathes 100% O₂; N₂ washed out of lungs and collected.
Uses: initial alveolar N₂ concentration (~81% fasting) and measured expired N₂ volume.
FRC = V_E × F_EN₂ / F_AN₂
Also underestimates in poorly-communicating regions.

3. Body Plethysmography (Gold Standard)

Based on Boyle's Law (P₁V₁ = P₂V₂).
Patient sits in a sealed box; at end-expiration shutter closes; patient pants against closed shutter.
Measures thoracic gas volume (TGV) = all intrathoracic gas, including trapped gas.
Advantage over dilution techniques: measures non-communicating (trapped) gas — therefore typically gives higher values than gas dilution in obstructive disease.
Three types of plethysmograph: pressure (most common), volume, and pressure-corrected flow. — Fishman's, Ch. 31

Changes with Age

Figure 18-8 — Lung volume changes with age (cross-sectional data):

Key age-related patterns:

TLC: little or no significant decline with age (cross-sectional data). Longitudinal studies suggest small but significant falls may exist (~25 mL/year in fit elderly men).
RV: increases consistently with age (airway closure and air trapping).
VC / FVC: declines with age — 21–33 mL/year in men, 18–29 mL/year in women (cross-sectional).
FRC: rises with age (loss of lung elastic recoil, chest wall stiffening).
Closing capacity (CC): rises with age and may exceed FRC, leading to airway closure during tidal breathing → V/Q mismatch.

— Fishman's, Ch. 18

Compliance and Elastic Recoil

Compliance (C) = ΔV_L / Δ(P_A − P_pl) — describes distensibility (ease of inflation). Its inverse is elastance (stiffness).

The pressure-volume (P-V) curve of the lung is nonlinear: compliance is greatest at low volumes (near RV) and least at high volumes (near TLC), where elastic elements approach their distensibility limits.

Hysteresis: the inflation and deflation P-V curves differ due to:

Surface tension forces (surfactant)
Sequential recruitment of closed lung units during inflation
Stress adaptation of lung tissue

Static elastic recoil pressure ≈ 5 cmH₂O at FRC and ≈ 30 cmH₂O at TLC. — Fishman's, Ch. 10

Interpretation of Abnormal Lung Volumes

Pattern	TLC	RV	FRC	RV/TLC
Restriction (e.g., IPF)	↓	↓	↓	Normal or ↑
Obstruction (e.g., COPD)	Normal or ↑	↑	↑	↑
Emphysema	↑	↑↑	↑	↑↑
Air trapping	Normal	↑	↑	↑

Important: TLC <5th percentile = restriction; TLC >95th percentile = hyperinflation. RV/TLC or FRC/TLC >95th percentile indicates air trapping even when TLC is normal.

All static lung volumes expressed at BTPS (body temperature, ambient pressure, saturated with water vapour).

Sources: Fishman's Pulmonary Diseases and Disorders (9th ed.), Chapters 10 (Respiratory System Mechanics), 18 (Aging Lung), and 31 (Pulmonary Function Testing).

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