Physiologists categorize the air space within the lungs to understand respiratory health. This classification is crucial for assessing the mechanical state of the lungs, muscle strength, airway resistance, and gas exchange efficiency. Medical professionals use these metrics to diagnose respiratory conditions through non-invasive, cost-effective tests.
What are the distinct Lung Volumes and Capacities?
Lung volumes are distinct components of respiration that fluctuate with breathing, while capacities are calculated combinations of these volumes that generally remain fixed.
Summary of Lung Volumes and Capacities
๐น Lung Volumes
| Type | Term | Definition | Average Volume |
|---|---|---|---|
| Volume | Tidal Volume (TV) | Air entering or exiting during normal quiet breathing | \(0.5\ \text{L}\) |
| Volume | Inspiratory Reserve Volume (IRV) | Extra air inhaled forcefully after a normal breath | \(2.5\ \text{L}\) |
| Volume | Expiratory Reserve Volume (ERV) | Extra air exhaled forcefully after a normal breath | \(1.5\ \text{L}\) |
| Volume | Residual Volume (RV) | Air remaining after maximal exhalation (prevents lung collapse) | \(1.5\ \text{L}\) |
๐น Lung Capacities
| Type | Term | Definition | Average Volume |
|---|---|---|---|
| Capacity | Vital Capacity (VC) | Maximum air exhaled after maximum inhalation (\(IRV + TV + ERV\)) | \(4.5\ \text{L}\) |
| Capacity | Inspiratory Capacity (IC) | Maximum air inhaled from quiet expiration (\(TV + IRV\)) | \(3.0\ \text{L}\) |
| Capacity | Functional Residual Capacity (FRC) | Air left after quiet exhalation (\(ERV + RV\)) | \(3.0\ \text{L}\) |
| Capacity | Total Lung Capacity (TLC) | Total air the lungs can hold (\(VC + RV\)) | \(6.0\ \text{L}\) |
What Is Respiratory Dead Space?
Dead space refers to air that does not participate in gas exchange.
๐น Types of Dead Space
- Anatomical Dead Space:
Air present in the conducting airways (trachea, bronchi) that never reaches the alveoli.
Typical volume: \(โ 150\ \text{mL}\) - Alveolar Dead Space:
Air that reaches alveoli but does not undergo gas exchange due to poor blood perfusion (e.g., pulmonary embolism).
How Do Doctors Measure Lung Function?
Clinicians use the following three primary methods:
1. Simple Spirometry
- Measures TV, IRV, and ERV
- Patient breathes into a closed circuit
- Cannot measure Residual Volume
- Values are standardized by age, sex, and height
2. Helium Dilution Technique
- Used to measure Total Lung Capacity
- Patient inhales inert helium gas
- The final concentration on exhalation reveals lung volume
- May underestimate volume in airway obstruction
3. Nitrogen Washout Method
- Used to calculate Anatomical Dead Space
- Patient inhales pure oxygen
- The nitrogen concentration in exhaled air is analyzed to estimate conducting airway volume
How Do You Read a Spirometry Graph?
Vitalograph (VolumeโTime Curve)
A vitalograph measures:
- FEVโ: Forced Expiratory Volume in 1 second
- FVC: Forced Vital Capacity
The diagnostic ratio is:
\(\text{FEV}_1/\text{FVC}\)FlowโVolume Loop
- Plots airflow rate vs lung volume
- Shape of the curve determines pathology type
- โSpooningโ of the expiratory curve indicates obstruction
Difference Between Obstructive and Restrictive Diseases
| Feature | Obstructive Disease (Asthma, COPD) | Restrictive Disease (Fibrosis, Obesity) |
|---|---|---|
| Primary Defect | Increased airflow resistance | Limited lung expansion |
| Key Metric | \(\text{FEV}_1/\text{FVC} < 0.7\) | \(\text{FEV}_1/\text{FVC} \geq 0.7\) |
| Lung Volumes | Residual Volume increases (air trapping) | Total Lung Capacity decreases (\(< 80%\)) |
| Graph Shape | โSpooningโ of expiratory curve | Smaller loop, normal shape |
| Notes | Often reversible with bronchodilators | Caused by scarring, muscle weakness, deformity |
