Key Concepts: Work of Breathing and Spirometry
The Work of Breathing
- Two opposing forces must be overcome to breathe: lung stiffness (compliance) and airway resistance.
- Lung stiffness: how easily the lungs expand; surface tension helps hold the lungs in place.
- Airway resistance: friction to move air from outside to alveoli; depends on airway radius.
Pressure, Volume, and the Breathing Cycle
- Boyle’s Law: the pressure of a gas is inversely related to its volume. P imes V = ext{constant} \,\rightarrow\, P \propto \frac{1}{V}
- Pressure gradient drives airflow: gas moves from high to low pressure.
- Before a breath: outside and inside lung pressures are equal; no air movement.
- Inhalation: chest volume increases → alveolar pressure decreases → air flows in.
- Exhalation: chest volume decreases → alveolar pressure increases → air flows out.
Lung Compliance (Stiffness)
- Compliance definition: C = \frac{\Delta V}{\Delta P}
- Normal lungs have a certain compliance; stiffness increases when elasticity decreases or surfaces tensions rise.
- Pulmonary fibrosis: thickening/scarring of alveolar membranes → reduced compliance (harder to expand).
Surface Tension and Surfactant
- Alveoli are lined with fluid that creates surface tension; must be overcome to expand alveoli.
- Surfactant production reduces surface tension, facilitating lung expansion during inspiration.
- Insufficient surfactant → difficulty expanding lungs and reduced oxygen uptake.
- Premature infants may lack surfactant → respiratory distress syndrome (RDS).
Airway Resistance and the Bronchial Tree
- Air moves through trachea, bronchi, bronchioles to alveoli; friction provides resistance.
- Airway radius affects resistance: smaller radius increases resistance dramatically.
- Resistance relationship: R \propto \frac{1}{r^4}
- Bronchioles have smooth muscle and can constrict/dilate; significant determinant of airway resistance.
Obstructive vs Restrictive Breathing Problems
- Obstructive: increased resistance to airflow; examples include asthma, chronic bronchitis.
- Restrictive: reduced lung capacity/compliance; examples include fibrosis.
- Distinguish by whether resistance (airflow) or capacity (volume) is primarily affected.
Spirometry: Pulmonary Function Test
- Spirometer measures: volume inspired/expired and the rate of airflow.
- Used to assess lung function and response to therapy.
Spirometry Trace: Volumes
- Tidal Volume (VT): volume moved with normal quiet breath.
- Inspiratory Reserve Volume (IRV): extra air that can be inspired with maximal inhalation.
- Expiratory Reserve Volume (ERV): extra air that can be exhaled with maximal effort.
- Residual Volume (RV): volume remaining after maximal exhalation.
- Minimal Volume: volume remaining if lungs collapsed.
Spirometry Trace: Capacities
- Vital Capacity (VC): IRV + VT + ERV
- Total Lung Capacity (TLC): VC + RV
- Inspiratory Capacity (IC): IRV + VT
- Functional Residual Capacity (FRC): ERV + RV
Key Spirometry Metrics
- Forced Expiratory Volume in 1 second (FEV1): volume expelled in the first second of a forced breath.
- FEV1/VC ratio: Normal ~0.80; <0.70 indicates obstructive disease.
- Spirometry traces reflect both volume and rate of airflow, aiding identification of obstructive issues.
Quick Summary
- Changes in chest volume create a pressure differential that drives airflow.
- Air to breathe must overcome lung stiffness (compliance) and airway resistance.
- Spirometry yields volumes and capacities; helps diagnose obstructive vs restrictive patterns.
- Surfactant lowers surface tension, enabling easier lung expansion; absence leads to RDS in preterms.
Practice exam style recall
- Which statement best describes a key feature of vital capacity? It is the total volume in the lungs when fully filled; i.e., the sum of IRV, VT, and ERV.
