Breathing works by changing the volume of the thoracic cavity, which changes intra-pulmonary pressure; spirometry measures the volumes and capacities moved.
How it's asked: Muscles of inspiration, TV/IRV/ERV/RV values, deriving VC, FRC, IC and TLC, and why RV cannot be measured by a spirometer.
Why this is true
Air moves only down a pressure gradient. During inspiration the diaphragm contracts and flattens and the external intercostal muscles lift the ribs and sternum, enlarging the thorax in the antero-posterior and dorso-ventral axes; the lungs follow, intra-pulmonary pressure falls below atmospheric and air flows in. Expiration is normally passive: the muscles relax, the thorax and lungs recoil, pressure rises above atmospheric and air leaves. Additional abdominal muscles can increase the force of both phases. Some air always stays in the lungs even after forcible expiration (the residual volume), which is why total lung capacity exceeds vital capacity and why RV cannot be read from a spirometer.
Key points
Key formulae
Vital capacity
VC = IRV + TV + ERV
Functional residual capacity
FRC = ERV + RV
Total lung capacity
TLC = VC + RV
Inspiration
- 1Diaphragm and external intercostals contractRibs and sternum lifted, diaphragm flattened
Common traps
- Residual volume cannot be measured by simple spirometry, so FRC and TLC also cannot be measured directly that way.
- Normal expiration is passive; the internal intercostals and abdominal muscles act only in forced breathing.
Worked example
A spirometer shows TV 500 mL, IRV 2500 mL and ERV 1000 mL; the subject's RV is 1200 mL.
High-yield
TV 500, IRV 2500–3000, ERV 1000–1100, RV 1100–1200 mL. VC = IRV + TV + ERV. FRC = ERV + RV.
Quick check
Q1.Functional residual capacity equals:
Q2.Inspiration begins when: