Process of gas exchange

Diffusion

Gas exchange occurs by passive diffusion across the alveolar-capillary membrane, driven by partial pressure gradients - investigated primarily via spirometry, lung volumes, and gas transfer (DLCO)

Pulmonary function testing is the core investigation distinguishing obstructive from restrictive ventilatory patterns and quantifying gas transfer efficiency

Diffusion depends on the surface area, thickness of the alveolar-capillary membrane, and the partial pressure gradient - disease increasing membrane thickness (fibrosis) or reducing surface area (emphysema, pneumonectomy) impairs gas transfer
DLCO (diffusing capacity for carbon monoxide) measures gas transfer efficiency using a tracer gas - reduced in ILD and emphysema (different mechanisms - thickened membrane vs lost surface area), normal or increased in pure asthma/chronic bronchitis and in conditions raising pulmonary capillary blood volume (e.g. early left heart failure, alveolar haemorrhage - paradoxically raised DLCO)

Spirometry: obstructive (reduced FEV1/FVC ratio) vs restrictive (reduced FVC with preserved/increased ratio) pattern
Lung volumes (plethysmography): confirms restriction (reduced TLC) vs distinguishes it from poor effort; identifies hyperinflation/gas trapping in obstructive disease (raised RV/TLC ratio)
DLCO differentiates causes of a restrictive pattern (ILD - reduced DLCO vs chest wall/neuromuscular restriction - normal DLCO) and causes of obstruction (emphysema - reduced DLCO vs asthma/chronic bronchitis - normal DLCO)
The combined pattern (spirometry + volumes + DLCO) narrows the differential substantially before imaging or biopsy, and is used serially to track disease progression/treatment response

DLCO decline over serial testing is a key marker of ILD progression, often preceding a measurable decline in FVC

5 more sections, plus exam facts

Premium unlocks every note across every specialty, and the full exam fact library behind it.