Pulmonary function tests (PFTs), such as spirometry, measure how well the lungs function by analyzing air flow. Spirometry yields measurements like FEV1, FVC, and their ratio (FEV1/FVC), which can help distinguish between obstructive and restrictive lung diseases. Obstructive diseases like asthma result in low FEV1/FVC due to airway narrowing, while restrictive diseases lower all lung volumes from reduced compliance. PFTs aid in diagnosis, monitoring treatment, and assessing pre-operative risk.