Minute ventilation is V̇E = RR × TV. Airway resistance (R = ΔP / flow) caps how much air can move in the time available, and compliance (C = ΔV / ΔP) caps how far the alveoli can stretch — so the achieved tidal volume can fall well short of the effort you dial in:
TV_effective = min( TV_target × compliance factor, max_flow / resistance × inspiratory time )
flow(t) = dV/dt (derivative of the breathing waveform — drawn live as the flow-volume loop)
- Breathing rate & effort — the drive behind ventilation; faster rates shorten inspiratory time, so flow-limited (obstructive) lungs get less air per breath.
- Condition — asthma narrows the airways (↑ resistance, bronchoconstriction, scalloped flow-volume loop); COPD destroys alveolar walls (↓ surface area, floppy over-compliant sacs, slow trapped exhalation); fibrosis stiffens the tissue (↓ compliance, small stiff breaths, tall narrow loop).
- Inspired O₂ — supplemental oxygen raises the O₂ gradient driving diffusion across the alveolar-capillary membrane, partly compensating for lost ventilation or surface area.
- Flow-volume loop — the classic spirometry readout: obstructive disease (asthma, COPD) shows a scooped-out, flow-limited expiratory curve; restrictive disease (fibrosis) shows a tall, narrow loop with reduced total volume.
Real-world relevance: pulmonary function tests (spirometry) measure exactly these curves — FEV1/FVC ratio for obstructive disease and reduced TLC for restrictive disease — to diagnose and stage lung disease.