Demand to oxygenation

Ventilation at Altitude

Set oxygen demand with body mass and activity. The ventilatory equivalent determines how much air must move for that demand, linking metabolism to arterial CO₂, alveolar oxygen, estimated arterial oxygen, and hemoglobin saturation.

Choose activity demand

Activity intensity4.0 METsSets oxygen consumption per kilogram112Body mass70 kgScales whole-body oxygen demand40120Oxygen ventilatory equivalent30 L VE / L VO₂Higher values mean more ventilation for the same oxygen demand2045Respiratory exchange ratio0.80 RERRelates metabolic CO₂ production to oxygen use0.71

Altitude2500 mStandard-atmosphere barometric pressure: 560 mmHg05500

1 · Oxygen demand

980 mL/min

4.0 METs × 70 kg

2 · Minute ventilation

29.4 L/min

VE/VO₂ 30

3 · Arterial CO₂

32.9 mmHg

Estimated pH 7.49

4 · Estimated saturation

90.4%

PaO₂ 58.2 mmHg

Alveolar ventilation

20.6 L/min

CO₂ production

784 mL/min

Alveolar PAO₂

66.2 mmHg

Arterial O₂ content

18.3 mL/dL

Ventilation can partly defend alveolar oxygen at altitude

At 2500 m, humidified inspired oxygen pressure is 107.3 mmHg. Increasing VE/VO₂ lowers PaCO₂, leaving more of that pressure for alveolar oxygen. It cannot restore the barometric pressure lost with altitude.

Demand sets oxygen use

VO₂ = 3.5 × mass × METs

980 mL O₂/min

Ventilation regulates CO₂

PaCO₂ = 863 × VCO₂ / VA

32.9 mmHg

CO₂ affects alveolar oxygen

PAO₂ = FIO₂(PB − 47) − PaCO₂/RER

66.2 mmHg

Fixed teaching assumptions

Alveolar ventilation is fixed at 70% of minute ventilation, postponing respiratory rate, tidal volume, and dead-space mechanics. Estimated PaO₂ uses a fixed 8 mmHg A-a gradient. Saturation uses the site's oxygen-hemoglobin model at 37 °C and normal 2,3-BPG; acute pH is estimated with 24 mmol/L bicarbonate. Oxygen content assumes 15 g/dL hemoglobin.

Teaching model, not an altitude-tolerance or clinical calculator. PaO₂, saturation, and arterial oxygen content describe oxygenation, not whether tissue oxygen demand is met. Delivery also requires cardiac output, regional blood flow, and tissue extraction. The model does not predict symptoms, acclimatization, diffusion limitation, or individual A-a gradients.

Upstream: inspired oxygen pressureIsolate how altitude and oxygen fraction establish inspired PO₂.Downstream: oxygen-hemoglobin bindingIsolate how PO₂, pH, CO₂, temperature, and 2,3-BPG alter saturation.

Model foundation

This algebraic teaching model links a standard MET estimate of oxygen consumption to ventilatory equivalents, the alveolar ventilation equation, the alveolar gas equation, and the site's oxygen-hemoglobin model. It exposes its closure assumptions rather than inferring an unobserved breathing pattern.

Chapter 12: Ventilation explains minute ventilation, alveolar ventilation, dead space, and ventilatory control.
Chapter 11: Respiration connects RER, gas partial pressures, and hemoglobin saturation.
NCBI: Alveolar Gas Equation supports the oxygen-pressure calculation and its simplifying assumptions.

Related live models

Altitude and Inspired Oxygen Pressure supplies the upstream altitude-pressure and airway-humidification relationships.
Oxygen-Hemoglobin Dissociation supplies the downstream saturation response to PO₂, pH, CO₂, temperature, and 2,3-BPG.
Cardiac Output and Frank-Starling adds the blood-flow term needed to move from arterial oxygen content toward systemic oxygen delivery.
Blood Pressure Equation Explorer connects stroke volume, heart rate, arterial compliance, and peripheral resistance.
Poiseuille's Law: Why Radius Matters isolates how pressure, vessel radius, viscosity, and length influence flow.