The rate of O₂ consumption (VO₂) is higher during muscular activity (including shivering), in pregnancy, in childhood and in states that increase metabolic rate (e.g. pyrexia, thyrotoxicosis). The extra O₂ is provided by an increase in:

• Cardiac output
• Ventilation
• Extraction of O₂ from the blood

Increasing the inspired O₂ may also be helpful during times of increased O₂ demand, particularly in patients with respiratory compromise.

During vigorous exercise in healthy individuals, the VO₂ can rise to 10 times the resting value (Fig 1).

The maximum extent to which an individual can increase his/her O₂ consumption during exercise (VO₂ max) is increasingly being used as a measure of cardiopulmonary fitness.