In a patient breathing air and with an FRC of 1900 ml, there would be just under 300 ml of oxygen, as the majority of the gas is nitrogen. By replacing the nitrogen in the lungs with oxygen, we dramatically increase the oxygen reserve available.

At induction of anaesthesia this is called preoxygenation, or sometimes denitrogenation.

Virtually no nitrogen is dissolved in the blood and so the nitrogen in the lungs can be removed rapidly during preoxygenation.

Because haemoglobin carries most of the oxygen in arterial blood and it is already 97% saturated, the oxygen content in blood is not increased to any great extent by preoxygenation.