When you exhale your breath, it goes in the exhale counterlung when you inhale a breath, it comes from the inhale counterlung.

This contributes to the inability to breathe out fully.

Air trapping, also called gas trapping, is an abnormal retention of air in the lungs where it is difficult to exhale completely.

The greatest reduction in air flow occurs when breathing out, as the pressure in the chest is compressing the airways at this time.

The 18O leaves the body in two ways (i) exhaled CO2, and (ii) water loss in (mostly) urine, sweat, breath.

FEV1 (forced expiratory volume in 1 second) should be above 80 of predicted values to be considered normal.

The National Institute for Health and Care Excellence criteria additionally require a FEV1 of less than 80 of predicted.

This means that the human body utilises only around 19 of the oxygen inhaled, leaving over 80 of the oxygen available in the exhalatory breath.

An analysis of arterial blood is used to determine the need for oxygen this is recommended in those with an FEV1 less than 35 predicted, those with a peripheral oxygen saturation of less than 92 and those with symptoms of congestive heart failure.

Two main components are measured to make the diagnosis the forced expiratory volume in one second (FEV1), which is the greatest volume of air that can be breathed out in the first second of a breath, and the forced vital capacity (FVC), which is the greatest volume of air that can be breathed out in a single large breath.