The feeling of not getting enough air, often connected with a low level of oxygen and high level of carbon dioxide in the blood. The latter condition stimulates the respiratory centre in the brain (causing a feeling of shortness of breath), which then causes the lungs to make respiratory movements. If one deliberately holds one’s breath, it is this mechanism that makes it impossible to avoid taking a breath in due course. One form of shortness of breath is familiar to everybody: being ‘out of breath’ as a result of physical exertion, but this should not occur as a result of light activity. Shortness of breath in such cases can occur in lung conditions.