breath

A breath is one round of breathing — drawing air in and letting it out again. It is the single unit you count when you watch someone's chest rise and fall, and the thing we take roughly twenty thousand times a day without ever thinking about it.

Each breath has two phases: inspiration, the active drawing-in of air as the diaphragm contracts and the chest expands, and expiration, the usually passive letting-out as the lungs and chest recoil. The volume of air moved in an ordinary, relaxed breath is the tidal volume, typically around half a litre in an adult at rest.

Breaths are mostly automatic, driven by the brainstem, but they can be overridden at will — you can hold a breath, take a deep one, or speak across one. Counting breaths over a minute gives the respiratory rate, one of the basic vital signs clinicians use to gauge how someone is doing.