ventilation

Ventilation is simply the act of moving air in and out — the bellows part of breathing. It is the bulk transport that brings fresh air down to the gas-exchange surfaces and pushes stale air back out, the way a fan or an open window keeps a room's air turning over.

Mechanically, breathing in lowers the pressure inside the chest as the diaphragm and rib muscles enlarge it, so air flows down into the lungs; breathing out lets the lungs spring back and pushes air out again. The amount of air moved per minute is the minute ventilation, and the portion that actually reaches the alveoli is the alveolar ventilation that counts for gas exchange.

It is worth keeping ventilation distinct from gas exchange. You can move plenty of air and still fail to oxygenate the blood, or exchange gas poorly even with vigorous breathing. The same word is also used for machine support — mechanical ventilation — where a device does the bellows work for a patient who cannot.