Gas exchange is a process in biology where gases contained in an organism and environment transfer or exchange.

In human gas-exchange, gases contained in the blood of human bodies exchange with gases contained in the atmosphere. Human gas-exchange occurs in the lungs. In unicellular organisms the respiratory surface and gas-exchange is governed by Fick's law.

The primary force applied in the respiratory tract is supplied by atmospheric pressure. Total atmospheric pressure at sea level is 760 mmHg (101 kPa), with oxygen (O₂) providing a partial pressure (pO₂) of 160 mmHg (21 kPa), 21% by volume, at the entrance of the nares, a partial pressure of 150 mmHg (20 kPa) in the trachea due to the effect of partial pressure of water vapor, and an estimated pO₂ of 100 mmHg (13 kPa) in the alveolar sac.^{[citation needed]} The pressure drop is due to conduction loss as oxygen travels along the transport passageway.

Atmospheric pressure decreases as altitude increases, making effective breathing more difficult at higher altitudes. Higher BPG^{[clarification needed]} levels in the blood are seen at higher elevations, as well.