Gas exchange in plants, animals, and insects

Introduction
Gas exchange is the process by which gases like oxygen are obtained from, and waste gases are returned to the environment. The surface across which gas exchange occurs is called a gas exchange surface. When a gas exchange surface is used for the diffusion of gases for respiration, the surface becomes known as a respiratory surface: a surface across which oxygen and carbon dioxide are exchanged between an organism and environment.

Gas exchange can be an internal or external process, as in some single celled organisms, while in larger organisms; gas exchange surfaces may be part of a specialised gas exchange organ, for example, lungs in mammals or gills in fi...