Gaseous exchange is the movement of oxygen and carbondioxide between the organism and its environment at the respiratory surfaces.
Breathing can be simply defined as the process of inhaling oxygen gas from theatmosphere and exhaling the carbon dioxide gas back into the atmosphere is termed breathing and the process is known as respiration. Oxygen goes in and carbon oxide goes out – These gases move in and out through diffusion.
Large animals cannot maintain gas exchange by diffusion across their outer surface. They developed a variety of respiratory surfaces that all increase the surface area for exchange, thus allowing for larger bodies.
A respiratory surface is covered with thin, moist epithelial cells that allow oxygen and carbon dioxide to exchange. Those gases can only cross cell membranes when they are dissolved in water or an aqueous solution, thus respiratory surfaces must be moist.