Respiratory gases the gases that animals must exchange are oxygen and carbon dioxide. cells need to obtain o2 from the environment to produce an adequate supply of atp by cellular respiration. co2 is an end product of cellular respiration, and it must be removed from the body to prevent toxic effect. diffusion is the only means by which respiratory gases are exchanged between the internal body fluids of an animal and the outside medium (air or water). there are no active transport mechanisms to move such gases across biological membranes. The concentrations of different gases in a mixture are expressed by the partial pressures of those gases. to do this, the total pressure must be known first (this is measured with a barometer). partial pressure of oxygen at sea level is 20. 9% of 760 mm hg.