Gas exchange occurs as a result of a difference in the partial pressures of oxygen and carbon dioxide in different body regions.

External respiration occurs at the lungs. Oxygen concentration (or partial pressure) is higher in the alveoli than it is in the blood of capillaries that surround the alveoli. As a result, oxygen moves from the alveoli to the blood. Conversely, carbon dioxide moves from the blood to the alveoli because its partial pressure is higher in blood.

Internal respiration involves the exchange of gases at the body tissue's cells. The concentrations of oxygen in the blood are higher than they are in the tissue's cells. Carbon dioxide concentrations are higher in the cells than they are in the blood. Consequently, gas exchange follows the normal patterns of diffusion-movement is from a higher concentration to a region of lower concentration.