In man the respiratory surface is the lungs and gaseous exchange take place in the alveoli and the cells. The lungs are spongy-like sacs and found in the thoracic cavity and the wall protected by the ribs, supported by the intercoastal muscle. The thoracic cavity separated by the abdominal cavity by the diaphragm. The lungs are protected by the pleura membrane which contains a pleural fluid which permit the lungs to move freely. The lungs opens outside as two nose tries.

(diagram of the alveoli in gaseous exchange)

• Air enters the alveoli during concentration of carbon dioxide.
• Some of the oxygen in the alveoli cavity dissolves in the moist inner surface of the alveoli.
• The oxygen concentration of blood in the capillary surrounding the alveoli is lower than in the cavity there is a step diffusion gradient
• Oxygen therefore diffuse through the epithelium of the alveolus into the blood capillaries where there is low concentration.