Four processes of Gas Exchange by the Human Lungs : 1.Ventilation : To and fro movement between the atmosphere and the gas exchanging units of the lung. 2.Circulation : Supply and distribution of blood through the pulmonary capillaries. 3.Diffusion : The movement of oxygen and carbon dioxide across the air-blood barrier between the alveoli and the pulmonary capillaries. 4.Ventilation-perfusion relationships