Ventilation, commonly called breathing, is the process of moving air into and out of the lungs. This movement is established by the balance among three pressures — atmospheric pressure, intrapulmonary pressure, and intrapleural pressure. The pressure gradients produced by contraction of the diaphragm and thoracic muscles drive air flow. Respiration is the more comprehensive process by which oxygen and carbon dioxide are exchanged between the atmosphere and body cells. Respiration includes ventilation (breathing), transfer of oxygen from lungs to blood (external respiration), transfer of oxygen from blood to cells (internal respiration), and finally the utilization of oxygen by cells for cellular metabolism (cellular respiration), and the corresponding processes for CO2 elimination.