Respiration is an involuntary activity initiated in the respiratory centers in the medulla. Diaphragm is the most important inspiratory muscle. The pressure required to generate air flows depends on airway caliber (or resistance), density and viscosity of the gas and nature of flow (turbulent or laminar). Expiratory air flows are independent of efforts at mid and lower lung volumes and depend on the mechanical properties of the parenchyma and the airways. Pulmonary circulation is a low pressure and high capacitance circulation. It provides perfusion across the ventilated alveoli for gas exchange. Net diffusion of oxygen and carbon dioxide occurs along concentration gradients between the alveolar air and capillary blood. The total oxygen (expressed as oxygen content) is transported in blood largely bound to hemoglobin and a small fraction is dissolved. The most important cause of hypoxemia is ventilation perfusion imbalance. Respiratory acidosis and alkalosis are the primary disturbances of acid-base balance and occur due to hypoventilation and hyperventilation, respectively.