The main function of the respiratory system is gas exchange which consists of the intake of oxygen and removal of carbon dioxide. This exchange takes place at the level of the alveolar membrane which is the aggregate of millions of alveolar units that form a large and thin surface where pulmonary blood meets the atmospheric gas. The inward transfer of air to the alveolar membrane is accomplished by the action of the respiratory muscles, predominantly the diaphragm, that generate a negative intrathoracic pressure that draws gas from the atmosphere to the lungs. Expiration, on the other hand, is a passive phenomenon in health, as the lung and the chest wall return to their resting state via mechanisms of elastic recoil. The exchange of gases occurs by passive diffusion, where a gas of a higher concentration passively diffuses towards the side of a lower concentration. Lung compliance and airway resistance are two cardinal mechanical properties of the respiratory system that can be assessed by pulmonary waveform monitoring as displayed in real time on the ventilator screen. Dynamic changes in pulmonary mechanics can thus be monitored such as an increase in compliance following the administration of an exogenous surfactant or a decrease in airway resistance following the administration of beta-2 agonists.