When positive pressure is used to inflate the lungs, the forces that must be overcome are primarily the elasticity of the lungs and chest wall and airflow resistance. From the resting expiratory point (functional residual capacity (FRC)), the lungs (whose normal tendency is to collapse) are held open by the chest wall (whose tendency is to expand); at this point there is no gas flow and the system is in balance. As the respiratory system expands and gas enters the chest, the movement of the system can be described mathematically by the equation of motion where the pressure at the airway opening (Pao) is related firstly to the compliance (Crs) of the system, secondly to the resistance to airflow (Rrs), and thirdly to the inertia of the gas (/rs). Inertia is a small contributor to the force required to generate lung inflation and is often omitted.
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