In simple means, the respiratory system can be reduced to a straw (endotracheal tube and airways exhibiting resistance) and a balloon on the end of this straw (network of alveoli exhibiting compliance). This balloon is placed somewhat in a semi-rigid cage (the thorax exhibiting elastic recoil forces) opposing the distending forces by the negative intrathoracic pressure effect during spontaneous breathing, or the positive pressure effect on the balloon volume during positive pressure application to the system (i.e. positive pressure ventilation). The movement of gas is determined by the force (i.e. pressure, or more precisely in physiologic terms pressure/surface area) applied, gas displacement kinetics, resistive forces exhibited against flow by the airways (resistance determined by the size and length of the straw, and the viscosity of the gas), and the rate of change of displacement of the components that are distensible (balloon). Alterations in the composition of the lung parenchyma and the airways due to alveolar and/or interstitial oedema, cellular infiltrates, or fibrosis will not only reduce distensibility of the lung but also increase elastic recoil forces exhibited by the lung parenchyma.