The lung is like a spring distended within the thorax. The thoracic cavity expands during inspiration due to the activity of diaphragm and intercostal muscles, thus expanding the lung. In stretching the lung an increasing elastic recoil pressure needs to be overcome; therefore the transpulmonary pressure (the pressure difference across the lung) needs to be increased to inflate the lung. It follows that pressure in the pleural space (Ppl) drops relative to alveolar pressure (Palv), just like in an inflated balloon where outside pressure is lower than inside pressure. The pressure difference reflects the elastic recoil pressure of the lung (PL,el) at that lung volume:
Palv - Ppl = PL,el
If after a deep breath, the breath is held with the glottis open, then the pressure in all airways (Pbr) and alveolar spaces equals barometric (Pbar) and mouth pressure (Pmo), and gas flow is nil:
Pbar = Pmo = Pbr = Palv
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The pressure outside intrathoracic airways is equal to that in the pleural space. During breathholding the pressure within airways is higher than pleural pressure by an amount equal to PL,el. Intrathoracic airways are therefore distended, the more so when the lung is stiffer and generates larger PL,el. As one expires to progressively lower lung volumes, the lung is stretched less and PL,el diminishes: the pull from elastic alveolar attachments diminishes and airway caliber decreases. On that account airway diameter is greatest at high lung volume, and smallest at low lung volume. Similarly airways resistance is highest at low lung volumes. In adults the aging process is associated with loss of lung elastic recoil, so that with increasing age airways are stretched somewhat less at the same lung volume. In pulmonary emphysema the lung resembles a spring with reduced recoil force, and airway distension is accordingly diminished.