Gas resorption from the pleural space is achieved by a simple diffusion from the pleural space into the venous blood. This diffusion, which can occur in both directions, is possible because the pleura and capillary walls are permeable to gases and the partial pressures of gases in the pleural space and those in the venous blood differ. Indeed, no active transport mechanisms for gas resorption exist; the only driving forces that determine gas resorption are pressure gradients.

The rate of gas resorption depends on four variables: the pressure gradients for the gases in the pleural space in relation to the venous blood, the area of contact between the pleural gas and the pleura, the permeability of the pleural surface (e.g., a thickened fibrotic pleura absorbs less than a normal pleura), and the diffusion properties of the gases present in the pleural space. Indeed, the solubility and diffusion properties of different gases vary considerably: oxygen is resorbed 62 times faster than nitrogen (N₂), and water vapor (H₂O) and carbon dioxide (CO₂) equilibrate almost instantaneously, CO₂ being 23 times more soluble than O₂.

Depending on the clinical situation, a pneumothorax can initially contain room air (i.e., a leak from the outside) or alveolar air (i.e., a leak through the lungs). The alveolar air contains different proportions of CO₂, O₂, or N₂, depending on the ventilation of the patient and the presence of supplemental O₂ given to the patient at the time of the leak into the pleural space. If a patient is receiving 100% O₂, the pleural gases will be composed mostly of this gas and contain no N₂, the slowest gas to be resorbed. Initial gas compositions in a pneumothorax when the air entry is from room air or from alveolar air with the patient breathing room air or 100% O₂ are presented in Figure 57-1.

Because the pressure gradients that favor gas resorption are those between the pneumothorax and the venous blood, it is also important to consider the venous blood partial gas pressures. A schematic approach is used here to explain the gas composition in venous blood. Dry room air contains, for all practical purposes, 80% N₂ and 20% O₂. Other gases (e.g., CO₂, argon) are present in minute quantities. The normal atmospheric pressure is 760 mm Hg at sea level (1,031 cm H₂O); therefore the partial pressure of N₂ in dry room air is 608 mm Hg, whereas that of O₂ is 152 mm Hg. When room air enters the alveoli, it gains H₂O vapor and CO₂ and loses O₂; the resulting gas composition is N₂ = 571 mm Hg, O₂ = 102 mm Hg, H₂O = 47 mm Hg, and CO₂ = 40 mm Hg. Because the alveoli are in close communication with the atmosphere, the total gas pressure at this level must equal 760 mm Hg. This alveolar gas composition is in contact with the blood at the alveolar capillary level, and gas exchange occurs. The resulting normal arterial gas composition is PaO₂ = 97 mm Hg, PaCO₂ = 40 mm Hg, and PaN₂ = 569 mm Hg, with water vapor remaining constant at 47 mm Hg; the total arterial gas pressure is 753 mm Hg. Note that a 7-mm Hg pressure gradient exists between the alveolar gas pressure and that of the arterial blood.