Pneumothorax: loss of the pleural seal and lung collapse
A pneumothorax is air in the pleural space (the gap between the lung covering and the chest wall lining), so the lung collapses inwards.
Remember
Once air gets into the pleural space, the pressure there is no longer negative (below atmospheric), nothing holds the lung against the chest wall, and its own elastic recoil, an inward pull, collapses it.
Memory aid
the lung is held open by suction, like a balloon held stretched by the chest wall
Once air gets in, the suction is lost and the lung springs back to a small size.
The pleural seal is a very thin film of pleural fluid between the two layers of the pleura, the double-layered membrane in which one layer covers the lung and the other lines the chest wall. A little like water holding two glass slides together, it makes the lung follow the chest wall as the chest expands.
At rest the lung recoils inwards (springs back towards a smaller size), while the chest wall springs outwards. These opposing pulls leave negative pleural pressure, which means a pressure below atmospheric pressure. That suction keeps the lung pressed against the chest wall.
In a pneumothorax, air enters the pleural space, either through the chest wall or from a leak in the lung itself. Air flows in until the pleural pressure is equal to atmospheric pressure. The suction is lost and nothing holds the lung out. Its own elastic recoil, the inward pull of the lung, then makes it collapse, either partly or completely.
The airways are open and the problem lies outside the lung. A blocked bronchus can collapse part of a lung, but that is a different process. Nothing keeps the lung attached to the chest wall, so when the chest wall expands the lung does not follow. Because a pneumothorax can happen suddenly, it can present with sharp pleuritic pain (pain that is worse on breathing in) and breathlessness.
In a tension pneumothorax, air can get in but cannot leave. The pressure in the pleural space can then rise above atmospheric pressure and push the mediastinum (the central chest structures, including the heart) away from the affected side.
Key facts
- Normal state
- At rest the pressure in the pleural space is negative (below atmospheric pressure), which holds the lung against the chest wall.
- Pleural seal
- A very thin film of pleural fluid between the pleural layers forms the seal that makes the lung follow the chest wall.
- Mechanism
- Air enters the pleural space, the pressure rises towards atmospheric, and the lung's elastic recoil collapses it inwards.
- Air entry
- Air can enter after injury to the chest wall, or from an air-filled cyst on the lung that can burst.
- Presentation
- A pneumothorax can happen suddenly and can present with sharp pleuritic chest pain (worse on breathing in) and breathlessness.
- Tension pneumothorax
- Air can get in but not out, so the pleural pressure can rise above atmospheric and the mediastinum is pushed away from the affected side.
Exam traps
- Tightening of airway muscle (bronchoconstriction) reduces airflow in asthma, but it does not detach the lung from the chest wall.
- A blocked airway can collapse part of a lung when the trapped air is absorbed, which is called atelectasis, but in a pneumothorax the airway is open and the problem lies outside the lung.
- In a pneumothorax the immediate cause of collapse is not air directly squeezing the lung from outside. Air raises the pleural pressure towards atmospheric, so the lung's own recoil is no longer opposed.
Read the full notes
Tested in 10 SimpleMed+ questions
Practice it in Respiratory System, with the answer and a full explanation after every question.
Practice these questions