Overview

Pleural fluid volume≈0.1–0.3 mL/kg per side

Intrapleural pressure at FRC≈ −5 cmH2O (≈ −4 mmHg)

End of quiet inspiration≈ −8 cmH2O

Normally each pleural cavity contains only ≈0.1–0.3 mL/kg of low-protein fluid, forming a layer ≈10–20 µm thick. Fluid is filtered from parietal pleural capillaries (systemic pressure) and removed by parietal lymphatic stomata; lymphatic capacity can increase many-fold before fluid accumulates.

Because the lungs tend to recoil inwards and the chest wall outwards, pressure in the pleural cavity is subatmospheric — ≈ −5 cmH2O at FRC, falling to ≈ −8 cmH2O at the end of quiet inspiration. In the upright lung it is more negative at the apex than at the base (gravity, ≈0.25 cmH2O per cm), so apical alveoli are more expanded at rest and basal alveoli are more compliant and better ventilated. If air or fluid enters the space, the coupling is lost and the lung collapses towards the hilum.

Location: Between the visceral and parietal pleura of each lung; separate right and left cavities.

Function

Couples lung and chest-wall movement.
Allows the lungs to slide freely during breathing.
Maintains negative intrapleural pressure that keeps the lungs expanded.

Physiology: Intrapleural pressure ≈ −5 cmH2O at FRC; transpulmonary pressure (alveolar − intrapleural) distends the lung. During forced expiration intrapleural pressure becomes positive (up to +100 cmH2O or more in coughing).

Clinical significance

Pneumothorax and tension pneumothorax

Air in the pleural cavity — spontaneous, traumatic or iatrogenic. Tension pneumothorax (one-way valve) shifts the mediastinum and obstructs venous return; immediate needle decompression is life-saving.

Haemothorax, chylothorax and empyema

Blood (trauma), chyle (thoracic duct injury, lymphoma) or pus (complicated pneumonia) in the pleural space all require drainage.

Chest drain insertion

Placed in the safe triangle — lateral border of pectoralis major, anterior border of latissimus dorsi, and a horizontal line at the level of the nipple (≈5th intercostal space) — passing just above the upper border of a rib to avoid the intercostal vein, artery and nerve in the costal groove.

Test yourself

When a needle or chest drain is passed through an intercostal space into the pleural cavity, it should run:

Just below the lower border of the rib above
Through the exact middle of the space, regardless of rib position
Along the costal groove
Just above the upper border of the rib below

Answer: Just above the upper border of the rib below

The intercostal vein, artery and nerve (VAN, from above down) run in the costal groove along the inferior border of each rib. Passing just above the upper border of the rib below avoids the main neurovascular bundle.