3.6. Additional Features of the Pleurae
Pleural Fluid, Recesses and the Pulmonary Ligament
The pleural cavity is filled with approx. 18ml of pleural fluid in the average human (weighing 70kg). This fluid has two functions:
- (i) Reduces friction during inspiration and expiration.
- (ii) Acts to increase surface tension between the two pleural layers, ensuring that the lungs are in a continually ‘expanded’ position (i.e. prevents lungs from collapsing during expiration).
A common question asked in exams is \"Where exactly do the lungs lie with respect to the thorax?”. The correct answer is “The lungs lie within the thoracic cavity, surrounded by the pleural membrane”. The wrong answer is “The lungs lie within the pleural cavity”. In fact, the only thing that should ever be in the pleural cavity (i.e. between the parietal and visceral layers) is a very small amount of pleural fluid.
Be aware of the presence of two pleural recesses…
- (i) Costodiaphragmatic recess - during normal, quiet inspiration, the lungs do not fully occupy the thoracic cavity, and this means there is a space between the visceral and parietal layers in the inferolateral corners of each side of the thoracic cavity. Remember, the visceral pleural layer doesn’t extend as far down as the parietal pleural layer. The accompanying diagram helps to show this better.
- (ii) Costomediastinal recess - the same principle as the recess mentioned above, but this time the recess is present between the costal pleura and the mediastinal pleura. Search online for various images of this recess. The recess actually lies close to the cardiac notch – more on this later in Section 9.
Know about the pulmonary ligament – this is a redundant extension of the parietal (mediastinal) pleura on each side. At this stage, it is of little significance.