3.1. Defining the 'Pleura' and the 'Pleural Cavity'
What is the 'Pleura'?
The pleura, which is also called the pleural membrane, is a serous* 2-layered membrane that lines most of the surface of a lung. There are two lungs and therefore two ‘pleurae’ in the human body.
*’Serous’ = fluid-secreting.
The pleura has two layers – an inner layer (visceral pleura) and outer layer (parietal pleura), shown in the diagram here. Actually, the two pleural layers are continuous with each other at the hilum of each lung (more of this in Section 8.3.) Understanding the layout here is key to understanding inspiration, expiration and the clinical case of a collapsed lung (i.e. pneumothorax). It might help to think about it like this: imagine it’s a freezing cold morning and you step out of the shower. You double over the towel to make it thicker and wrap it around you to dry off. So, all the way around your body (i.e. the lung) is the inner layer of the towel (the visceral pleural layer) and the outer layer of the towel (the parietal pleural layer). Remember that the two layers of the towel are continuous with each other – at some point there is a fold in the towel at which the outer layer becomes continuous with the inner layer. It's also worth noting that the two pleurae that are present in the body are separate.
The pleural cavity is located between the parietal and visceral pleural layers. The only thing it contains is pleural fluid. The lungs do not lie in the pleural cavity! In the analogy of the shower and towel, the pleural cavity is the space between the two layers of the towel, not the space between the inner layer of the towel and your body. However, you could say that the lungs lie within the thoracic cavity, as do the pleural layers.