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The parietal pleura is a membrane that lines the cavity in the chest that contains the lungs. Another membrane, called the visceral pleura, closely covers the lung surfaces. The narrow gap between the parietal and the visceral pleura is known as the pleural cavity or space. Within this space, pleural fluid forms a thin layer on the pleural membranes, preventing friction between the surfaces.
Cavities inside the body are generally lined by what are known as serous membranes. Those that line cavity walls are called parietal layers and those that cover organs or viscera are known as visceral layers. Serous membranes produce serous fluid, which lubricates the space between the two different layers. The parietal pleura, which lines the thoracic cavity, is an example of a serous membrane and the type of serous fluid it produces is known as pleural fluid. This helps the surfaces of the parietal and visceral pleurae to glide over one another when the lungs expand and relax during respiration.
Parietal serous membranes generally have rich nerve supplies and the pleura is no exception, being well supplied by the intercostal nerves. These nerves ensure that the membrane is sensitive to pain and touch. The visceral pleura has a separate nerve supply and is only sensitive to stretch.
Normally, there is no air in the pleural space and, if air should enter, perhaps as the result of a stab wound or a diseased lung, the increase in pressure causes the lung to collapse. Air in the pleural space is known as pneumothorax. If the pleural space becomes continuous with the air outside, after a piercing injury for example, this is called an open pneumothorax.
In the case of an open pneumothorax, the patient's clothes and the chest wall may occasionally accidentally form a valve that allows air in but prevents air from leaving. Each time the patient breathes in, the pressure increases, eventually compressing the lung on the uninjured side. This is known as a positive-pressure or tension pneumothorax, and the danger is that the patient may end up with both lungs unable to function. Treatment involves administering oxygen and piercing the chest wall with a needle to allow the air to escape.
A condition known as pleurisy, where the pleural space becomes inflamed, may follow lung infections such as pneumonia. This can lead to roughening of the pleural membranes, causing them to rub together, a phenomenon that can often be heard through a stethoscope. Pleurisy can cause breathlessness and sharp pains in the chest, especially when breathing in or coughing, and treatment depends on the underlying cause.