Pleural fluid is secreted by the mesothelial cells that line the pleura, a two-layer membrane that covers the lungs and lines the chest cavity. The fluid is secreted through a process called active transport, which requires energy.
Pleural fluid has several functions, including:
Lubrication: Pleural fluid helps to lubricate the pleura, so that the two layers of the membrane can glide smoothly over each other during breathing.
Protection: Pleural fluid helps to protect the lungs from injury and infection.
Waste removal: Pleural fluid helps to remove waste products from the lungs.
Pleural fluid is also important for maintaining the negative pressure in the pleural cavity. This negative pressure helps to keep the lungs expanded, even during exhalation.
A normal adult has about 5-30 mL of pleural fluid in each pleural space. However, too much pleural fluid can build up, a condition known as pleural effusion. This can happen for a variety of reasons, including heart failure, pneumonia, and cancer. Pleural effusions can make it difficult to breathe and can be life-threatening if not treated.