Pleural effusion

“Pleural” refers to the space between the chest cavity and the lungs, and “effusion” refers to a collection of fluid, so a pleural effusion is when a disease process causes fluid to start to collect in the pleural space, which can sometimes restrict lung expansion.

The pleural cavity or pleural space lies between the parietal pleura which is stuck to the chest wall and the visceral pleura which is stuck to the lungs.

Because the lungs fit snugly inside the chest cavity, the visceral and parietal pleura lie right next to each other, and the very very thin space between them contains a layer of fluid that acts as lubrication to allow the lungs to slide back and forth as they expand and contract.

This pleural fluid is similar to interstitial fluid and is made slippery by proteins like albumin.

It’s so similar to interstitial fluid because it--essentially--is interstitial fluid.

There is always a tiny bit of plasma that leaks out of capillaries and gets into the interstitial space, and since these capillaries are so close to the edge of the pleural space, that fluid makes its way into that space and collects there.

If there were no way out of the pleural space, then it would fill up with fluid, but fortunately, there are lymphatic vessels in the pleura then drain the fluid away and deliver it back into the circulatory system.

A pleural effusion is when there’s excess fluid in the pleural space either because too much pleural fluid is produced by the body, which can be due to either a transudative or exudative effusion or because the lymphatics can’t effectively drain away the fluid, called a lymphatic effusion.

A transudative pleural effusion occurs when too much fluid starts to leave the capillaries either because of increased hydrostatic pressure or decreased oncotic pressure in the blood vessels.

Hydrostatic pressure is what we normally think of as blood pressure; it is the force that blood exerts on the walls of the blood vessel, and can be thought of as a pushing force.

A common cause of increased hydrostatic pressure in the lung capillaries is heart failure.

That’s because when the heart can’t effectively pump blood out to the body, it backs up into the pulmonary vessels and causes the blood pressure in those vessels to rise. The high pressure forces fluid out of the capillaries and into the pleural space.

Oncotic pressure results from the the inability of solutes like large proteins - albumin for example - to move across through the capillary.

By the process of osmosis - the process, not the company - fluid moves from areas of low solute concentration to high solute concentration.

Fluid therefore flows out of capillaries and leaks into the pleural space when there is decreased oncotic pressure in the blood vessels.

Two causes of low oncotic pressure are cirrhosis, where the liver makes fewer proteins and nephrotic syndrome, where proteins are lost through the urine.

An exudative pleural effusions is due to inflammation of the pulmonary capillaries which makes them much more leaky.

The larger spaces between endothelial cells allows fluid, immune cells and large proteins like lactate dehydrogenase (LDH) --which is found in all cells, to leak out of the capillaries.

The causes can vary - trauma, malignancy, an inflammatory condition like lupus, or an infection like pneumonia.

If the underlying reason is an infection, like a bacterial or mycobacterial infection, then it’s also possible for that infection to spread into the pleural space which is a walled off space - a bit like an enormous abscess.

Just like an abscess, the infected pleural space can develop fibrinous walls and have loculations.

Finally, there can be a lymphatic pleural effusion, called a chylothorax.