What is Pleural Fluid?

What is Pleural Fluid and What Happens When There's Too Much?

diagram of the lungs in the body
What is pleural fluid?. Istockphoto.com/Stock Photo©Janulla

Definition: Pleural Fluid

Pleural fluid is the fluid that is found between the layers of the pleura, the membranes that line the thoracic cavity and surround the lungs. The space containing the fluid is referred to as the pleural cavity or pleural space. Normal pleural fluid consists of a small amount of a thin (serous) fluid that functions as a lubricant during breathing.

Anatomy and Structure

Pleural fluid is a thin translucent fluid that fills the cavity between the parietal (outer) and visceral (inner) pleural layers surrounding the lungs.

The amount of fluid is small, roughly 20 cc, or 4 teaspoons total.

Function of Pleural Fluid

Pleural fluid functions to lubricate the space between the pleura, allowing the pleura to glide smoothly during inspiration and expiration. In this way the pleural fluid cushions the delicate lung tissue against friction from the ribs and chest wall.

Conditions with Increased Pleural Fluid

There are several conditions which may involve the pleural cavity.  Some of these include:

  • Pleural effusion - A pleural effusion is an excess build-up of pleural fluid in the pleural cavity. There are many causes of pleural effusions, including congestive heart failure, pulmonary embolism, kidney conditions, cancer, and autoimmune diseases such as lupus and rheumatoid arthritis.
  • Malignant pleural effusion - When a build up of fluid contains cancer cells, it is called a malignant pleural effusion.  This most commonly occurs with lung cancer (a malignant pleural effusion defines lung cancer as stage 4), but may also occur with other cancers that metastasize (spread) to the region of the lungs, such as breast cancer and ovarian cancer.


    When fluid builds up in the pleural space it can compress the underlying lung. This, in turn, can cause symptoms of shortness of breath and chest pain.


    To access pleural fluid, a few approaches may be taken.

    • Thoracentesis  - If your doctor finds that you have too much pleural fluid, she may recommend that a sample of the fluid be removed by a procedure called thoracentesis.  In this procedure, a needle is placed through the skin on the chest into the pleural space to obtain a sample of the fluid, which can then be analyzed in the lab.
    • Chest tube placement - A chest tube is a flexible tube that is placed from outside the body and into the pleural space.  The tube may be left in place for varying amounts of time depending on the reason it is placed.

    Laboratory Tests

    • Pleural fluid analysis - Pleural fluid obtained via a thoracentesis is first examined for things such as protein.  There are two primary types of pleural fluid found in pleural effusions.  One is a transudate which can be thought of as a "thin" clear fluid most commonly found in congestive heart failure.  The other is an exudate which is a thicker "pus-like" fluid more commonly found when an infection is present.
    • Pleural fluid cytology - Pleural fluid is then evaluated for the presence of white blood cells (a sign of infection,) red blood cells, and bacteria (a gram stain)  If an infection is suspected the fluid is then cultured.


    With lung cancer, an excess amount of pleural fluid (pleural effusion) is quite common, and can be either benign (non-cancerous) or due to the spread of lung cancer cells into the pleural cavity (malignant pleural effusion).


    American Society of Clinical Oncology. Cancer.Net. Fluid Around the Lungs or Malignant Pleural Effusion. 09/2014. http://www.cancer.net/navigating-cancer-care/side-effects/fluid-around-lungs-or-malignant-pleural-effusion

    Dixon, G., de Fonseka, D., and N. Maskell. Pleural controversies: image guided biopsy vs. thoracoscopy for undiagnosed pleural effusions?. Journal of Thoracic Disease. 2015. 7(6):1041-51.

    U.S. National Library of Medicine. MedlinePlus. Pleural Fluid Analysis. Updated 12/02/15. https://medlineplus.gov/ency/article/003624.htm

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