The Functions and Disorders of the Pleural Fluid

Pleural fluid is defined as the fluid that is found between the layers of the pleura, the membranes of which line the 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.

Woman laughing on patio during winter
Hero Images / Getty Images

Changes in the volume of pleural fluid can be caused by infection, trauma, or other causes and can lead to respiratory problems and other adverse conditions. Extraction of pleural fluid allows doctors to diagnose causes of these changes or to investigate for signs of infection or disease. When a large amount of pleural fluid is present, it is usually removed due to symptoms. Often times, however, the fluid recurs, and procedures to prevent further recurrences are then needed.

Structure and Function of Pleural Fluid

Pleural fluid is a thin translucent fluid that fills the cavity between the parietal (outer) and visceral (inner) pleural layers surrounding the lungs. The volume of fluid is small, roughly 1-10 ml.

Pleural fluid functions by lubricating the space between the pleura, allowing the pleura to glide smoothly during inhalation and exhalation. In this way, it cushions delicate lung tissues against friction from the ribs and the chest wall itself.

Conditions Involving the Pleural Cavity

There are several conditions that can affect the pleural cavity and, by default, the pleural fluid. Among these:

  • Pleural effusion is a condition wherein excess fluid accumulates in the pleural space. 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. Approximately 1.5 million people in the U.S. are affected by pleural effusion each year.
  • When a buildup of fluid contains cancer cells, it is called a malignant pleural effusion. While this most commonly occurs with during stage 4 lung cancer, it can also occur with other cancers that have spread (metastasized) from other parts of the body, including the breast and ovaries.

Types of Pleural Fluid

There are two primary types of pleural fluid found in pleural effusions.


One is a transudate, which is a thin, clear fluid. Causes of a transudate may include:

  • Congestive heart failure
  • Chest surgery
  • Cirrhosis of the liver
  • Pulmonary emboli (blood clots in the legs that break off and travel to the lungs)


The other type of fluid is an exudate, a thicker, pus-like fluid. an exudate may be found in:

  • Cancer (with a malignant pleural effusion)
  • Pneumonia
  • Inflammatory or autoimmune diseases such as lupus and rheumatoid arthritis
  • Hemothorax (with bleeding into the pleural cavity)
  • Chylothorax (when chyle fills the pleural cavity)

Symptoms of Pleural Disorders

When fluid builds up in the pleural space, it can compress the underlying lung. This, in turn, may cause symptoms such as:

  • Chest pain, especially pain with a deep breath (pleuritic chest pain)
  • Shortness of breath
  • Coughing (usually dry and non-productive)
  • Orthopnea or difficulty breathing when lying down (people with increased pleural fluid are often more comfortable sitting up than reclining)
  • Lightheadedness or fainting if the amount of fluid is large


An excess amount of pleural fluid (pleural fluid) may be noted on a chest X-ray or chest CT scan, though these imaging tests are unable to determine what the fluid is.

In a procedure referred to as a thoracentesis, a fine needle is inserted into the pleural space, and a sample extracted. If a large amount of fluid is present, this procedure can also remove fluid in order to improve breathing.

To determine the type of fluid, it is examined under the microscope (cytology) and other tests are done to check for the composition of the fluid. Cytology can detect the presence of certain white blood cells (their presence indicates an infection), bacteria (using a gram stain), and other substances that shouldn’t be there. If an infection is suspected, the fluid would then be cultured to identify the specific organism.


If excess pleural fluid accumulates in the pleural space, it can result in increasing shortness of breath, chest pain (often worsening with a deep breath,) and may eventually compress the heart leading to heart failure.

To remove the fluid, a chest tube is usually placed. Chest tube placement involves the insertion of a flexible tube into the pleural space. The tube may be left in place to drain excess fluid, blood, or air which has accumulated. It can be kept in place for varying amounts of time depending on the condition or cause.

Sometimes, fluid continues to accumulate, making it difficult to remove the chest tube. There are a few procedures that can be done if this occurs. In a pleurodesis, the two membranes of the pleura are forced to adhere to each other by injecting a chemical (such as talc) into the pleural space. This creates inflammation and eventually scarring which serves to glue the two layers of the pleura together, obliterating the pleural space.

Another option is to place a one-way valve that lets air and fluid out of the pleural space but blocks air entry from outside into the pleural space. The fluid can then be withdrawn periodically, even in the comfort of your home. This is done most often for pleural effusions related to advanced cancer.

Finally, a pleurectomy may be performed. In this procedure the pleural membranes are removed, effectively removing the pleural space.

A Word From Verywell

Pleural fluid plays an important role in cushioning the movements of the lungs, but when increased in quantity, can limit this movement and cause symptoms such as pain and shortness of breath. Not all excesses of pleural fluid are the same, and determining the composition and the underlying cause are important in order to determine the best treatments.

4 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Ibitoye BO, Idowu BM, Ogunrombi AB, Afolabi BI. Ultrasonographic quantification of pleural effusion: comparison of four formulaeUltrasonography. 2018;37(3):254–260. doi:10.14366/usg.17050

  2. Kookoolis AS, Puchalski JT, Murphy TE, Araujo KL, Pisani MA. Mortality of hospitalized patients with pleural effusionsJ Pulm Respir Med. 2014;4(3):184. doi:10.4172/2161-105X.1000184

  3. Karkhanis VS, Joshi JM. Pleural effusion: diagnosis, treatment, and managementOpen Access Emerg Med; 4:31–52.

  4. Porcel JM. Biomarkers in the diagnosis of pleural diseases: a 2018 updateTher Adv Respir Dis. 2018;12:1753466618808660. doi:10.1177/1753466618808660

Additional Reading

By Lynne Eldridge, MD
 Lynne Eldrige, MD, is a lung cancer physician, patient advocate, and award-winning author of "Avoiding Cancer One Day at a Time."