What Are the Two Pleurae of the Lungs?

The visceral pleura adheres directly to the outer surface of each lung. It expands into the lung’s horizontal and oblique fissures, lining their opposite surfaces. It is significantly thinner and more fragile than the parietal pleura.

The visceral pleura covers most of the lung’s surface except in the hilum.

• The hilum is a triangular section at the midpoint of both the lungs where the pulmonary artery and vein, as well as the bronchi, nerves, and lymphatics, enter and exit the lungs.
• The mediastinal part of the parietal pleura connects with the visceral pleura to produce the pleural sleeve, which covers the base of the lung.
• The pulmonary ligament is formed by the continuation of the visceral and parietal pleurae inferior to the lung's hilum.

The pulmonary plexus innervates the visceral pleura through visceral afferent (autonomic) nerves. It does not experience pain in general although it is sensitive to strain sensations.

The parietal pleura covers the inside of the chest wall and the diaphragm. It is essential for the formation and removal of pleural fluid. Parietal pleura has direct connections between the pleural space and the underlying lymphatic network. These stomata (connections) allow big particles to be removed from the pleural space. Stomata are only found in the parietal pleura. The parietal pleura is thicker than the visceral pleura.

The parietal pleura is divided into four parts:

1. The cervical part of parietal pleura
   • The cervical part of parietal pleura is also called pleura cupola or dome of pleura.
   • It is a dome-shaped layer that lines the upper part of the thoracic cage and borders the apex of the lung.
   • It is located between the first rib and 2 to 3 cm superior to the middle part of the clavicle and extends toward the root of the neck.
   • The suprapleural membrane is a fascia in the neck that covers the cervical part of the parietal pleura.
2. The costal part of parietal pleura
   • The costal part of the parietal pleura covers the inner surface of the ribs. Ribs are a major part of the thoracic wall.
   • The sternum, costal cartilages, intercostal muscles, and sides of the thoracic vertebrae are other parts of the thoracic wall, which are covered by the coastal section of the parietal pleura.
3. The mediastinal part of parietal pleura
   • The mediastinum is the middle part of the thoracic cavity between the left and right pleural cavities and contains the heart, thymus gland, and some parts of the esophagus and trachea.
   • This part is covered by the mediastinal part of the parietal pleura.
   • It connects to the costal pleura both anteriorly and posteriorly.
4. The diaphragmatic part of the parietal pleura
   • The exposed superior surface of the diaphragm, other than parts of the diaphragm attached to the ribs, costal cartilages, and pericardium, is covered by the diaphragmatic part of the parietal pleura.
   • It is a thin layer that attaches strongly to the diaphragm and connects to the costal pleura anteriorly, posteriorly, and laterally, as well as to the mediastinal pleura medially.
   • The parietal pleura contains pain receptors and receives sensory impulses from the intercostal nerves and the phrenic nerve.

The lungs do not occupy the complete pleural cavity. For example, the lungs do not reach the inferior part of the pleural cavity, leaving a gap. Hence, two layers of parietal pleura are directly opposed to each other in the pleural cavity, separated by pleural fluid. This is the lowest part of the costal pleura, which is continuous with the diaphragmatic pleura. These gaps are called pleural recesses.

Pleural recesses provide additional space for the lungs to accommodate during forced inspiration.

Pleural recesses are of two types:

1. The costodiaphragmatic recesses
   • The costodiaphragmatic recesses or costophrenic angles are the largest of the recesses between the costal and diaphragmatic pleura of the right and left pleural cavities.
   • They appear where the costal pleura joins the diaphragmatic pleura and are located between the inferior margins of the lung and the pleural cavity.
2. The costomediastinal recesses
   • The costomediastinal recesses are located where the costal pleura meets the mediastinal pleura. Because of the existence of the cardiac notch of the left lung, this recess is greater on the left side.

Pleural space has a continuous circulation of the pleural fluid. This is formed by the parietal circulation and drained by the stomata on the parietal pleura. You must have 10 to 20 mL of pleural fluid within the pleural space for both inspiration and expiration. It reduces the friction between the parietal and visceral pleura during lung movements. Normal pleural fluid is clear plasma with some proteins and white blood cells.

During certain circumstances, more amount of pleural fluid is accumulated within the pleural space, and this condition is called pleural effusion.

Pleural effusion interferes with normal breathing and cause:

• Dry cough
• Chest pain
• Orthopnea (breathing is easier with sitting than lying down)
• Dyspnea (shortness of breath)

Pleural effusion is caused by infections, trauma, inflammatory diseases, heart and lung diseases, cancer, and autoimmune diseases.

The underlying cause of the pleural effusion determines the severity of the problem, such as compromised breathing, and indicates if it can be adequately treated. Pleural caused by a viral infection, pneumonia, or heart failure, can be efficiently treated or managed.

Pleural effusion is confirmed by various radiological imaging, including X-rays, ultrasound examinations, and computed tomography scans of the chest. Once you are diagnosed to have pleural effusion, thoracocentesis (a procedure where a needle is inserted into the chest cavity to remove excess pleural fluid) is done. The collected fluid is analyzed to determine the cause.

Sometimes you may have very little pleural effusion and gain relief from breathlessness with thoracocentesis without the need for further intervention.

Pleural fluid analysis can give a picture of the type of pleural effusion.

1. Transudate
   • Transudate effusion is low in proteins. It is caused by mechanical issues, such as an imbalance of pressure in specific blood arteries. Therefore, extra fluid leaks into the pleural space. Heart failure or liver cirrhosis are the most common causes of transudate pleural effusion.
2. Exudate
   • Exudate occurs when the pleura is injured or irritated. It is high in proteins. This can cause extra fluid to flow from certain blood arteries. There are several reasons for exudate pleural effusion, such as pneumonia, cancer, renal illness, and autoimmune disorders. It usually only affects one side of the chest.

The treatment choices for transudate and exudate effusions are different.