The pleural cavity is a fluid filled space that surrounds the lungs. It is found in the thorax, separating the lungs from its surrounding structures such as the thoracic cage and intercostal spaces, the mediastinum and the diaphragm. The pleural cavity is bounded by a double layered serous membrane called pleura.
Pleura is formed by an inner visceral pleura and an outer parietal layer. Between these two membranous layers is a small amount of serous fluid held within the pleural cavity. This lubricated cavity allows the lungs to move freely during breathing.
In this article we will learn about the anatomy and function of the pleural cavity.
|Location||Surrounding the lungs
Between parietal and visceral layers of pleura
Superior: root of neck (above rib 1)
Inferiorly: diaphragm and costal margin
Laterally: thoracic wall
|Parietal pleura||Lines the boundaries of the cavity
Costal: lines thoracic wall
Diaphragmatic: lines diaphragm
Mediastinal: lines mediastinum
|Visceral pleura||Lines the surface of the lung itself|
Costomediastinal: anterior, between the costal and mediastinal layers of parietal pleura
Costodiaphragmatic: inferior, between the costal and diaphragmatic pleura
|Pleural effusions||Excess build-up of fluid in the pleural cavity|
The pleural cavity surrounds the lungs in the thoracic cavity. There are two pleural cavities, one for each lung on the right and left sides of the mediastinum. Each pleural cavity and it’s enclosed lung are lined by a serous membrane called pleura. The right and left pleural cavities are completely independent compartments. This is important for containing the spread of infection from one lung to the other. It also means that the function of one lung is not compromised by damage to the other. The left pleural cavity is smaller than the right one. This is because the heart in the mediastinum projects to the left hand side of the thoracic cavity, limiting the space available for the left lung and its corresponding pleural cavity.
The boundaries of the pleural cavity are:
- Superior: root of the neck, 2-3 cm above the level of rib one
- Inferior: The thoracic surface of the diaphragm inferiorly
- Medial: The mediastinum medially
- Anterior, posterior and lateral: The internal surface of the ribs, costal cartilage and intercostal muscles anteriorly, posteriorly and laterally
The lung almost entirely fills the space in the thorax. Therefore, the pleural cavity is described as a potential space, meaning that under normal circumstances, there is no actual space present, and the visceral and parietal pleura are in contact with each other. This potential space is also important as the unfilled spaces of the pleural cavity form recesses, which allow extra space for expansion of the lung. These will be discussed later in this article.
The pleurae are two layers of serous membrane that form the boundaries of the pleural cavity. There are two types of pleura; parietal and visceral. The parietal pleura is the thicker and more durable outer layer that lines the inner aspect of the thoracic cavity and the mediastinum. The visceral pleura is the more delicate inner layer of pleura that lines the outer surface of the lung itself. The parietal and visceral layers are not entirely separate, rather they are continuous with each other at the hilum of the lung. Each layer consists of a single layer of mesothelial cells and supporting connective tissue including collagen, elastin, blood vessels and lymphatics. The pleural cavity containing a small amount of pleural fluid is contained between the parietal and visceral layers of pleura.
A common way of describing the location of the parietal and visceral pleura relative to one another is by thinking of pushing your fist into an underinflated balloon, a useful analogy for the developing lung. Your fist represents the developing lung and the balloon, the pleural cavity. The balloon itself is the pleura, and the space inside is the thoracic cavity that will become the pleural cavity. As you push your fist into the balloon, it becomes almost entirely covered in pleura. This ‘pleura’ covering your fist would be the visceral pleura, and all that is remaining (not covering your fist) would be the parietal pleura. Both are continuous with each other. The space between the parietal and visceral pleura is the pleural cavity. The lung itself is not located within the pleural cavity, rather it is surrounded by it.
The function of the pleura is to allow optimal expansion and contraction of the lungs during breathing. The pleural fluid acts as a lubricant, allowing the parietal and visceral pleura to glide over each other friction free. This fluid is produced by the pleural layers themselves.
The parietal pleura is the layer of pleura associated with the walls of the pleural cavity. It lines the internal aspect of the thoracic wall, the thoracic surface of the diaphragm and separates the pleural cavity from the mediastinum.
The parietal pleura is divided into four parts. The cervical part (also called the dome of pleura or pleura cupola) is a dome shaped layer that lines the upper aspect of the thoracic cage. It extends superiorly through the superior thoracic aperture into the root of the neck following the projection of the pleural cavity that lines the apex of the lung. It extends between the level of the first rib and 2-3cm superior to the middle third of the clavicle. It is covered by fascia in the neck called the suprapleural membrane.
As the name suggests, the costal part lines the internal surface ribs, which are a large contributor to the thoracic wall. Other structures of the thoracic wall lined by the costal part of parietal pleura are the sternum, ribs, costal cartilages, intercostal muscles and sides of the thoracic vertebrae. It is separated from the thoracic wall by a layer of endothoracic fascia.
The mediastinal part covers the lateral aspect of the mediastinum located in the centre of the thorax between right and left pleural cavities. It is continuous with the costal pleura anteriorly and posteriorly.
The diaphragmatic part covers the exposed superior surface of the diaphragm on each side of the mediastinum (i.e. excluding its attachments to the ribs and costal cartilages and the pericardium). It is a thin layer that is tightly adhered to the diaphragm and is continuous with the costal pleural anteriorly, posteriorly and laterally and the mediastinal pleura medially.
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There are a number of areas in the thorax where the parietal pleura changes direction as it passes from one surface onto another. These areas are known as pleural reflections. The sternal line of pleural reflection is a sharp turn where the costal pleura becomes the mediastinal pleura anteriorly. The costal line of pleural reflection is another abrupt turn where the costal pleura becomes the diaphragmatic pleura inferiorly. The vertebral line of pleural reflection is a more rounded turn where the costal pleura becomes the mediastinal pleura posteriorly. Sometimes, these reflections of pleura can form a pocket-like space in the thoracic cavity called a pleural recess, which will be discussed in more detail further in this article.
Innervation of the parietal pleura
The parietal pleura receives somatic afferent (sensory) innervation from two different sources; the intercostal nerves (T1-T11) and the phrenic nerve (C3-C5). The intercostal nerves (T1-T11) provide innervation to the costal pleura and peripheral diaphragmatic pleura. The mediastinal pleura and the central parietal pleura are innervated by the phrenic nerve (C3-C5).
The visceral pleura is the serous membrane that is directly adhered to the outer surface of each lung. It extends into the horizontal and oblique fissure of the lungs, lining the opposing surfaces of these fissures. It is much thinner than the parietal pleura, making it more delicate.
The visceral pleura covers the lung on all surfaces except at the hilum of the lung, where the structures that form the root of the lung enter and exit its mediastinal surface. These structures include the pulmonary artery and vein, the bronchi, nerves and lymphatics. Between the levels of T5-T8, the mediastinal pleura reflects off the mediastinum forming a tubular sleeve of pleura. Here, it is continuous with the visceral pleura forming a covering over the root of the lung called the pleural sleeve. Inferior to the hilum of the lung, the continuation of the visceral and parietal pleurae forms the pulmonary ligament.
The visceral pleura is innervated by visceral afferent (autonomic) nerves via the pulmonary plexus. It generally does not feel pain, but is sensitive to stretch sensation.
In some areas of the thorax, the lungs do not completely occupy the pleural cavity. This is especially true for the inferior region of the pleural cavity, where the inferior margin of the parietal pleura extends approximately two ribs inferior to the lung. This results in an area of the pleural cavity where two layers of parietal pleura are directly opposed against each other, separated by pleural fluid (i.e. where the lowest part of the costal pleura is continuous with the diaphragmatic pleura at the costal line of pleural reflection). The potential pleural spaces that are formed are called pleural recesses.
There are two pleural recesses. The costodiaphragmatic recesses (also called costophrenic angles) are the larger of the recesses located between the costal and diaphragmatic pleura of right and left pleural cavities. They occur at the costal reflection where the costal pleura becomes continuous with the diaphragmatic pleura. They are essentially gutters that surround the convexity of the diaphragm against the thoracic wall. They extend between the inferior margin of the lung and the inferior margin of the pleural cavity.
The smaller costomediastinal recesses are found anteriorly at the sternal reflection where the costal pleura is in contact with the mediastinal pleura. This recess is larger on the left side due to the presence of the cardiac notch of the left lung.
During forced inspiration, these recesses provide spaces into which the lungs can expand. They are also important in the clinical setting, which will be described later in this article.
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As previously mentioned, the inferior border of the parietal pleura is located more inferiorly to the lung, resulting in the formation of pleural recesses. The surface relations of the inferior margin of the parietal pleura can be pinpointed on the surface at three different areas around the thorax; the midaxillary line, the midclavicular line and the vertebral column. These external reference points are important to know for accessing the pleural cavity without causing damage to the lung.
Procedures that would require this knowledge include draining of a pleural effusion, a medical condition involving the pleural cavity that will be discussed further on in this article. The inferior margin of the parietal pleura takes an oblique path laterally from the level of rib 8 in the midclavicular line to rib 10 in the midaxillary line and the T12 vertebra at the vertebral column.
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As previously mentioned, the pleural recesses provide additional space for the lungs to expand during forced inspiration. However, they can also be a potential site for fluid accumulation, and from here this fluid can sometimes be aspirated.
Pleural effusion is an excessive build-up of fluid in the pleural cavity. They can be bi- or uni- lateral. Effusions can restrict space within the pleural cavity, compressing the lung and reducing its ability to expand for inspiration. Due to gravity, the excess fluid is likely to collect in the costodiaphragmatic recess, clinically referred to as the costophrenic angle. The build-up of fluid in the pleural space causes the lung on the same side to be pushed upwards. Because the right and left pleural cavities are separate from each other, the opposite lung will not be affected unless its surrounding pleural cavity is also compromised.
Radiological approaches are frequently used to diagnose a pleural effusion. It can be seen on a chest x-ray and is known as a costophrenic angle ‘blunting’. The costophrenic angle will appear more obtuse and blurred in this case, rather than sharp and distinct under normal circumstances. Pleural effusion has multiple etiologies. It can be caused by a number of things, such as infection, cancer, cardiac failure, liver disease and pulmonary embolism. Depending on its pathophysiology, a pleural effusion can be classed as transudate or exudate. Transudates are caused when there is an imbalance in osmotic pressure, which can occur in heart failure and liver disease. They contain a small amount of protein and immune cells (mainly macrophages and lymphocytes). In comparison, exudates are associated with inflammation and are abundant with immune cells and proteins. These occur more often in infection or cancer. To treat pleural effusion, a chest tube may be inserted through the intercostal space into the pleural cavity to drain the excess fluid thus allowing the lungs to expand again.
When blood, pus or air accumulate in the pleural cavity, these conditions are referred to as hemothorax, pyothorax and pneumothorax respectively.