Pleural Cavity and Membranes - Anatomy & Physiology

Introduction

The surface of the inner wall of all of the body cavities is lined by a serous membrane which consists of a single layer of flat epithelium with a thin underlying propria (connective tissue). Within the thoracic cavity, this is known as the pleura.

The visceral pleura which coats the outer surface of the lung is derived from the splanchnic mesoderm. The parietal pleura lining the thoracic cavity is derived from somatic mesoderm

The pleural cavity is a potential space between the two areas of pleural membrane, which normally are adhesed to each other.

Pleural Membranes

Structure of the Pleural Membranes

Schematic Diagram of the Pleural Membranes (Cross section through thoracic cavity at the level of the heart)

• Each lung is placed within a separate layer of membrane, thus there are two pleural sacs.
• The space between the two sacs is known as the Mediastinum, and is almost in the midline of the thorax.
• The pleura covering the surface of the lung is known as Pulmonary Pleura or Visceral Pleura. This becomes continuous with the Mediastinal Pleura as it wraps around the lung.
• The Diaphragmatic and Costal Pleura are continuous with the Mediastinal Pleura. Together, these three membranes are the Parietal Pleura.
• Between the Parietal and Visceral Pleura is the Intrapleural Space, or Pleural Cavity. This contains a small amount of serous fluid which establishes adhesion between the layers and allows smooth movement between the lung and chest wall, and between individual lobes of the lungs.
• The Heart is invested in a Parietal and Visceral layer of Pericardium with the Pericardial Space separating the two membranes.
• Cranially the Costal and Mediastinal Pleura adjoin to form a 'dome' of pleura which extends cranially to the thoracic inlet. This is known as the Cupulae Pleurae. This region is only protected by soft tissue, and thus is vulnerable to injury.
• Within the pleural sac encasing the right lung, there is a fold which encases the Vena Cava. It is known as the Plica Venae Cavae.
• The Pleural Membranes are larger than the lungs which they encase, thus there are areas where the facing surfaces of parietal pleura touch each other. These are known as Pleural Recesses. One example is the Costodiaphragmatic Recess, the space between the costal and diaphragmatic pleura. The volume of the recess varies to a point due to the phase of inspiration/expiration.

Pleural Membrane Function

• The Pleural Cavity is filled with a small amount of serous fluid which forms a thin film of liquid between the pleural layers. This is vital in that it prevents separation of the two pleural layers and lubricates the surface, so the lungs can move easily within the thoracic cavity.
• The Pleural Fluid also provides surface tension, keeping the lung suitably close to the wall of the thorax, despite the the lungs not being directly fixed to it. The pleurae thus allow the volume of the lungs to change with the volume of the thoracic cavity, enabling ventilation.
• The left and right pleural sacs are separate so that in the event of one being punctured, the other lung can continue to function effectively.

Vasculature

• The Pleural membranes are supplied by the Bronchial and Pulmonary Arteries.

Innervation

• Parietal Pleura is innervated by the intercostal nerves, with the Mediastinal and Diaphragmatic pleura also innervated by the Phrenic Nerve.
• Visceral pleura is innervated by the Vagus nerve and sympathetic fibres. It is insensitive to pain stimuli.

Species Differences

The mediastinum is visible in this dog dissection, left lung removed. ©RVC 2008

• In cattle the Mediastinum is a tough membrane, in contrast to horses, dogs and cats where it is rather thin and more delicate.
• Birds do not have a pleural cavity as Avian lungs do not have the capacity to inflate.
• The Respiratory Systems of non-Homeotherms are also very different to that of mammals.

Links

Pleural Cavity and Membranes - Pathology

References

• Dyce, K.M., Sack, W.O. and Wensing, C.J.G. (2002) Textbook of Veterinary Anatomy. 3rd ed. Philadelphia: Saunders.

• Budras, K.D., McCarthy, P.H., Fricke, W. and Richter, R. (2002) Anatomy of the Dog. 4th ed. Hannover: Schlutersche GmbH & Co. KG, Verlag und Druckerei.