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Function of Pleural Cavities and Pleural Membranes

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  • 0:07 Pleural Cavities and Membranes
  • 1:01 Pleural Cavities and Pressures
  • 2:40 Pneumothorax
  • 3:25 Lesson Summary
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Lesson Transcript
Instructor: John Simmons

John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.

Each lung is contained within a pleural cavity, the space between the outside of the lung and inside of the chest wall. Pleural membranes cover the outside of the lungs and line the inside of the chest wall. The lungs remain expanded when we breathe due to a vacuum effect within the pleural cavity.

Pleural Cavities and Membranes

The thoracic cavity contains the lungs along with other organs, such as the heart. Each lung is contained within a pleural cavity. Each pleural cavity is defined by a space surrounding each lung and is lined by a pleural membrane. The pleural membrane is made up of two layers. These layers can be illustrated in a transverse section of the thoracic cavity:

Transverse section of the thoracic cavity
transverse section of thoracic cavity

The visceral or pulmonary pleura covers the outside of the lungs, and the parietal or costal pleura lines the inside of the chest wall, and it extends over the top of diaphragm. The pleural membranes secrete a lubricating fluid which allows them to move freely against each other during ventilation, like pistons within a cylinder of an engine. Without this fluid, breathing would be very difficult and even painful.

Pleural Cavities and Membranes

Lungs within the thoracic cavity
lungs within thoracic cavity

In order to understand how we breathe, we need to understand the different spaces and pressures within our thoracic cavity. The illustration above shows the lungs within the thoracic cavity. The space between the outside of the lungs and inside of the chest wall is exaggerated to illustrate the pleural cavity, or sometimes referred to as the pleural space. The pressure within this pleural cavity is referred to as the intrapleural pressure, and it is less than the intrapulmonary pressure - that's the pressure within the lungs. That's so important I'm going to say it again. The intrapleural pressure is less than the intrapulmonary pressure.

Transpulmonary pressure is the difference between the intrapleural pressure (outside of the lungs) and the intrapulmonary pressure (that's inside the lungs). Let's consider the implications of the transpulmonary pressure. The lower intrapleural pressure outside the lungs creates a vacuum that keeps the lungs expanded and prevents them from collapsing.

Let's do a little experiment. Go ahead and take a deep breath and force out as much air as possible. Even at the end of the forced expiration, your lungs were still expanded. They did not collapse because the intrapleural pressure is lower than intrapulmonary pressure. If not for this transpulmonary pressure, the lungs would collapse and we could not breathe.

Pneumothorax

Wounds that create a hole in the chest wall can cause a pneumothorax.
Pneumothorax Image

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