The Pleural Cavities and Pleural Membranes

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Chapter: Anatomy and Physiology for Health Professionals: Respiratory System

The pleurae are relatively thin, even though they have two layers. Although they can slide across each other with ease, their separation is highly resisted by the surface tension of the pleural fluid.

The Pleural Cavities and Pleural Membranes

The pleurae are relatively thin, even though they have two layers. Although they can slide across each other with ease, their separation is highly resisted by the surface tension of the pleural fluid. Each pleura is a potential space and not an open structure.

The visceral pleura consists of a layer of serous membrane attached to each lung surface that folds back to become the parietal pleura. The visceral pleura covers the external lung surfaces and lines their fissures. The parietal pleura forms part of the mediasti-num and lines the inner thoracic cavity (FIGURE 21-7). It also forms the superior face of the diaphragm. The slit-like potential space between the visceral and parietal pleurae is called the pleural­cavity. It contains a thin film of serous or ­pleural fluid that reduces friction as the pleurae move against each other during breathing.

Pulmonary Ventilation

Pulmonary ventilation is a mechanical process thatconsists of inspiration and expiration. Alveolar venti-lation is the movement of air into and out of the alveoli. Pulmonary ventilation is based on volume changes in the thoracic cavity. You should remember the following:

Volume changes always lead to pressure changes.

Pressure changes lead to flow of gases to equalize pressure.

The relationship between pressure and volume of a gas is explained by Boyle’s law, which states that at a constant temperature, the pressure of a gas changes inversely with its volume:

P1V1= P2V2

where P is the pressure of the gas, V is its volume, and 1 and 2 indicate the initial condition and the resulting condition, respectively.

Gases always fill their containers, so the size of a container influences the space between gas mole-cules. A larger container will cause gas molecules to be farther apart and, therefore, pressure to be lower. Reducing container volume brings the gas molecules closer and increases the pressure. This is a simple way to understand how gases work inside our lungs. An example that helps explain this principle is a car tire. The air inside the tire is compressed to about one-third of its atmospheric volume, providing high pres-sure that can handle the weight of the car.


Normal individuals utilize quiet expiration in a passive process, which is based more on the elasticity of the lungs than upon muscle contraction. The rib cage descends and the lungs recoil as the inspiratory muscles relax, resuming their resting lengths. Therefore, thoracic and intrapulmonary volumes are decreased. This decrease in volume compresses the alveoli. Intrapulmonary pressure rises to approximately 1 mm Hg higher than atmospheric pressure. When the intrapulmonary pres-sure is greater than the atmospheric pressure, the pres-sure gradient forces gases out of the lungs.

Forced expiration is active and caused whenabdominal wall muscles contract. These are mostly the oblique and transversus muscles. There are two results of these contractions:

Increased intra-abdominal pressure: This forcesabdominal organs superiorly against the diaphragm.

­Depression of the rib cage: The internal intercostalmuscles assist in this depression and decrease thoracic volume.

The accessory muscles of expiration require con-trol in order to accurately regulate air flow from the lungs. The ability of a singer to hold a long note is based on coordination of several muscles that are usu-ally used for forced expiration.

1. Describe the force that controls pulmonary ventilation.

2. Identify the actions of the diaphragm and intercostal muscles in inspiration.

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