Pressure Relationships in the Thoracic Cavity

Atmospheric Pressure (Patm)

Atmospheric pressure is the pressure exerted by the air surrounding the body. It serves as the reference point for other respiratory pressures.

• Definition: The force exerted by the weight of air in the atmosphere on the surface of the body.

• Standard Value: 760 mm Hg at sea level, equivalent to 1 atmosphere (atm).

• Relative Pressures: Respiratory pressures are described relative to Patm:

  • Negative respiratory pressure: Less than Patm

  • Positive respiratory pressure: Greater than Patm

  • Zero respiratory pressure: Equal to Patm

Intrapulmonary and Intrapleural Pressures

Understanding the pressures within the lungs and pleural cavity is essential for grasping the mechanics of breathing.

• Intrapulmonary Pressure (Ppul): The pressure within the alveoli (also called intra-alveolar pressure). It fluctuates with breathing but always equalizes with atmospheric pressure between breaths.

• Intrapleural Pressure (Pip): The pressure within the pleural cavity. It is always negative relative to Ppul (typically about -4 mm Hg).

• Transpulmonary Pressure: The difference between intrapulmonary and intrapleural pressures.

Formula:

• This pressure keeps the lung spaces open and prevents lung collapse.

• The greater the transpulmonary pressure, the larger the lungs will be.

• Lungs will collapse if:

  • or

• Negative Pip must be maintained to keep lungs inflated.

Diagram: Pressure Relationships

The diagram illustrates the relationships between atmospheric, intrapulmonary, and intrapleural pressures, as well as the anatomical structures involved (parietal pleura, visceral pleura, pleural cavity, lung, diaphragm).

Homeostatic Imbalance: Atelectasis and Pneumothorax

Disruptions in pressure relationships can lead to lung collapse.

• Atelectasis: Collapse of lung tissue due to:

  • Plugged bronchioles, causing collapse of alveoli

  • Pneumothorax (air in the pleural cavity)

• Pneumothorax: Air enters the pleural cavity, equalizing intrapleural and atmospheric pressures, leading to lung collapse.

  • Can result from a wound in the parietal pleura (e.g., knife wound) or rupture of the visceral pleura (often spontaneous).

  • Treatment involves removing air with chest tubes; the lung reinflates as the pleurae heal.

Diagram: Pneumothorax Mechanism

The diagram shows how puncture of the parietal or visceral pleura allows air into the pleural cavity, causing the intrapleural pressure to equal atmospheric pressure and resulting in lung collapse.

Pulmonary Ventilation

Mechanics of Breathing: Inspiration and Expiration

Pulmonary ventilation consists of two phases: inspiration (inhalation) and expiration (exhalation). These are mechanical processes that depend on volume changes in the thoracic cavity.

• Boyle's Law: The relationship between pressure and volume of a gas. If the volume of a container increases, the pressure decreases, and vice versa.

Formula:

• Volume changes lead to pressure changes.

• Pressure changes cause gases to flow to equalize pressure.

Summary Table: Key Pressures in the Thoracic Cavity

Clinical Relevance

• Maintaining negative intrapleural pressure is essential for lung inflation.

• Disruption of this pressure (e.g., pneumothorax) can lead to atelectasis (lung collapse).

• Understanding these pressures is fundamental for interpreting respiratory pathologies and interventions such as chest tube placement.

Additional info: The above notes are based on standard concepts in respiratory physiology and are essential for understanding the mechanics of breathing and related clinical conditions.