Question

Explain how a subatmospheric intrapleural pressure is produced, and how this relates to collapse of a lung when a person suffers an open chest wound.

Short answer

Subatmospheric intrapleural pressure is produced due to opposing forces of lung elasticity and chest wall expansion, resulting in a slightly negative pressure that keeps the lungs partially expanded. This pressure is crucial for preventing lung collapse and maintaining lung volume. In the case of an open chest wound, the normal pressure balance in the intrapleural cavity is disrupted as air from the atmosphere enters the cavity. This causes the intrapleural pressure to equalize with the atmospheric pressure, consequently leading to lung collapse and severe respiratory distress due to the loss of forces keeping the lungs expanded.

Step-by-step solution

1. Introduction to Intrapleural Pressure

Intrapleural pressure is the pressure within the intrapleural cavity, which is the space between the visceral pleura (membrane covering the lungs) and the parietal pleura (membrane covering the inner walls of the thoracic cavity). The intrapleural cavity contains a thin layer of fluid that, in normal cases, prevents the lung from collapsing and allows it to expand and contract smoothly.

2. The Creation of Subatmospheric Intrapleural Pressure

Subatmospheric intrapleural pressure is a pressure lower than the atmospheric pressure in the intrapleural cavity. It is produced due to opposing forces of lung elasticity and chest wall expansion. The lungs' natural tendency is to collapse due to their elasticity, and the chest wall's natural tendency is to expand. This results in a slightly negative intrapleural pressure, which keeps the lungs partially expanded.

3. Role of Intrapleural Pressure in Maintaining Lung Volume

The negative intrapleural pressure serves as a force that keeps the lungs distended and prevents their collapse. The pressure difference between the intrapleural cavity and the lung itself maintains the contact between the pleural membranes and ensures that the lungs follow the movement of the chest wall during respiration.

4. Lung Collapse in an Open Chest Wound

When a person suffers an open chest wound, also called a "sucking" chest wound or a pneumothorax, the normal pressure balance in the intrapleural cavity is disrupted. Air from the atmosphere can enter the intrapleural cavity through the wound, causing the intrapleural pressure to rise and equalize with the atmospheric pressure.

5. Loss of Subatmospheric Intrapleural Pressure and Its Consequences

When the intrapleural pressure increases and becomes equal to atmospheric pressure, the forces keeping the lungs expanded are lost. Consequently, the lung elasticity brings about a collapse, thereby reducing the effective lung volume. This may lead to difficulty in breathing and can become life-threatening if not treated immediately. In conclusion, subatmospheric intrapleural pressure is crucial for maintaining lung volume and preventing lung collapse. When a person suffers an open chest wound, this pressure balance is disrupted, leading to lung collapse and severe respiratory distress.