What is the Difference Between Alveolar Pressure and Pleural Pressure

The main difference between alveolar pressure and pleural pressure is that the alveolar pressure removes carbon dioxide and uptakes oxygen, while pleural pressure is responsible for inhalation.

Alveolar and pleural pressure are two pressure types responsible for pulmonary ventilation. Alveolar pressure is the pressure inside the lungs’ alveoli, whereas pleural pressure is the pressure within the pleural cavity. 

Key Areas Covered

1. What is Alveolar Pressure
– Definition, Facts, Features
2. What is Pleural Pressure
– Definition, Facts, Features
3. Similarities Between Alveolar Pressure and Pleural Pressure
– Outline of Common Features
4. Difference Between Alveolar Pressure and Pleural Pressure
– Comparison of Key Differences
5. FAQ: Alveolar Pressure and Pleural Pressure
– Answers to frequently asked questions

Key Terms  

Alveolar Pressure, Pleural Pressure

What is Alveolar Pressure

Alveolar pressure is the pressure of the air inside the lungs’ alveoli. Alveolar pressure is equal to atmospheric pressure when the glottis is opened. Therefore, no air flows in or out of the lungs. Inhalation results in an increased volume of alveoli. Also, it decreases the intra-alveolar pressure below the atmospheric pressure. This negative pressure is equal to the -1 cmH₂O. It moves air into the lungs in 2 seconds. In addition to that, 500 ml of air moves into the lungs within 2 seconds.

Figure 1: Alveolar Pressure

Furthermore, during exhalation, the expelling of air results in the collapse of the lung’s alveoli. Also, it increases the alveolar pressure to +1 cmH₂O. 500 ml air moves out of the lungs within 2-3 seconds. Alveolar pressure becomes equal to atmospheric pressure after exhalation.     

What is Pleural Pressure

Pleural pressure is the pressure within the pleural cavity. It is slightly less than the atmospheric pressure. Therefore, it is known as negative pressure. Thoracic pressure is different from intrapleural pressure. Thoracic pressure occurs in the thoracic cavity, which includes the pleura, lungs, and heart. It is negative throughout the ventilatory cycle, while the intrapleural pressure is negative at rest. 

Moreover, the ventilation phase, the volume of the intrapleural cavity, and the atmospheric pressure are the key factors that determine the pleural pressure. Regarding the ventilation phase, there are four phases of mechanical ventilation. They are the trigger phase, the inspiratory phase, the cycling phase, and the expiratory phase. The intrapleural cavity is the potential space between the pleurae of the pleural sac that surrounds each lung. Further, atmospheric pressure is the main pressure that allows the flow of air into the lungs. It allows the transpulmonary pressure, which is the pressure difference between the pleural space and the alveolar space, to expand the lung.

Figure 2: Pleural Pressure    

Similarities Between Alveolar Pressure and Pleural Pressure

• Alveolar and pleural pressure are two pressure types in the lungs.
• They are important for the ventilation of the lungs. 

Difference Between Alveolar Pressure and Pleural Pressure

Definition

Alveolar pressure refers to the pressure of air inside the lung alveoli. In contrast, pleural pressure refers to negative pressure that occurs inside the pleural cavity.

Occurrence

Alveolar pressure occurs in the alveoli, while pleural pressure occurs in the pleural cavity.

Function

Alveolar pressure removes carbon dioxide and uptakes oxygen, while pleural pressure is responsible for inhalation.

Type of Pressure

Alveolar pressure is negative during inhalation and positive during exhalation, while pleural pressure is a negative pressure. 

FAQ: Alveolar Pressure and Pleural Pressure

What is the pressure difference between intra-alveolar and intrapleural pressure called?

Transmural pressure is the difference between the alveolar and intrapleural pressure. It occurs across the lungs. It occurs across the chest wall.   

Why is pleural pressure negative?

The intrapleural pressure is negative. It maintains proper inflation, preventing the lungs from collapsing. The negative pressure is generated in the pleural cavity by less intrapleural pressure than the atmospheric pressure. 

What happens when pleural pressure is positive?

Pleural pressure becomes positive during active expiration. Abdominal muscles contract to force the diaphragm up, temporarily collapsing the bronchi. Also, it limits the airflow. 

Conclusion

In brief, alveolar and pleural pressure are two pressure types in the lungs. Alveolar pressure occurs in the alveoli. Also, it is a positive pressure that helps to remove carbon dioxide from the lungs. In comparison, pleural pressure is negative pressure. It removes air from the lungs. Therefore, it is responsible for the inhalation. Hence, the main difference between alveolar and pleural pressure is their occurrence and function.  

References:

1. Mechanics of ventilation. Mechanics of Ventilation | SEER Training. (n.d.-a). 

Image Courtesy:

1. “Gaz pressures in the alveoli and capillaries” By Glafoululle des Alpes – Own Work (CC-BY SA 4.0) via Commons Wikimedia
2. “2315 Intrapulmonary and Intrapleural Pressure” By OpenStax College – Own Work (CC-BY 3.0) via Commons Wikimedia