Question

Using specific values for each of the pressures, explain air movement into the out of the lungs during the respiratory cycle and why we are defined as negative pressure breathers.

Pressures: Atmospheric, inter-alveolar, intrapleural, transpulmonary.

Answer 1

Intrapleural pressure:

This is pressure within the pleural space (also called intrathoracic pressure). Mid-thoracic oesophageal pressure measures intrapleural pressure. The volume of fluid in the intrapleural “space” is small (2.0–10 mL), but closely regulated by Starling forces.

It is essential that intrapleural fluid volume remains small so that the cohesive forces linking the lung to the inner chest wall are preserved and thoracic cage enlargement results in lung expansion.

A net Starling force of about 9 cm H2O favors fluid filtration from the parietal-side circulation into the intrapleural “space,” whereas an oncotic force of about 10 cm H2O favors the reabsorption of fluid from the intrapleural “space” into the visceralside circulation. These interacting Starling forces function to keep the intrapleural space from excessive fluid accumulation.  

Intrapleural pressure:

At the beginning of quiet inspiration, it is – 2.5 mm Hg (– 5 cm H2)
at the base of the lung, (that is, 2.5 mm Hg less than atmospheric pressure of 760 mm Hg).

At the end of inspiration, it becomes – 6.0 mm Hg. Then, it returns to -2.5 mm Hg at the end of expiration.

The transmural pressures are:

Transpulmonary pressure: The pressure difference between intrapulmonary and intrapleural pressure.

Transthoracic pressure: The pressure difference between the intrapleural pressure and atmospheric pressure.

Transrespiratory: The pressure difference between the intrapulmonary pressure and atmospheric pressure.

intra-alveolar pressure:

It is also called as transpulmonary pressure. It is the pressure in the airways or in the alveoli.

At the peak of inspiration, it is – 1 mm Hg. At the peak of expiration, it is + 1 mm Hg. At the beginning, at the end of both inspiration and expiration, the intraalveolar pressure is zero.

Strong inspiratory efforts reduce intrapleural pressure to values as low as – 30 mm Hg.

Pressure changes during inspiration:

The thoracic volume increases as the chest expands. Intrapleural pressure becomes more negative.
Normally, it is – 2.5 mm Hg at the beginning of inspiration and it decreases to about – 6 mm Hg.
Alveoli expand in response to the increased transmural pressure gradient. This causes an increase in the alveolar elastic recoil. Alveolar pressure falls below atmospheric pressure as the alveolar volume increases. This establishes a pressure gradient for airflow (at the peak of inspiration, it is – 1 mm Hg). Airflows into the alveoli until alveolar pressure balances with atmospheric pressure. As the negative pressure draws the air inside the lungs, we are said to be negative breathers

Pressure changes during expiration:

There is a decrease in the thoracic volume. This causes the intrapleural pressure to become less negative and decreasing the alveolar transmural pressure gradient. The decrease in the alveolar transmural pressure gradient allows the increased alveolar elastic recoil to return the alveoli to their pre-inspiratory volumes. The decrease in the alveolar volume, increases alveolar pressure above atmospheric pressure, thereby establishing a pressure gradient for airflow. The air will flow out of the alveoli until the alveolar pressure balances with atmospheric pressure.