Question

During passive expiration, the intrapleural pressure is

A greater than atmospheric pressure

B the same as atmospheric pressure

C determined by the diaphragm

D greater than the intra-alveolar pressure

E lower than the intra-alveolar pressure

Answer 1

Answer) option E - lower than the intra-alveolar pressure

Explanation- Intrapleural pressure is pressure within the pleura cavity.

It is also known as intra- thoracic pressure.

The lungs and the chest wall are elastic structures. At end expiratory position i.e during expiration, the tendency of the lungs to recoil from the chest wall is just balanced by the tendency of the chest wall to recoil in the opposite direction.

This causes a slight subatmospheric pressure to develop i.e about -2 mmHg (negative pressure) between the two layers of pleura at the start of inspiration,called intrapleural pressure.

The negative intrapleural pressure is directly proportional to the amount of thoracic expansion which occurs.

Therefore, during quiet inspiration, the lungs are pulled into a more expanded position causing intrapleural pressure to decrease to about -6mmHg.

At the end of inspiration, the lung recoil begins to pull the chest back to the expiratory position; and at end expiratory position i.e during expiration where the recoil pressure of the lungs and chest wall balance intra - pleura pressure returns back to - 2 mmHg.

Compared to inspiration, during expiration, intrapleural pressure increases. But intrapleural pressure always remain less than the atmospheric and intra alveolar pressure.

If the intrapleural pressure becomes positive (rather than negative), the lungs collapses. So negative intrapleural pressure i.e less than atmospheric and intra alveolar pressure.

Normal atmospheric pressure is 760 mmHg.

During expiration, expulsion of air occurs from the lungs.

So during expiration, intra alveolar pressure raises to 761 mmHg as air flow from high pressure areas to low pressure areas.

Intra pleural pressure, during expiration, is 758 mmHg.

So, intrapleural pressure is less than intra alveolar pressure during expiration.

Other options are incorrect -

Option A and B are incorrect - As intrapleural pressure is negative than atmospheric pressure i.e intrapleural pressure is less than atmospheric pressure.

Option C is incorrect - As intrapleural pressure is determined by amount of thoracic expansion.

Option D is incorrect - As during expiration, intra alveolar pressure is greater than intrapleural pressure.