Question

Lungs do not collapse between breaths and some air always remains in the lungs which can never be expelled because.
(A) Their is positive intrapleural pressure
(B) Pressure in the lung is higher than the atmospheric pressure
(C) There is negative pressure in the atmosphere
(D) There is negative intrapleural pressure pulling the walls of the lungs

Answer 1

Hint: During expiration, we come across volume that is paid back to functional residual capacity(FRC) by the elastic recoil forces produced within the lung tissues during the prior inspiration and hence the Lungs do not collapse.

Complete answer:
Your lungs do not collapse completely for the following reasons:
-Nitrogen is non-absorbent at normal sea level pressures comprises 78% of the air and remains as a “filler” for the alveoli which in turn has Surfactant, a “non-stick" layer, develops it easy for alveoli to stay open.
-The bulk of airways have semi-rigid patterns that engage with each other and are mutually reluctant to collapse at end exhalation.
-The thoracic wall is quite rigid, and it's attached to the lungs via a vacuum seal between the pleura which, when strong, and does not permit inspiratory negative pressure and expiratory positive pressure to withstand.
> Option (A) There is positive intrapleural pressure: This option is not correct.
> Option (B) Pressure in the lung is higher than the atmospheric pressure: This option is not correct.
> Option (C) There is negative pressure in the atmosphere: This option is not correct.
> Option (D) There is negative intrapleural pressure pulling the walls of the lungs
Lungs do not sink between breaths as some air always remains in the lung which can never be expelled because the negative intrapleural pressure is developed by two opposing elastic forces pulling in the intrapleural space. The elastic lungs tend to fall while the elastic chest wall tends to push outward. These two opposing forces build a negative intrapleural pressure which in turn contradicts the natural tendency of lungs to collapse

Thus, the correct option is (D) There is negative intrapleural pressure pulling the walls of the lungs.

Note:
Pulmonary surfactant enables to reduce the surface tension so that the alveoli do not collapse during expiration. The capacity of the lungs to stretch is called lung compliance, which plays an important role in gas flow.