Question

Lungs do not collapse between breaths and some air always remains in the lungs which can never be expelled because

• A. pressure in the lungs is higher than the atmospheric pressure
• B. there is a negative pressure in the lungs
• C. there is a negative intrapleural pressure pulling at the lung walls
• D. there is a positive intrapleural pressure

Answer 1

The correct option is C there is a negative intrapleural pressure pulling at the lung walls

During respiration, there are pressure and volume changes in the alveoli. The lungs are lined by the visceral pleura, whereas, the thoracic wall is lined by the parietal pleura.

Lungs do not collapse between breaths as some air always remain in the lung which can never be expelled because the negative intrapleural pressure is created by two opposing elastic forces pulling in the intrapleural space. The elastic lungs tend to collapse while the elastic chest wall tends to thrust outward. These two opposite forces create a negative intrapleural pressure which in turn opposes the natural tendency of lungs to collapse.

The intrapleural pressure is – 2.5 mmHg at the beginning of inspiration and becomes – 6 mmHg at the end of inspiration. During expiration, it becomes less negative and reaches the preinspiratory level. Forceful inspiration causes intrapleural pressure to become more negative, reaching up to – 30 mmHg. The negative intrapleural pressure prevents the collapse of alveoli during expiration. Further, it makes the alveoli distended during inspiration. This is the major function of intrapleural pressure, which gives stability to the lungs.

Therefore, the correct answer is "option C".