MCQ 49 The intrapleural pressure is negative both during inspiration and expiration because:

A.Intrapulmonary pressure is always negative
B.Thoracic cage and lungs are elastic
C.Transpulmonary pressure determines the negativity
D.Surfactant prevents the lungs to collapse

The correct answer is : B.Thoracic cage and lungs are elastic

NOTE: 

• Both the thoracic cage and lungs are elastic structures.
• Being elastic structure they both tend to recoil but in opposite direction
• The thoracic cage has inherent tendency to expand,while trying to expand,it draws the parietal pleura with it.
• The lungs have an inherent tendency to collapse because of their elasticity and surface tension of thin layer of fluid lining the alveoli.
• As the visceral pleura is strongly attached with the lungs,the pull in opposite directions of the visceral and parietal pleura,creates a negative intra-pleural pressure.

Negative Intrapleural pressure