Negative intrapleural pressure is due to ?
Ans. is 'c' i.e., Absorption by lymphatics The pleural pressure is negative, more negative during inspiration, less negative during expiration, but always negative during quiet breathing. This is because both the thoracic cage and lungs are elastic structures; therefore, both tend to recoil, but in opposite direction. This creates negative intrapleural pressure. "A negative force is always required on the outside of the lungs to keep the lungs expanded. This is provided by negative pressure in the normal pleural space. The basic cause of this negative pressure is pumping of fluid from the space by the lymphatics (which is also the basis of the negative pressure found in most tissue spaces of the body)." - Guyton 12/e p 483 So, two impoant reasons of negative intrapleural pressure are :- i) Elasticity of lungs and thoracic cage in opposite direction. ii) Lymphatic drainage of pleural fluid.
Uniform distribution of surfactant over alveoli
Negative intraalveolar pressure
Absorption by lymphatics
Presence of cailage in the upper airway