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2 out of 5

Negative intrapleural pressure is due to? (AIIMS NOV 2010)

A Lymphatic drainage from plural cavity

B Equally distributed surfactant

C Negative pressure in alveoli

D presence of cartilaginous ring at the upper airway

Ans. A Lymphatic drainage from plural cavity

Ganong - Review of Medical Physiology 23rd Ed &West JB. Respiratory physiology—the essentials. Baltimore (MD): Williams and Wilkins;

Pleural cavity is a closed cavity. And the pressure is always negative due to the following factors:

I. The lungs have a tendency to recoil inwards, therefore visceral pleura is pulled inwards.

II. Surface tension also help by creating a inward force in alveoli.

III. The thorax has a tendency to recoil outwards, pulling parietal pleura with it. Therefore the pleural cavity is stretched on both sides leading to increase in its volume and decrease pressure inside. And it becomes negative.

IV. Also there is a continuous Lymphatic drainage from pleural cavity, which creates a suction force in pleural cavity, keeping the pressure negative.

Note: Surfactant by decreasing Surface tension actually opposes the negative pressure.. And pressure in alveoli is same as atmospheric pressure, taken as ZERO.