Question

This question is about the interpleural pressure.
When we breath in, the lungs expand therefor putting more pressure on the interpleular pressure, right? why does the interpleural pressure go down instead when we breath in? We are not losing the pleural fluid when we breath in right? so why is the pressure going down. Thank you

Answer 1

Let us first understand intrapleural pressure.

Intrapleural pressure is the pressure of the air in the pleural cavity. Normal intrapleural presure (in resting state) is negative (-2.5mm Hg).

This negative pressure is because of presence of vaccume in intrapleural cavity. Pleural cavity is subjected to two opposite forces. The thoracic wall exert the outward force and the alveoli exert the inward force. These two forces act in opposite direction and thus a negative pressure is created in pleural cavity.

Now when we breath in, the volume of air in the lungs increases. So according to Boyle's law, ( which states that pressure is inversely proportional to volume) the pressure will decrease.

This is the reason why pressure goes down when we breath in.

Pleural fluid has no role in this lowering down of pressure. It is purely due to Boyle's law as explained above.