Is intrapleural pressure 4 mm Hg less than the in alveoli?

Yes, intrapleural pressure is indeed about 4 mm Hg less than the pressure in the alveoli. This pressure difference is what keeps the lungs inflated. If the intrapleural pressure were to equal the pressure in the alveoli, the lungs would collapse.

Here's a more detailed explanation:

* Intrapleural pressure is the pressure inside the pleural cavity, which is the space between the lungs and the chest wall.

* Alveolar pressure is the pressure inside the alveoli, which are the tiny air sacs in the lungs where gas exchange takes place.

* Normally, intrapleural pressure is about 4 mm Hg less than alveolar pressure. This pressure difference creates a force that pulls the lungs outward, keeping them inflated.

If the intrapleural pressure were to increase, it would compress the lungs and make it more difficult to breathe. This can happen with conditions such as pleural effusion (a buildup of fluid in the pleural cavity) or pneumothorax (a collapsed lung).

If the intrapleural pressure were to decrease, the lungs would expand and become overinflated. This can happen with conditions such as emphysema (a chronic lung disease that causes the alveoli to lose their elasticity).