Originally Posted by MedPR
1. Intrapleural pressure changes as you breathe. When you inhale, intrapleural pressure increases and intrapulmonary (alveolar) pressure decreses. Opposite when you exhale.
2. In order to get air into and out of the alveoli, there must be a pressure gradient with the atmosphere. When you inhale you are creating a vacuum within your alveoli. By decreasing the alveolar (intrapulmonary) pressure, you allow air to flow into your lungs. When you reach the end of your inhale, intrapulmonary pressure (alveoli) equals atmospheric pressure, but now your intrapleural pressure is higher than both atmospheric and intrapulmonary and it is pressing onto your lungs, thus making you exhale.
Intrapleural pressure decreases during insipiration. Intrapleural pressure increases during expiration.
Intrapleural pressure is always less than alveolar pressure unless during forced expiration. Here is a link that shows you the ventilation cycle and how each thing changes during inspiration and expiration.