Pulmonary ventilation involves three different pressures:
Atmospheric pressure
Intra Alveolar (intrapulmonary) pressure
Intrapleural pressure
Atmospheric pressure is the pressure of the air that is exerted outside the body. Intrapulmonary pressure is the pressure exerted inside the alveoli of the lungs. Intrapleural pressure is the pressure within the pleural body cavity.
You have to first make some observations and then ask a question. Then you make a hypothesis (or guess) what the answer is - do the experiment to see if you're right!
It is referred to as a selective semipermeable membrane because it only allows certain things through, due to the size and shape of whatever is trying to get through.
