Intrapulmonary pressure in the
a. pressure within the pleural cavity.
b. pressure within the alveoli
c. negative pressure in the intrapleural space
d. difference between atmospheric pressure and respiratory pressure

The correct answer is option B- pressure within the alveoli.

Explanation:

Intrapulmonary pressure is the pressure within the lungs especially in the alveoli of the lungs so is also known as "intra-alveolar pressure".

This pressure helps in the breathing process as :

1. During inspiration - The thoracic cavity volume increases due to which the intra-pulmonary pressure becomes less than the atmospheric pressure which causes the lungs to inhale about 500 ml of air.

2. During expiration- the thoracic volume decreases which cause the intra-pulmonary pressure larger than atmospheric pressure which causes the lungs to exhale the air.

Thus, option B- pressure within the alveoli is the correct answer.