Question

Expiration occurs when the intrapulmonary pressure is_________________ the atmospheric pressure.

• A. higher than
• B. lower than
• C. equal to
• D. none of the above

Answer 1

The correct option is A higher than

Expiration is a process in which the air is drawn out of the lungs. During expiration, the diaphragm relaxes and returns back to its usual position. The volume of the thoracic cavity (chest cavity) decreases in the longitudinal direction. The external intercostal muscles also relax and the chest returns to its original position by moving downwards and inwards. As the chest moves downwards and inwards, the thoracic cavity contracts in the dorsoventral direction.
Due to the relaxing of these muscles the volume inside the lungs decreases. It is known from Boyle’s law that for a gas at constant temperature the volume is inversely proportional to pressure. So when the volume decreases, the pressure increases and goes above the atmospheric pressure. This causes the movement of gases from the lungs to the external environment.