Final answer:
Passive expiration is facilitated by the elastic recoil of the lungs. As the diaphragm and intercostal muscles relax after inhalation, the elastic tissues of the lungs cause them to shrink back to their original size, creating a pressure gradient that pushes air out without the need for ATP.
Step-by-step explanation:
The question is, what allows passive expiration without ATP? When we analyze the process of how air leaves the lungs during the passive phase of breathing, we can find the answer. Passive expiration primarily relies on elastic recoil of the lungs. When we breathe in, or inhale, the diaphragm and intercostal muscles contract, expanding the chest and pulling air into the lungs. For expiration, the situation reverses; the diaphragm and intercostal muscles relax. The elasticity of the lung tissues then comes into play, causing the lungs to return to their original, smaller size.
This lung recoil increases the pressure inside the lungs, which becomes higher than the pressure outside, creating a pressure gradient that facilitates the movement of air out of the lungs. This process does not require ATP, because no muscular effort is exerted to push air out – the lungs and thoracic cavity return to their original state due to their inherent elasticity, thus completing a passive expiration.