In which phase of breathing does the diaphragm relax?

During expiration, the diaphragm relaxes, which is a crucial aspect of the breathing cycle. When we inhale, the diaphragm contracts and moves downwards, creating a vacuum that allows air to flow into the lungs. This contraction is what facilitates the process of inhalation (or inspiration).

However, during expiration, which can be passive during quiet breathing, the diaphragm relaxes and moves back to its dome-shaped position. This relaxation reduces the volume of the thoracic cavity, increasing the pressure in the lungs compared to the atmospheric pressure, leading to the expulsion of air. In more active or forced exhalation, other muscles come into play to assist with this process, but the fundamental action of diaphragm relaxation during normal expiration is essential for air to be pushed out of the lungs effectively.

The options that suggest the diaphragm relaxes during inspiration or at both phases do not align with the physiological mechanics of breathing. Thus, the focus on expiration as the phase in which the diaphragm relaxes is accurate.