Final answer:
The passive phase of breathing when the diaphragm and intercostal muscles relax refers to expiration, during which the elastic recoil of lung tissue expels air out without requiring energy.
Step-by-step explanation:
The passive phase of breathing where the diaphragm and intercostal relax refers to expiration or exhalation. During this phase, the diaphragm and intercostal muscles return to their resting positions after contracting during inhalation. The elasticity of the lung tissue allows the lungs to recoil, aiding in the reduction of the thoracic cavity volume, which in turn increases the intrapulmonary pressure. As the pressure inside the lungs becomes higher than that of the atmosphere, air flows out of the lungs in a passive process without requiring additional energy.
This passive outflow of air is a natural sequence during normal expiration where relaxation of the muscles leads to the return of the thoracic cavity and lungs to their pre-inhalation size. This causes a rise in intrapulmonary pressure as compared to the atmospheric pressure, enabling air to leave the lungs.