What does expiration refer to in the breathing process?

Expiration specifically refers to the process of breathing air out of the lungs. During expiration, the diaphragm and intercostal muscles relax, causing the thoracic cavity to decrease in volume. This reduction in space leads to an increase in pressure within the lungs, forcing air out. This process is crucial for removing carbon dioxide from the body, a waste product of metabolism, and plays an essential role in maintaining respiratory function and homeostasis.

While the other options relate to aspects of the respiratory process, they describe different mechanisms. Breathing in fresh air relates to inhalation, which is the intake of oxygen necessary for cellular metabolism. Gas exchange in the alveoli occurs during the overall breathing process but is distinct from the act of expiration itself. Lastly, oxygen diffusion into blood is part of the gas exchange process where oxygen passes from the alveoli into the bloodstream but is not representative of expiration. Therefore, identifying expiration as the act of breathing air out of the lungs is key to understanding respiratory physiology.