What distinguishes inspiration from expiration in respiration?

The distinction between inspiration and expiration lies primarily in their definitions and the physiological processes involved. Inspiration refers to the process of bringing air into the lungs, facilitating the intake of oxygen from the environment. This occurs through the contraction of the diaphragm and the intercostal muscles, leading to an increase in thoracic cavity volume, which reduces lung pressure and allows air to flow in.

Expiration, on the other hand, is the act of expelling air from the lungs, which typically involves the relaxation of the diaphragm and intercostal muscles, causing a decrease in thoracic cavity volume. This increase in pressure in the thoracic cavity forces air out of the lungs.

Thus, understanding these definitions clarifies why the correct answer identifies inspiration as the intake of air and expiration as the expulsion of air. The other options involve inaccuracies about the physiological processes or conditions under which they occur, which do not accurately represent the basic mechanics of respiration.