What occurs during the process of expiration?

During the process of expiration, the diaphragm relaxes, which leads to a decrease in thoracic volume. This decrease in volume creates a higher pressure in the thoracic cavity compared to the atmospheric pressure, causing air to flow out of the lungs. When the diaphragm is at rest or relaxes, it moves upwards into a dome shape, which reduces the space in the thoracic cavity. As a result, air is pushed out of the lungs and expelled into the environment.

The other options present alternate processes or outcomes that do not align with the mechanics of expiration. The contraction of the diaphragm, for instance, occurs during inhalation and increases lung volume, rather than during expiration. Drawing air into the alveoli from the environment is also a characteristic of inhalation, while an increase in carbon dioxide levels typically would not be a direct result of expiration itself; rather, expiration is the process by which carbon dioxide is expelled from the body. Understanding these distinctions clarifies why the relaxation of the diaphragm leading to decreased thoracic volume is the correct aspect of expiration.