What happens during exhalation?

During exhalation, the diaphragm and intercostal muscles indeed relax, which causes a decrease in the volume of the thoracic cavity. As the volume decreases, the pressure within the thoracic cavity, known as intrathoracic pressure, increases. This increase in pressure forces air out of the lungs and into the atmosphere. The mechanism relies on the principles of gas laws, where gases move from areas of higher pressure to lower pressure.

The other options describe processes that do not occur during exhalation. Contraction of the diaphragm and intercostal muscles, for example, occurs during inhalation when the volume of the thoracic cavity increases and intrathoracic pressure decreases, allowing air to flow into the lungs. Meanwhile, stating that the lungs fill with air during exhalation contradicts the fundamental definition of the respiratory process, as exhalation involves the expulsion of air rather than intake.