Is exhalation considered to be a passive or active process?

Exhalation is considered a passive process primarily due to the mechanics of breathing. During exhalation, especially at rest, the diaphragm and intercostal muscles relax after contraction. This relaxation allows the lungs to deflate and air to be expelled from the thoracic cavity as the elastic recoil of the lung tissue and the surface tension within the alveoli come into play.

As the diaphragm rises and the rib cage lowers, a decrease in lung volume occurs, causing the pressure within the lungs to become greater than the atmospheric pressure, allowing air to flow out. This process does not require energy or muscle contraction under normal circumstances, which is why it is categorized as passive.

While certain situations, such as during exercise or in cases of respiratory distress, can involve active exhalation (where muscles such as the abdominal muscles are engaged to forcefully expel air), the standard exhalation that occurs during normal restful breathing is passive. This distinction helps in understanding respiratory physiology and the dynamics of ventilation.