Which of the following describes inhalation?

Inhalation is primarily described as an active process because it involves the contraction of specific muscles, particularly the diaphragm and intercostal muscles. When these muscles contract, the volume of the thoracic cavity increases, leading to a decrease in pressure within the lungs compared to atmospheric pressure. This pressure differential causes air to flow into the lungs, allowing for the exchange of gases essential for respiration.

During inhalation, energy is expended to enable this muscular contraction and the associated changes in lung volume. Consequently, inhalation cannot be classified as passive, as passive processes typically do not require energy expenditure and rely instead on natural pressures or diffusion.

In contrast, exhalation can be a passive process under normal resting conditions, as it often occurs when the diaphragm and intercostal muscles relax, allowing the lungs to recoil and expel air without active muscle contraction. However, during vigorous activity or forced breathing, exhalation can also become an active process.

In summary, inhalation necessitates the active engagement of muscles and energy consumption to draw air into the lungs, making it distinctly an active process.