Final answer:
The true statement about breathing mechanics is that C. functional residual capacity refers to the air remaining in the alveoli after a normal exhalation. Inspiration requires muscle contraction, while expiration normally involves muscle relaxation.
Step-by-step explanation:
The mechanics of breathing involve two major processes: inspiration (or inhalation) and expiration (or exhalation). During inspiration, the diaphragm and external intercostal muscles contract, which expands the rib cage and moves it outward, while also expanding the thoracic cavity and lung volume. This lowers the pressure within the lungs, causing air to enter.
During expiration, these muscles relax, allowing the thorax and lungs to recoil due to their elasticity. This increases the pressure within the lungs, forcing air out. Gentle exhalation is typically a passive process, but during forced exhalation, internal intercostal and abdominal muscles may contract to help expel air.
The true statement regarding the mechanics of breathing from the options provided is that Functional residual capacity is the air remaining in the alveoli after a normal exhalation.