Two primary characteristics of the lungs?

• A. Elasticity and Viscosity
• B. Volume and Pressure
• C. Compliance and Capacitance
• D. Compliance and Resistance

Answer: (D)

Ventilation depends on two key mechanical properties: how easily the lungs expand (compliance) and how much resistance air encounters as it moves through the airways. Compliance describes the distensibility of the lung tissue and alveolar walls—high compliance means the lungs expand readily with a small change in pressure, while low compliance means the lungs are stiff and require more pressure to achieve the same volume. Airflow, on the other hand, is governed by airway resistance—the opposition to flow that comes from the airways themselves. Narrower or obstructed airways raise resistance, making it harder to move air in and out and increasing the effort required to breathe.

These two factors together explain why breathing becomes more difficult in different situations: stiff lungs (low compliance) demand more pressure to inflate, whereas narrowed airways (high resistance) impede airflow, especially during rapid breathing or expiration. Understanding them also helps interpret disease patterns: fibrosis lowers compliance, while bronchospasm or obstruction raises resistance.

Elasticity is related to how the lungs recoil, but it’s not typically described as one of the two main characteristics alongside resistance for general lung mechanics. Viscosity refers to fluid properties and isn’t a primary descriptor of lung mechanics. Volume and pressure describe what you measure rather than inherent properties, and capacitance is not the standard term used for lung mechanics.