Which factor primarily determines resistance in the respiratory system?

• A. Size of the airway
• B. Length of the airways
• C. Type of gas
• D. Temperature of the air

Answer: (A)

The size of the airway is the primary factor that determines resistance in the respiratory system due to the principles of fluid dynamics, specifically Poiseuille's law. This law states that resistance to airflow is inversely related to the fourth power of the radius of the airway. This means that even small changes in airway size can lead to significant changes in resistance.

In the respiratory system, larger airways provide a wider passage for airflow, which decreases resistance and facilitates easier breathing. Conversely, narrower airways increase resistance, making it more difficult for air to flow. Conditions such as asthma or bronchoconstriction can significantly alter airway size, illustrating how crucial airway dimensions are to respiratory function.

Other factors, such as the length of the airways, type of gas, and temperature of the air, do influence resistance and airflow but to a much lesser extent compared to the size of the airway. Length has a linear relationship to resistance, while the effects of gas type and air temperature are more nuanced and context-dependent, focusing primarily on their density and viscosity rather than directly affecting the mechanics of resistance in the same manner as airway size.