What is the site of highest airway resistance in the respiratory system?

The medium-sized bronchi are indeed where the highest airway resistance in the respiratory system occurs. This is primarily due to their specific anatomy and structure. The medium-sized bronchi have a relatively small diameter compared to the trachea, leading to a greater resistance to airflow. Additionally, the branching nature of the airways means that as bronchi become smaller, they create more total surface area but also increase resistance because of the decrease in diameter.

In contrast, the trachea has a larger diameter, allowing for less resistance overall. Terminal bronchioles, though narrower than medium-sized bronchi, exhibit decreased resistance due to the parallel arrangement and number of airways in the lower respiratory tract which allow for a lesser resistance combined. Alveoli, being the final structures in the respiratory tree, are responsible for gas exchange and do not contribute significantly to airway resistance. This anatomical arrangement and the laws of fluid dynamics help explain why medium-sized bronchi are the site of highest airway resistance.