How do alveolar ducts and alveolar sacs connect to the respiratory bronchioles?

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Medical illustration of the human chest showing the lungs, diaphragm, ribs, trachea, bronchus, and bronchioles, with one lung cut away to reveal internal structures. — human respiratory system The lungs serve as the gas-exchanging organ for the process of respiration.

In the human respiratory system, the transition from the conducting airways to the gas-exchange region occurs as the respiratory bronchioles branch out from the terminal bronchioles.1

As these respiratory bronchioles extend distally, their walls begin to feature cuplike outpouchings known as alveoli. Over the course of two to four generations of branching, the frequency of these alveolar outpocketings increases rapidly until the airway wall is composed entirely of alveoli. At this stage, the airway is referred to as an alveolar duct. These ducts represent the final segments of the airway tree, eventually ending blindly in alveolar sacs, where the gas-exchange process is facilitated.12