Final answer:
The smallest airspace of the respiratory tree is the alveolar duct, which connects the respiratory bronchioles to the alveolar sacs.
Step-by-step explanation:
The smallest airspace of the respiratory tree is the alveolar duct. The alveolar ducts are small tubes that connect the respiratory bronchioles to the alveolar sacs. The alveolar sacs consist of numerous alveoli, which are responsible for gas exchange.