Where is the gas-exchange surface in the bird respiratory system, i. e. where does oxygen enter the blood and carbon dioxide leave the blood

the lungs

Explanation:

The avian respiratory system consists of three sets of organs that perform respiration: lungs (where gas exchange occurs), anterior air sacs and posterior air sacs. The lungs have static structures with surfaces that enable gas exchange, and air sacs that enable the unidirectional flow of air through the lungs. Birds have a very efficient respiratory system that obtains oxygen when they breathe in and breathe out. The inhaled air reaches the primary bronchus where enters to the lungs for gas exchange and also fills the air sacs.

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