Final answer:
Bird lungs have a cross-current exchange mechanism allowing for efficient gas exchange to meet the high metabolic demands of flight.
Step-by-step explanation:
The respiratory system that has a cross-current exchange mechanism is the bird lungs. Bird lungs are uniquely designed to supply the high oxygen demands required for flight. Airflows unidirectionally through their respiratory system, moving from the posterior sacs into the lungs, then into the anterior air sacs. This flow-through system includes air capillaries where gas exchange occurs. The counter-current exchange system between air capillaries and pulmonary blood provides birds with an efficient means of acquiring oxygen and getting rid of carbon dioxide.
In contrast, fish gills use a countercurrent exchange mechanism, where water flows opposite to the direction of blood flow. Mammalian lungs, like those in humans, involve a bidirectional flow of air where gas exchange occurs in the alveoli. Salamander gills and the skin in lungless salamanders facilitate gas exchange, but not through a cross-current exchange mechanism.