Final answer:
The respiratory systems that use countercurrent exchange are fish gills and bird lungs, which allow for efficient gas exchange to support their metabolic needs in water and flight, respectively.
Step-by-step explanation:
The respiratory systems that mainly involve countercurrent exchange are fish gills and bird lungs. In fish, water flows over the gills in one direction while blood within the gill capillaries flows in the opposite direction. This adaptation allows for a more efficient transfer of oxygen into the blood and carbon dioxide out, supporting the energetic demands of fish. Similarly, bird lungs employ a countercurrent exchange system where the airflow is unidirectional through their lungs, and the blood flows in the opposite direction, maximizing oxygen absorption to meet their high metabolic rate necessary for flight.