What are the two features of a respiratory system that make fish efficient in gaseous exchange?

1. Gills: Fish have gills, which are highly vascularized structures that are composed of many thin filaments. These filaments are covered in tiny blood vessels called capillaries, which allow for the efficient exchange of oxygen and carbon dioxide between the water and the bloodstream.

2. Countercurrent System: The countercurrent system in the gills helps to maximize the efficiency of gas exchange. The blood in the capillaries of the gill filaments flows in the opposite direction to the water flowing over the gills. This ensures that the water with the highest oxygen concentration comes into contact with the blood with the lowest oxygen concentration, and vice versa.