Complex sentence: Countercurrent flow is when deoxygenated blood moves through gill in one direction while oxygenated water moves in the gill in the opposite direction maintaining the concentration gradient and so increasing the efficiency of the process in comparison to cartilangenous fish.
Split into simpler sentences: Countercurrent flow is when deoxygenated blood moves through gill in one direction while oxygenated water moves in the gill in the opposite direction. Maintaining the concentration gradient and so increasing the efficiency of the process in comparison to cartilangenous fish.