Counter current flow - Respiration
Each filament has an upper and a lower row of flat lamellae. The lamellae of successive filaments are in close contact. Gas exchange takes place in these lamellae as water flows between them in one direction and blood within them in the other direction. This we call a counter current flow. This type of flow has an important consequence. It permits the fish gills to have the highest possible oxygen levels.
Figure: Counter current flow