Compare O₂ partial pressure in blood and water across the exchange surface under a countercurrent arrangement (fish gills) vs. a co-current arrangement (if flow ran the same direction).
Countercurrent: blood keeps meeting less-depleted water at every point, so the gradient never collapses — blood can approach water's own starting O₂ level. Co-current: blood and water equilibrate partway across, capping how much O₂ blood can ever pick up at roughly the midpoint value.