Figure 31.7: Fish Breathing (A) Water flows unidirectionally over the gills of a fish. (B) Gill filaments have a large surface area and thin tissues. (C) Blood flows through the secondary lamellae in the direction opposite (left to right) to the flow of water (right to left) over the lamellae. Blood flow is depicted by small blue and red arrows. Water flow is depicted by large blue arrows. “Afferent” here means flowing into the gills, and “efferent” means flowing out of them.