Countercurrent Gas Exchange in Fish Gills

Fish gills are one of biology's most efficient gas exchangers, and the secret is geometry, not chemistry: water flows over the gill lamellae in the opposite direction to the blood inside them. This simulator models a single lamella as a discretized exchanger, solving real advection and diffusion equations for both the water and blood streams every frame, so you can toggle between countercurrent and concurrent (parallel) flow and watch the oxygen-extraction efficiency change live as you adjust flow rates, membrane permeability and ambient water oxygen — the same physics that makes countercurrent exchange the dominant design in gills, kidneys and the limbs of cold-water animals.