The gills of fish remove oxygen from water with extreme efficiency because water flows countercurrent to capillary blood flow.

References

Book
Fowler's Zoo and Wild Animal Medicine, Volume 8, 1eW. B. Saunders PhiladelphiaJune 26, 2014
RE Miller, FE Fowler

“Water flow over the secondary lamellae is countercurrent to capillary blood flow, resulting in extremely efficient oxygen extraction. Gills also function in monovalent ion regulation (via specialized chloride cells) and nitrogenous waste excretion (ammonia).” (Fowler and Miller 2003:4)

Book
Zoo and Wild Animal MedicineW.B. Saunders Co.ME Fowler, RE Miller

Living System/s

Organism
ChordatesChordataPhylum

Organism
FucaceaeFamily

Organism
FishPiscesSuperclass