Gills

Gills are respiratory organs that exchange gases between an animal and water, typically absorbing dissolved oxygen and releasing carbon dioxide. Their thin, extensive surfaces support this exchange as water flows across them.

Countercurrent exchange: A transport arrangement in which two fluids flow in opposite directions, maintaining a gradient for exchange along their contact surface. Opposing water and blood flows let fish gills extract oxygen across nearly the entire exchange surface.
Branchial arch: A skeletal support in the throat region of many aquatic vertebrates that bears gill structures. Gill filaments attach to these arches, which support and position the exchange surfaces.
Aquaculture: The farming of aquatic organisms, including fish, shellfish, and aquatic plants. Farmed fish depend on adequate water flow and oxygen at their gill surfaces.
Lung: An internal respiratory organ that exchanges gases between air and blood or another body fluid. Lungs exchange gases with air, while gills are adapted to exchange gases with water.
Gill lamella: A thin plate of gill tissue that carries blood close to flowing water for gas exchange. Lamellae multiply the surface area available for oxygen and carbon dioxide exchange.
Gill filament: An elongated gill structure bearing many thin lamellae for gas exchange. Filaments carry the lamellae that create much of a fish gill's exchange area.
Fish respiration: The process by which fish exchange oxygen and carbon dioxide, chiefly through their gills. It describes how gills function as part of a whole-animal respiratory system.
Tracheal system: A network of air-filled tubes that delivers oxygen directly to tissues in many arthropods. Unlike gills, tracheal systems move air through branching tubes rather than exchanging gases across a water-facing surface.
Operculum: A bony flap covering and protecting the gills of most bony fishes. Its movements help pump water across gills when a fish is stationary.
Fish: Aquatic vertebrates with gills throughout most or all of their lives. Fish are the best-known animals whose respiratory systems are built around gills.

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