Fish gill
A respiratory organ that exchanges gases between water and blood. Its thin, extensive surfaces absorb dissolved oxygen and release carbon dioxide.
Countercurrent exchange: A process in which fluids flow in opposite directions across an exchange surface, sustaining a concentration gradient. Water and blood flow oppositely across gill surfaces, preserving oxygen transfer along their length.
Gill arch: A curved skeletal support that bears gill filaments and, in many fishes, gill rakers. It organizes and supports the structures that filter water and exchange gases.
Aquaculture: The farming of aquatic organisms, including fish, in controlled or managed environments. Farmed fish depend on adequate dissolved oxygen and water flow across their gills.
Lung: An internal respiratory organ that exchanges gases between air and blood or another circulatory fluid. Lungs exchange gases with air, whereas fish gills are adapted to flowing water.
Vertebrate gill: A respiratory structure in vertebrates that exchanges gases with water, especially during aquatic life stages. Fish gills are one major form of the broader vertebrate gill system.
Gill filament: A slender projection from a gill arch that bears rows of gas-exchanging lamellae. Filaments provide the broad surface that carries the gill's oxygen-exchange structures.
Gill raker: A projection on a gill arch that helps keep particles or prey from passing through the gill openings. Rakers protect delicate gill surfaces from debris and can help retain food.
Dissolved oxygen: Molecular oxygen present in water, available for aquatic organisms to use in respiration. It is the oxygen supply that gills must extract from the surrounding water.
Tracheal system: A network of air-filled tubes that delivers oxygen directly to tissues in insects and some other arthropods. Unlike gills, it can deliver air to tissues without transporting oxygen in blood.
Shark gill slit: An external opening behind a shark's mouth through which water exits after passing over the gills. Sharks typically expose separate gill slits, unlike the bony opercular cover of most fishes.