Gill

A gill is a respiratory organ found in many aquatic animals, used to extract dissolved oxygen from water and release carbon dioxide. In vertebrates, gills typically occupy a pharyngeal region and are protected by an operculum in bony fish; in some amphibians, they are external during larval stages. In most aquatic invertebrates, gills are specialized structures within branchial chambers or mantle cavities.

Structure and mechanism: Gill arches support two or more rows of gill filaments, and each filament bears numerous lamellae, increasing surface area. Blood in the gill capillaries flows in the opposite direction to water passing over the gills (countercurrent exchange), maximizing oxygen uptake and carbon dioxide diffusion. This arrangement, along with thin, moist membranes, facilitates gas exchange across a small diffusion distance.

Function and operation: Water enters the mouth and exits through gill slits or an opercular opening while the fish ventilates by pumping water. The rate of water flow and the partial pressure gradient determine the amount of oxygen that can be extracted.

Variation and evolution: In amphibians, larvae often have external gills that protrude from the head; in adult aquatic crustaceans and mollusks, gills are located within gill chambers. Gills have evolved in various forms reflecting habitat and physiology, including specialized structures in different taxa.

Importance and ecology: Gills are essential for aquatic respiration but are sensitive to pollution, temperature changes, and low oxygen conditions; impairment can reduce gas exchange and threaten survival.