In the past 50 years, it has become clear that the branchial epithelium is the primary site of transport processes that counter the effects of osmotic and ionic gradients, as well as the principal site of body fluid pH regulation and nitrogenous waste excretion. For example, the high surface area of the gills that enhances gas exchange between the blood and environment can exacerbate water and ion fluxes that may occur due to gradients between the fish's extracellular fluids and the aquatic environment. The characteristics of the gill that make it an exceptional gas exchanger are not without trade-offs. The entire cardiac output perfuses the branchial vasculature before entering the dorsal aorta and the systemic circulation. The fish gill evolved into the first vertebrate gas exchange organ and is essentially composed of a highly complex vasculature, surrounded by a high surface area epithelium that provides a thin barrier between a fish's blood and aquatic environment ( Fig. Aerial-breathing species may use the gill, swim bladder, or other accessory breathing organs (including the skin). Regardless of lineage, the majority of fish species uses the gill as the primary site of aquatic respiration. They have evolved into three major lineages: Agnatha (hagfish and lampreys), Chondrichthyes (sharks, skates, and rays usually referred to as elasmobranchs), and Actinopterygii (bony fishes, with teleosts being the most prevalent). Fishes are aquatic vertebrates that are members of the largest and most diverse vertebrate taxon (∼25,000 species), that dates back over 500 million years.
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