Fishes are the most phylogenetically ancient vertebrates, and their cardiovascular system presumably resembles the ancestral prototype. Nearly half of all vertebrate species are fishes, and they are also the most physiologically diverse. For example, hagfish have the highest plasma sodium and chloride concentration of any vertebrate (>400 mM), elasmobranchs (sharks skates and rays) adjust plasma osmolarity with 300 mM urea, and bony fishes maintain plasma osmolarity ∼300 mOsm in either saltwater (1,000 mOsm) or freshwater (<1 mOsm). Body temperature varies with the environment and can range from supercooled –1.8°C in Antarctic icefish to 40°C for desert pupfish. Ambient oxygen (O2) varies diurnally, seasonally, and with strata from supersaturation to anoxia, and various mechanisms have evolved to deal with hypoxia; the crucian carp is unaffected by up to 5 days in anoxic water, and many fishes breathe air, either using modified gills or as lungfish with a primitive lung. Hagfish have the lowest blood pressure yet measured in any vertebrate (7 to 10 mm Hg), whereas ventral aortic pressure approaches 90 mm Hg in tuna, similar to mammals. The gill and systemic circulations of fishes are arranged in series thus, unlike most other vertebrates, delicate respiratory tissues are exposed to the highest blood pressures. Also unique to fishes is the apparent lack of a lymphatic system; the impact of this on transcapillary fluid balance is largely unknown.

Despite this variability, the general features of cardiovascular systems in all extant vertebrates are surprising similar (1). Thus, it seems likely that the piscine cardiovascular system can provide clues into the factors that shaped the evolution of the vertebrate cardiovascular system and defined its plasticity.