Introduction

Animal life first arose in the sea roughly 700 million years ago, and the earliest forms - much like modern sponges - had every cell bathed directly in seawater. As tissue organisation grew more complex, animals developed specialised outer tissue layers that separated an internal extracellular fluid from the external environment, letting a cell hold an internal ionic makeup different from its surroundings even while immersed in seawater. This separation made active regulation possible, and major advances in controlling that internal fluid's osmotic and ionic composition arose during chordate evolution - regulation that was essential for animals to spread into brackish water, fresh water and eventually onto land. Moving onto land brought its own problem: without water nearby to carry nitrogenous waste away directly, animals needed an alternate route to dispose of it, which is where excretion - the process of removing the nitrogenous by-products of protein metabolism - becomes essential.

Most animals rely on the kidney to manage this ionic and water balance, though some instead use external tissues such as gills, skin or digestive mucosa. Together these organs regulate three related processes: osmotic regulation (controlling tissue osmotic pressure, which drives the movement of water across membranes), ionic regulation (controlling the ionic composition of body fluids) and nitrogen excretion. Freshwater vertebrates keep higher salt concentrations in their body fluids than their surroundings, marine vertebrates keep lower salt concentrations than the seawater around them, and terrestrial animals - carrying more water in their bodies than the dry air around them - constantly risk losing water through evaporation. Animals that let their internal osmotic concentration shift along with the environment are called osmoconformers (marine molluscs and sharks, for instance), while osmoregulators actively hold their internal concentration steady regardless of the environment (as otters do). A related distinction separates stenohaline animals, which can only tolerate a narrow range of salt concentration (goldfish, for example), from euryhaline animals, which tolerate wide swings (such as Artemia, tilapia and salmon). Whatever the strategy, the nitrogenous waste itself is mostly ammonia, urea or uric acid, alongside smaller amounts of other compounds such as trimethylamine oxide in marine bony fish or guanine in spiders.