The kidneys in fish and mammals are excretory and osmoregulatory organs and their main functions are:
Marine Fish – converse water, excrete excess salts and nitrogenous wastes
Freshwater fish – excrete excess water and nitrogenous wastes (produce large amounts of dilute urine), conserve salt
Mammals – conserve water and salts when required and excrete excess nitrogenous wastes.
Live in rivers and lakes, where the water potential is very high (water potential is the tendency of a solution to lose water by osmosis, generally these solutions have high water concentrations) – these habitats contain few dissolved salts and hence water is freely available. Freshwater fish tend to urinate frequently, as water tends to accumulate in their tissues as a result of passive movement by osmosis (water moves from high concentrations to lower concentrations inside the fish). These fish are face with the problem of there being too much water in their bodies – as a result their kidneys excrete excess water, as well as nitrogenous wastes. Their kidneys are structurally suited to this as they have large glomeruli for the filtration of blood in large volumes. Their kidneys are not involved in salt balance, as there is no salt accumulation in freshwater fish.
Marine fish urinate less. They tend to lose body water (by osmosis), across the body surface and gills, into their salty surroundings. Excess salt tends to accumulate in their bodies, moving in by diffusion from the surrounding sea water. The main function of the kidneys is therefore, to remove excess salt. Marine fish drink the sea water, extract the salt, use the water for metabolism, then excrete the extracted salt to keep their bodies salt levels at a minimum. The kidneys also tend to conserve water rather than excrete it – and as a result the kidneys of marine fish have small glomeruli as well as a mechanism for removing excess salt from sea water. The kidney is also responsible for removing nitrogenous wastes.
Lose water and solutes as a result of evaporation from the lung surface during respiration. The kidneys of mammals excrete urine, which is composed mainly of water and nitrogenous wastes as well as some excess salts. The mammalian kidney can adjust the reabsorption of nitrogenous wastes, water and salts – varying the concentration of urine produced. Mammals have a complex control mechanism to ensure that a balance is maintained between the amounts of sweat and urine excreted. For example in hot weather, more water is excreted as sweat (since sweat is evaporative cooling, lowering the bodies temperature) and as a result less urine is produced. In colder weather, more water is lost in urine and very little as sweat. A relatively large quantity of salts is also lost during sweating and needs to be replaced, in order to maintain a stable osmotic pressure. Any adjustment to the water or salt concentration in body fluids is brought about by the hormone ADH and aldosterone. Urine may be dilute or concentrated depending on the needs of the body.