Maintaining a Balance Topic 19: Excretion Systems in Fish and Mammals Biology in Focus, HSC Course

DOT POINT Identify the role of the kidney in the excretory system of fish and mammals

Introduction Water and solutes are continually exchanged between the environment and a living organism’s body fluids. Water accumulates within the body of an animal when it eats or drinks, or as a bi-product of cellular metabolism. Nitrogenous wastes accumulate in animals as a result of metabolism. flickr.com

Introduction The water potential of living cells is similar to that of sea water, but lower than that of fresh water or the surrounding air. Water potential is the tendency of a solution to lose water by osmosis, typical of solutions that have a high water concentration. ocean.nationalgeographic.com

Introduction The concentration of water in the immediate environment of an organism determines its need to conserve (retain) water or lose it. In aquatic animals such as fish, the concentration of solutes in the surrounding aquatic environment has a direct influence on the direction of movement of water – whether it will move into or out of the body of the fish. australia.com

Freshwater Fish Freshwater fish live in rivers and lakes, where the water potential is high – these habitats contain very few dissolved salts and water is therefore freely available. legacy.owensboro.kctcs.edu

Freshwater Fish Freshwater fish urinate frequently, as water tends to accumulate in their tissues as a result of passive movement by osmosis from a higher concentration in the surroundings to a lower concentration in the animal. bio-diversity-nevis.org

Freshwater Fish These fish are faced with the problem of too much water being present in their bodies. The kidneys in freshwater fish therefore excrete excess water (gained from their surroundings), as well as nitrogenous wastes (ammonia). Their kidneys are structurally suited to this role by having large glomeruli for the filtration of blood in large volumes. biologicalexceptions.blogspot.com

Freshwater Fish Their kidneys are not involved in salt balance, since these fish do not face the problem of salt accumulation from their freshwater environment. Any excess salts that they consume in their diets are excreted via the gills. todayifoundout.com

Marine 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. scuba-equipment-usa.com

Marine Fish The main function of the kidneys in these fish is therefore to remove excess salt. Marine fish tend to drink sea water, extract the salt from it and then use the water for metabolism. They excrete the extracted salt to keep salt levels in the body to a minimum. legacy.owensboro.kctcs.edu

Marine Fish Their kidneys tend to play a role in conserving water rather than excreting it. To meet this need, their kidneys tend to have small glomeruli as well as a mechanism for removing excess salt taken in with sea water. The kidney is also responsible for excreting nitrogenous wastes (usually in the form of urea) in marine fish. tutorvista.com

Terrestrial Mammals Terrestrial mammals lose water and solutes from the body as a result of evaporation from the lung surface during respiration and as a result of excretion. For example, the production of sweat and urine. blog.kimblechartingsolutions.com

Terrestrial Mammals The kidneys of mammals excrete urine that is composed mainly of water and nitrogenous wastes (urea), 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. beastlair.com

Terrestrial Mammals Mammals have a complex control mechanism to ensure that a balance is maintained between the amounts of sweat and urine excreted. In hot weather, more water is excreted as sweat and as a result less urine is produced. In cold weather, more water is lost in urine and very little in sweat. en.wikipedia.org

Terrestrial Mammals A relatively large quantity of salts is also lost during sweating and needs to be replaced to maintain a stable osmotic pressure within body fluids and cells in an organism. Any adjustment to water and salt levels in urine is brought about by the action of the hormones ADH and aldosterone on the kidney tubules. winergy.com

Activity -Students to complete activity 3.11 Urine concentrations of terrestrial mammals, marine fish and freshwater fish.