BIOLOGY Volume 3 - A Guide to General Biology - 2004

20. EXCRETION AND OSMOREGULATION

20.3. Nitrogen Excretion and Osmoregulation in Selected Animals

20.3.4. Freshwater Fish

In fish, the Organs of excretion and osmoregulation are the gills and Kidneys. The gills are in contact with the external environment and are permeable to Water, nitrogenous wastes, and salts. They possess a large surface area that facilitates efficient gas exchange, but this creates certain challenges for osmoregulation, particularly in freshwater.

In freshwater teleost fish, the internal environment is a more concentrated solution than their surrounding habitat. Although the outer body covering of mucus-covered scales is relatively impermeable, a significant amount of water enters the body by osmosis through the highly permeable gills, and salts are simultaneously lost through them. The gills also serve as excretory organs for nitrogenous waste products such as ammonia. To maintain the steady state of their internal Body Fluids, freshwater fish must continuously eliminate large volumes of water (Fig. 20.8). They achieve this by producing copious amounts of dilute urine (with a higher water potential than Blood) containing ammonia and various other dissolved solutes. The volume of urine excreted daily can reach up to one-third of the fish's total body mass. The loss of salts in the urine is compensated for by electrolytes obtained from food and through the active uptake of salts from the surrounding water by specialized Cells located in the gills.

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Fig. 20.8. Excretion and osmoregulation in a freshwater teleost fish.