Osmoregulation of Freshwater Fish

Freshwater fish are hypertonic to their water environment, meaning water from the outside diffuses into them through their gills. The gills are permeable to water, but they are also permeable to waste products. Allowing the fish to expel respiratory gasses, ammonia, and ions.

The water flows in through the gills in favor of the osmotic pressure in the blood; while sodium and chloride ions diffuse out of the fishes system into the external environment. These fish have to use energy to regulate the water increase and ion decrease of their bodies.

The fish have special lamellae (dictionary.com: a thin layer, membrane, scale, or platelike tissue or part) in their gills that contain sodium and chloride enzymes. These special enzymes use energy to push sodium and chloride ions up their concentration gradient to keep higher concentration in the fishes body.

The kidneys regulate the large amount of water intake by producing a lot of dilute urine. Even though the kidneys are very helpful in maintaining the high amount of water intake, a lot of salt is lost through the urine and the gills.

It is very important for freshwater fish to have low stress levels and proper nutrition; this keeps them healthy and reduces damage to kidneys. Kidney damage through bacterial infection is detrimental to a fishes life.

Some examples of fish who strive in their native environment are Angelfish and Discus. These two fish have very efficient osmoregulatory systems because of the type of water in the Amazon River Basin. However, when these fish feel stressed their systems are much more likely to fail.

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