What are nitrogenous wastes?
Nitrogenous wastes are the nitrogen-containing metabolic waste products of the breakdown of proteins and nucleic acids, both biomacromolecules which contain nitrogen. Excretion is the removal of nitrogenous wastes. in mammals, the nitrogenous waste urea is removed as part of the urine.
The elimiation of nitrogenous wastes is essential, and there are 3 main nitrogenous wastes with varying levels of toxicity and solubility.
Ammonia is the most toxic, and the most soluble in water
Urea is moderately toxic, and moderately soluble in water
Uric acid is the least toxic, and the most insoluble in water
A build-up of ammonia in cells can affect their pH, making them more basic (increasing pH levels), which can denature enzymes and reduce metabolic activity as their function has been compromised. Various organisms have different ways of coping with this waste product. Some animals excrete ammonia directly, while many others expend energy to convert ammonia to a less toxic form, urea or uric acid, prior to excretion.
Comparing nitrogenous wastes in various vertebrate groups
What are the problems for fish regarding nitrogenous wastes?
Fish, such as salmon, are faced with physiological challenges when living in freshwater environments and in seawater environments, and have methods in order to overcome these challenges.
In freshwater environments, the salt concentration is hypertonic in fish (higher) compared to their environment, and so as a consequence, the fish tends to gain water.
To combat this, the kidneys produce large volume of dilute urine in order to eliminate excess water. Alongside this, fish actively gain salt from water through gills (via active transport), and fish do not drink freshwater.
In seawater environments, the salt concentration is hypotonic in fish (lower) compared to their environment, and so as a consequence, the fish tends to lose water.
To combat this, the fish drinks seawater to gain water and actively expels excess salt through specialised gill cells. The fish also produces small amounts of concentrated urine in order to reduce water loss/preserve water levels.