When differences occur in salt concentration between an aquatic animal's body fluids and the surrounding salt or fresh water, respiration via the gills has the potential to disrupt salt and water balance. The solute concentration of a solution of water is known as the solution's osmolarity, expressed as milliosmoles/litre (mOsm/L). The number of dissolved solute molecules determines a solution's osmolarity. For example, a 150-mM NaCl solution has an osmolarity of 300 mOsm/L, because each NaCl molecule dissociates into two molecules, one Na+ and one Cl- (2 × 150 = 300). The internal fluid osmolarity of most fish is usually within the range of 225–400 mOsm/L, similar to that of most other vertebrates. However, because freshwater lakes and rivers have very little salt content (usually <25 mOsm/L), a strong concentration gradient for salts could promote the loss of salts from a fish's body into the fresh water. Likewise, a strong osmotic gradient favours the movement of water into a freshwater fish. Water flows across the gills and into the underlying capillaries and bloodstream.
Freshwater fish, therefore, gain water and lose salt when ventilating their gills. If left uncorrected, this would cause a dangerous decrease in blood salt concentrations. Freshwater fish avoid this problem using two different strategies. First, their kidneys are adapted to producing copious amounts of dilute urine—up to 30% of their body mass per day (an amount that would be equivalent to about 25 L per day in an average-sized man). Second, specialized gill epithelial cells actively transport Na+ and Cl- from the surrounding water into the fish's capillaries. Thus, these two important ions are recaptured from the water. As the preceding discussion suggests, freshwater fish rarely if ever drink water, except for any that might be swallowed with food.
https://biology-forums.com/index.php?action=gallery;sa=view;id=1270Salt and water balance in water-breathers.Water breathing creates osmoregulatory challenges because of diffusion of salts and osmosis of water across gills. These challenges differ between (a) freshwater and (b) saltwater fish and are met by drinking or not drinking water, by active transport of salts across the gills, and by alterations in urine output.Other freshwater animals, such as frogs and other adult amphibians, have body surfaces that are permeable to water and used in gas exchange. Like freshwater fish, therefore, they tend to gain water by osmosis and compensate by excreting copious dilute urine. Epithelial cells of the skin actively transport necessary electrolytes from the water into the blood.
Saltwater fish have the opposite problem. They tend to gain salts and lose water across their gills, because seawater has a much higher osmolarity (about 1,000 mOsm/L) than that of their body fluids. The gain of salts and the loss of water from the body are only partly offset by the kidneys, which in marine fish produce very little urine so that water can be retained in the body. The urine that is produced has a higher salt concentration than that of freshwater fish.
To prevent dehydration from occurring, marine fish must drink. However, the only water available to them to drink is seawater, which has a very high salt content. Paradoxically, therefore, marine fish drink seawater to replenish the water lost by osmosis through their gills. This creates a new problem: What does the fish do with all the salt it has ingested?
The ingested salt must be eliminated, and this process is accomplished by gill epithelial cells. In contrast to the gills of freshwater fish, which pump salt from the water into the fluids of the fish, gills of marine fish pump salt out of the fish and into the ocean. Thus, marine fish drink seawater to replace the water lost through their gills by osmosis and then expend energy to transport the excess salt out of the body.
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