0x668143c4…385fsent to0x9e4e9d99…a81d·#10,748,997·view on Etherscan
Aquatic organisms are bathed in water. In the same way water is pulled with particles when excreted from the kidney, water is pulled into the organism when its osmolarity increases as amino acids are degraded into HCO3- and NH4+. To the aquatic organism, there is no water cost to excrete these byproducts, water will first flow inwards to neutralize the osmotic balance, and then follow along outwards for the same reason. The terrestrial organism on the other hand will not have the inwards osmotic equilibration that aquatic organisms have. To the terrestrial organism, the byproducts from amino acid catabolism have a large cost in water. This is the reason terrestrial organisms have evolved ways to lower that cost, by turning the HCO3- and NH4+ into CO2 and urea. Urea is a weaker base than ammonia, and will free the proton on NH4+ to combine with HCO3-, forming carbon dioxide that can be exhaled with no water cost, removing 50% of the cost. Urea also combines two NH3 into a single molecule, lowering the osmotic pressure by 2x, reducing the total osmotic cost with another 25%. The end result is that only 1/4th as much water is needed.