Shark Ion and Water Balance

To the tune of "When I Kissed the Teacher" by ABBA
Sharks, skates, and rays are osmoconformers
They do not regulate their osmotic pressure
Their blood plasma
Is slightly hyper-osmotic to the surrounding media
However
They are very strict regulators of ion concentration
High levels of NaCl can be devastating
For macromolecules
Ions damage proteins and bind lipids and enzymes too
This is the importance of ion regulation
Concentration gradient
For ions to enter through the gills
They're excreted
Through the rectal gland so they don't damage the cells
Sharks use urea, as an osmolyte
To help maintain the osmotic pressure
It is synthesized
From ammonia in the liver and the muscle by
The Ornithine-Urea cycle
Urea
Is reabsorbed from urine in the kidney
Urea loss
Is prevented in the gills in these species
But with urea proteins will unfold
So they have high levels of TMAO
And some enzymes
Are less sensitive to urea because they're modified
TMAO comes from the diet—it's not synthesized