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For marine vertebrates, the concentration of salts outside their body (in the water, that is) is more than that inside their body. So, there is a natural tendency for exosmosis to occur.
To minimise this natural tendency for exosmosis to occur, these vertebrates would need to retain more salts inside their body.
I understand that the Proximal Convoluted Tubule (PCT) of the nephrons of the kidneys is involved in selective reabsorption of salts. So, making the PCT more coiled would increase the surface area of absorption, thus increasing the absorption, and therefore retention of salts in the body.
So, my question is: is the PCT of the nephrons of the kidneys of marine vertebrates more coiled than that of terrestrial vertebrates?
First off, I agree with @WYSIWYG - marine life has adapted to conserve water, not electrolytes. Osmosis in sea water draws water from the body and hence their problem is to keep the water in, not the salts.
Osmoregulation in marine mammals is accomplished mainly via differences in hormonal regulation of the kidneys. The histology and morphology of the kidneys are different, but the adaptations have more to do with adaptations to diving/pressure than anything else. Instead, most marine mammals take care not to drink too much sea water and obtain their water from metabolism and foods (Ortiz, 2001).
- Ortiz, J Exp Biol (2001); 204: 1831-44