Science Questions

Sun, 28th Nov 2004

Part of the show Safe Sex, & Cervical Cancer Vaccines

Question

Alex asked:

Do fish ever get thirsty, and do they sweat?

Answer

Bony fish that live in the sea have blood that is less concentrated (in terms of salt) than the water they live in. By a process called osmosis, these fish will lose lots of water into the sea. To stop them from dehydrating, bony fish drink lots of water. Although drinking salt water is never a good idea if you're a thirsty human, fish have special mechanism on their gills that enables them to excrete the excess salt they are drinking. By pumping the salts back out again, they are able to drink water without become even more dehydrated. In contrast, sharks make their bodies as concentrated as the water around them. They do this by building up levels of urea in their blood. This is why dead sharks smell like urine. Having highly concentrated blood also means that sharks won't do very well if put into freshwater - water will rush into the shark by osmosis and it might even explode! Sweating is a process designed to help organisms cool down. On land, sweat on the skin evaporates and takes heat away from the body, making you feel cooler. In the sea, water can't evaporate off the fish because they are already in water! So no, fish don't sweat.

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