Sharks

Fish living in the ocean are in danger of dehydrating because water moves out of their body into their salty environment through the process of osmosis. Basically, this occurs because the salt concentration in the ocean is much higher than that in the blood of fish. In part, sharks solve their dehydration problem by having a relatively high internal concentration of salts and other molecules. In addition to the salts naturally present, sharks have additional solutes (i.e., dissolved substances) in their blood, so the total osmotic activity of dissolved substances is similar to that in seawater. They maintain their blood at this concentration by excreting the excess salt they ingest in their diet. A special gland near the end of the intestine, called the rectal gland, absorbs extra salt from the blood and passes it into the intestine to be excreted. These two adaptations function together to ensure that sharks do not dehydrate.