Why do sharks have high urea concentrations?

Why do sharks have high urea concentrations in blood?

• Simple one sentence explanation please?? I've read the book and it gives me a paragraph of mumbo jumbo. There's a chance this question will be on the test and I need to answer it in one sentence. All i know is that it makes them taste nasty when you try to eat them, and that butting vinegar on them someone makes the urea go away.

• Answer:

  Sharks- some other fish too- use urea to keep osmotic balance with the seawater they live in. That means that their blood (inside the skin) and the seawater outside must have approximately the same ion-density (dissolved chemicals), or fluids will migrate back and forth becuase of osmosis. Urea decomposes to ammonia (and other stuff) real quickly, hence, the unique stink of dead shark.