Kidneys in ANY animal function to filter chemicals out of the blood system and "flush" them out of the body by adding water. In short terms, you pee.
filter and rid of waste and excess materials from body
Same as the function of the kidneys in every other animal - to filter out toxins from the blood stream and pass them into the bladder for expulsion from the body, and to help regulate the blood's water levels.
to dilute fat
The kidneys of a frog, like many other animals, filter the blood and excretes excess water. The ureters then carry the urine from the kidneys to the urinary bladder.
To filter blood and secrete impurities.
Kidneys (or ~ nephridia) appear quite early in evolution, and are responsible for disposing of waste liquids, salts and some other chemicals not wanted by the body.