Why kidney is known as osmoregulator???
Your friend is correct, Apart from this I would like to add more points,
As the kidney helps to keep the amount of water and dissolved solutes in balance in our body called osmoregulation, so it is also known as osmoregulator.
@amritpreet.singh...Good answer...Keep posting!
Osmoregulation is the active regulation of the osmotic pressure of an organism's fluids to maintain the homeostasis of the organism's water content; that is, it keeps the organism's fluids from becoming too diluted or too concentrated. Osmotic pressure is a measure of the tendency of water to move into one solution from another by osmosis. As Kidney helps in the above functions it is known as osmoregulater