The kidneys are a pair of red-brown organs attached to the back of the abdominal cavity. They play a role in osmoregulation - regulation of water balance - and the excretion of nitrogenous waste such as urea.The kidney filters blood from the renal artery at a rate of 180dm3 a day. Blood is returned to the heart via the renal vein. The filtrate is collected by the collecting ducts and removed from the kidneys through the ureters and stored in the bladder. Urine passes out of the body through the urethra.