To illustrate this we show the excretion of urea (which contains nitrogen which is toxic to the body): The amino acids are used for growth and excess amino acids are excreted as urea. This re-enters into the blood stream and which then enters both kidneys. The kidneys will filter the urea from the blood and the urea will be added to water to form urine and travel down the ureter to collect in the bladder which is stored there as the form of urine. The filtered blood travels back to the blood stream without any urea.
Osmo (osmosis) regulation (to control), hypertonic (too concentrated), hypotonic (too dilute)
We want to keep the tissue fluid isotonic (the amount of water into is equal to the output) which is achieved by controlling the composition of blood. The kidney is the organ which controls the composition of blood. Blood circulates through our kidney and excess water or salt is removed and excreted through the ureter here by controlling the content of water and salts in the blood, the kidney can keep the blood and therefore the tissue fluid can be maintained as isotonic which maintains the function of the cells.