The kidneys play several important roles in maintaining homeostasis, including maintaining the proper blood volume and ion balance as well as removing nitrogenous wastes from the blood. The University of New Mexico explains that the kidneys also play an important role in regulating the body’s pH. The kidney primarily accomplishes these tasks by filtering impurities, metabolic wastes and salt from the blood.
According to the BBC, the kidneys are able to maintain homeostasis in part by varying the concentration of urine. For example, when the amount of water in the blood plasma is low, the kidneys reabsorb water from the urine, returning it to the blood stream. Conversely, when the amount of water in the blood plasma is high, the kidneys do not reabsorb much water, which produces highly dilute urine. Both responses help to keep the body's water balance within the range of tolerance.
The kidneys must compensate for other biological functions and stimuli that may alter the water balance of the body, thus disrupting homeostasis. The BBC lists several such stimuli, including the external temperature, exercise and salt intake, as having the potential to change the amount of water in the body. For example, during exercise, the body's temperature rises. The body attempts to compensate for this by sweating. However, sweat results in a net loss of water, which in turn, triggers the kidneys to reabsorb more water from the urine and return it to the bloodstream.