<Li> Maintain pH and osmotic concentration of the extracellular fluid . </Li> <Li> Excrete toxic metabolic by - products such as urea, ammonia, and uric acid . </Li> <P> The way the kidneys do this is with nephrons . There are over 1 million nephrons in each kidney; these nephrons act as filters inside the kidneys . The kidneys filter needed materials and waste, the needed materials go back into the bloodstream, and unneeded materials becomes urine and is gotten rid of . </P> <P> In some cases, excess wastes crystallize as kidney stones . They grow and can become painful irritants that may require surgery or ultrasound treatments . Some stones are small enough to be forced into the urethra . </P>

Which structure of the excretory system mainly functions in filtration