Editing Excretory system (section)

==== Kidneys ====
{{Main|Kidneys}}

The kidney's primary function is the elimination of waste from the bloodstream by production of urine. They perform several homeostatic functions such as:-

# Maintain volume of extracellular fluid
# Maintain ionic balance in extracellular fluid
# Maintain pH and osmotic concentration of the extracellular fluid.
# Excrete toxic metabolic by-products such as urea, ammonia, and uric acid.

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. Needed materials go back into the bloodstream; unneeded materials become urine and are expelled through the urethra.

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.