Your kidneys are all about filtration. Picture a water filter, or air filter. It as very tiny holes to pass to allow air or water to pass but because the holes are so tiny, it traps bigger particles and debris. This is exactly how the nephrons of the kidney work. There are millions of these tiny units within the kidney. By filtering out and keeping blood, and nutrients like proteins, nephrons are able to remove all the wastes and create urine. The creation of urine is essential to maintaining homeostasis.
The Urinary Bladder