The excretory system is the system in the body that contains skin, lungs, kidneys, and their associated organs. The purpose of course, is to excrete.
The Kidneys
If the kidneys cut in half two distinct sections can be seen. The inner section is called the renal medulla and the outer section is called the renal cortex. Renal corex is where the main function of the kidneys happen.
A kidney has 1,000,000 nephrons, which are like little mini-organs. They have there own circulation system, meaning they have their own blood supply, an arteriole, a venule, and some capillaries. It also has a tube connecting it to the ureter (the tube to the urethra).
When the blood enters the arteriole, impurities are filtered out. It can be explained simply, but the actual process is very complicated. It involves two processes:
Filtration
The blood is pushed into a network of arteries called the glomerulus. Then it goes to a thing called the Bowman capsule. This is where the filtration happens. It is a very complicated chemical process, so it won't be explained here.
Reabsorbtion
Most of the stuff collected in the capsule is reabsorbed back to the capillaries. The rest of what can't be reabsorbed is sent to the ureter, then to the urethra to be excreted.