As your body performs the many functions that it needs in order to keep itself alive, it produces wastes. These wastes are chemicals that are toxic and that if left alone would seriously hurt or even kill you.
For example, as your cells break down amino acids, they produce a dangerous toxin known as urea. The cells of your body excrete this urea into your blood.
As your blood travels along within your body, it becomes more and more polluted with urea and other wastes. Eventually the blood enters a special filter, an organ known as your kidneys. As the blood enters your kidneys it is cleansed. Your kidneys remove the urea from the blood, sending it to your bladder for storage in the form of urine, commonly known as pee. It takes about 45 minutes for your kidneys to completely filter all of the blood in your body.
Slowly your bladder fills up with the urine being produced by your kidneys. Eventually as it becomes full, you will feel a sensation telling your brain that you need to remove it from your body. Urine leaves your body through the process of urination, commonly known as peeing.