Your kidneys are the primary filtration and removal plant in your body. They are your blood cleaning organs. Most of your kidneys consist of nephrons. Each one is a capillary cluster with a coiled tube attached to it. There are over a million of them in your kidneys! As the blood passes through the capillary cluster, water and waste products filter through the capillary walls and into those tubules. Most of that waste water is cleaned and returned to the blood. Your kidneys, then, are like a million little thinking machines, each one of which knows just what to remove from the blood and what to leave in it.
The waste fluid drains out into a collecting basin in each kidney called the renal pelvis. From each one, a tube leads down into the bladder. When the bladder fills to about 200 cc [12.2 cu inches], it sends a signal to the brain to void the urine. How can a bag send a signal? How does it know to do it at the right time?