A drain or drain pipe is about your elimination system. It can be the colon, particularly if it connects to a sewer. It can also represent your bladder.
An overflow drain pipe connected to a heating or water system is about the immune system. Here the drain pipe kicks in when required. This is how the immune system works. It is connected to the circulatory system and drains infection from it. You will notice this if you get a cut on your shin. A swelling bellow the knee at the back of your leg is where the immune system drains the infection.