Station 3: Wallace realizes it's time to go to the bathroom, so he raises his hand for permission. The teacher tells him it is okay to leave the room. Urine is a waste product made up of salts, water, and urea which would be dangerous if they built up in your bloodstream. Your body systems work together to collect these waste products and turn them into urine and then let you know when it's time to eliminate them. Which of Wallace's systems are working together to help him eliminate these dangerous waste products?