As for how that affects your heart, imagine that your kidneys suddenly stop working. All that water in your blood has nowhere to go, so it sticks around in the blood. Maybe after a gallon of extra water is in your blood you notice your feet are swelling a bit, and after a little more you notice that you can press on your calf and you can see your fingerprint indentation in your skin. Drink even more water and your body has so much fluid that your heart has to pump as hard as it can just to get the blood out into the vessels and eventually blood is going to back up. Unfortunately for your lungs, this is where blood is going to back up if your heart can't pump blood out into the body. Pressure builds up in the tiny vessels there, and fluid starts getting pushed out into the little air sacs in your lungs. It makes sense if you think of blood vessels as porous; they have to be, or else blood would never leave and the only way your tissues could get nutrients and exchange gases is if they were inside the vessels themselves, which is a scary thought.