How will other body systems be affected if the kidneys begin to shutdown?

The cardiovascular system would be the most affected. Salt from the blood would not be able to be filtered, then backing up your urine. Waste products (feces and urine) would not be able to be disposed of. The buildup in toxins in your body can also cause damage to your liver, and possibly your blood stream if toxins do enter. If that does happen, your entire body is now affected because the toxins will travel throughout. 

Small intestine because that is the organ which absorbs nutrients