In addition to waste removal, water balance, and sodium removal, the kidneys interact with other systems to maintain the body's balance

1. Kidneys filter blood forming urine. a watery fluid of urea, excess salts, and other wastes.  Useful materials are returned to the body as the blood vessels exit the kidneys.
2. Ureters are tubes that carry urine to the bladder.
3. The bladder collects and holds urine in preparation for excretion.
4. Urine is excreted through the urethra.

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The kidneys interact with the skeletal system by helping the body maintain calcium balance and forming a substance needed for red blood cell formation.

The kidneys use vitamin D produced by the skin to help with calcium balance. (movie)

Potassium is necessary for cardiac muscle contractions.  Too little potassium and the heart cannot beat.  Too much and the heart will stop beating.  The kidneys assist the heart by maintaining the potassium balance.

Regulation of water and sodium levels helps maintain a safe pressure level in blood vessels.