The kidneys in human beings are a part of the system for

The correct answer is Excretion.

Key Points 

• In humans, the kidney is one of the main organs of the excretory system.
• The kidney's function is the filtration of the blood and removing waste(toxic) products.
• Waste products and excess fluid are removed through the urine.
• If the kidney fails person needs to undergo dialysis.
• Dialysis is a treatment that filters and purifies the blood using a machine
• The human excretory system includes certain organs that eliminate the wastes such as salts, carbon dioxide, urea, uric acid and water produced by the body.
• The path of the urine in the human excretory system is -
• After filtering from the kidney → The urine through Ureter → Stored in the Urinary bladder → Exits through the urethra.

Additional Information 

• Respiration- It is the movement of oxygen from the environment and the transport of carbon dioxide back to the environment.
• Transportation- The blood is responsible to transport hormones, oxygen, food, and waste materials in human bodies.