I can't write your whole essay for you here on eNotes, but I can get you pointed in the right direction.
Your kidneys serve a very important function in the body, and without them it is impossible to live. While it is true that people can donate one o their two kidneys, or have one removed for medical reasons and still live a normal live, the body does have two of them for a good purpose: the filtration of wastes from the blood, including urea, drugs and ammonia, three things that build up in our tissues and cause problems in concentrations.
Most people also don't realize the kidneys are very important in maintaining fluid levels in the body, adjusting the pH balance of those fluids and regulating a person's blood pressure.
What's more, kidneys work 24 hours a day, in continuous effort to achieve these roles, and once the function of the kidneys is limited, or if the sole remaining kidney is damaged, a person will either need a transplant or dialysis on a machine for life.