The kidneys, two bean-shaped organs located on either side of the spine at the level of the lowest ribs, serve a variety of vital functions. At the junction of the circulatory and urinary systems, the kidneys' primary function is the removal of excess fluid and waste material from the bloodstream, beginning the process of eliminating them from the body as urine flowing through the ureters, into the bladder and, finally, exiting through the urethra. The kidneys also regulate the body's levels of sodium, potassium, calcium, and other chemicals which can become toxic if left to build up. In addition, the kidneys function as glands, producing hormones to help maintain normal blood pressure, enable the production of red blood cells, and aid the formation of bone.
The kidneys have a remarkable operating capacity—a single kidney, working at just 20% of its capacity, can still provide all needed renal filtration and regulation. And, the kidneys are capable of altering their activities from day to day, constantly adjusting to the variety and amounts of foods and liquids that are consumed. As we fast one day, or overindulge the next, the kidneys compensate to keep the body's tissues from bloating with fluids or dehydrating.
Through a network of millions of nephrons in each kidney, blood entering from the renal arteries is filtered, and through complex biochemistry, substances not required by the body (typically urea, creatinine and ureic acid) are removed, and others, such as blood cells, proteins, and some amount of water, are reabsorbed. Thus, the blood that leaves the kidney through the renal veins contains necessary salts, protein, sugar, calcium and other vital substances in the right proportion, while excess chemicals, toxins and fluids are sloughed off into the urinary tract.
Congenital anomalies, inherited disorders, injury, hypertension, exposure to toxins, kidney stones, tumors, and even infections in other parts of the body can result in compromised renal function. Many kidney diseases have no noticeable symptoms until substantial and irreparable damage has been done. When the kidneys can no longer function well enough on their own—termed "end-state renal disease" or ESRD—hemodialysis or peritoneal dialysis will be required to help eliminate waste and excess fluids from the blood. Dialysis cannot, however, replicate the kidneys' hormonal benefits. Transplantation has proven to be the one way to recover the quality of health, and quality of life, enjoyed before the onset of end-stage renal disease.