By Ken Bodziak, MD

The kidneys are most commonly viewed as the organs responsible for the production of urine - the liquid waste product of daily metabolism. Though true, however, this rather simple definition implies that the kidneys are somewhat passive role players in our lives. This article is intended to further define the multiple functions of the kidneys and the consequences that occur when they become diseased.

The two kidneys are located on either side of the backbone (or vertebrae) in the back, at the level of the lowest of the rib cage. They are about the size of an adult's fist (4.5 - 5 in.) in length and weigh less than one-half of one percent of the total body weight. Yet, together the two kidneys receive approximately one-fifth of the blood that is pumped out from the heart every minute. That disproportionate amount of blood flow to size of the organs is required in order for the kidneys to perform a primary task, removing potentially harmful waste products from the blood, which are formed as the result of our bodies breaking down (metabolizing) foodstuffs, drugs and the contents of dying cells.

If it were not for the adequate removal of these toxic substances in the urine (as well as in the stool), we would fall ill. Symptoms such as malaise (general feeling of tiredness), nausea and vomiting would arise. Our muscles would weaken and our mental capacity could decrease. Seizures may occur and/or a coma-like state followed by death would ensue without proper treatment.

In addition to removing harmful waste products from the body, the kidneys also aid in the maintenance of the local environment around the cells of the body. The different cells, such as nerves, muscles, blood, etc., all require a very precise liquid environment surrounding them. That environment needs to be within a narrow range of pH (acid content) and have a salt composition comprised of sodium, potassium, calcium and other elements, which also must be within discreet ranges of concentrations in order for the cells to function properly. Otherwise, the cells may suffer a number of potentially lethal fates, such as shrinking, swelling or bursting.

Cellular function in response to an abnormal environment may actually prove fatal to the person. For example, if the potassium level in the blood fluid is too high, heart cells responsible for the coordinated beating of the heart may fire out of synchrony and lead to the heart's inability to pump out blood, leading to death. The kidneys act to fine tune the composition of that extracellular fluid by adjusting the final concentration of certain elements, like acid, in the urine.

For the kidneys to play such a vital role, they need to be more than passive filters of blood but also active regulators of the final composition of the urine. The filtering components of the kidney are called glomeruli, which are tiny blood vessels intimately located around kidney cells that act to produce the initial urine by forming a filtrate (fluid that has passed through a filter). This filtrate lacks the blood cells and most of the protein found in the blood and is passed next through a series of tubules. Along the tubule, water and salts are added to or removed from the filtrate until the final urine is emptied into the bladder where it is stored until the person voids.

Together, a single glomerulus and its tubule are referred to as a nephron. Each kidney has approximately one million nephrons with a wide range in actual number, which work to filter the blood and produce the urine. How much water and salt (sodium, potassium, calcium, etc.) is added or removed along the tubule is dependent upon the quantity of each in the body, which is dependent on the food ingested. Imagine a person on a desert island with limited access to water. Certainly, his or her urine composition should be drastically different from that of a person who is at a neighborhood bar drinking beer and eating salted peanuts. If the urine compositions were the same, then the person on the desert island would become quickly dehydrated or the person in the bar would swell.

Such consequences do occur in the setting of kidney diseases. The person cannot adequately concentrate their urine (like the person on the desert island) or dilute the urine (like the person in the bar). In addition, salts such as sodium and potassium may accumulate and lead to potentially harmful consequences.

But that's not the end of the story! The healthy kidneys do much more than produce urine and prevent the loss of important substances like protein. They play a vital role in the manufacturing and processing of a number of hormones (chemicals which have specific actions upon different organs of the body). For example, they produce a substance called erythropoietin (Epo), which acts on the bone marrow to produce red blood cells. Without Epo, blood production drops and the person becomes anemic, lethargic and weakened.

The kidneys also produce substances that regulate local blood flow and hence the filtering of blood and urine production. A number of drugs, such as over-the-counter analgesics called NSAIDs (for non-steroidal anti-inflammatory drugs) or certain blood pressure medications, e.g. ACE inhibitors, can affect the actions of these substances. They may result in a decline in blood flow and the making of urine in the susceptible (abnormal functioning) kidney.

Bone disease may arise depending on the severity of kidney disease. Phosphorus, which is present in many foodstuffs we eat, accumulates with kidney disease. Four glands in the neck, called the parathyroid glands, produce a hormone in response to the elevated phosphorus level in the blood to drive the kidney to excrete more phosphorus in the urine. However, the same parathyroid hormone causes calcium to leave the bones, causing the bones to weaken and leaving the person prone to fractures.

Drugs and hormones may be broken down completely or partially and then passed through the kidney to be filtered in the urine. In the setting of kidney disease, changes in the dosing of certain drugs may be called for in order to avoid overdose and side effects. Certain hormones may also occur which can impact on daily metabolism and functioning. For example, insulin, the hormone responsible for the uptake of sugar into cells and which is deficient in diabetes mellitus, requires dose adjustments when taken by the diabetic patient who has kidney disease. Otherwise, there is a chance low blood sugar may arise, which could potentially lead to a coma.

Thus, the kidney has multiple functions in addition to the making of urine. The consequences of severe kidney failure can prove fatal if left untreated. Treatment methods include dialysis and transplantation.

Dr. Bodziak is an Assistant Professor of Medicine in the Division of Nephrology at University Hospitals of Cleveland.

This article originally appeared in the August 2002 issue of Kidney Beginnings: The Magazine, Vol. 1, No. 1.

Back