- The Kidneys
- The kidneys are fist-sized, bean-shaped organs located near
the middle of the back, just below the rib cage. Their primary function is to filter
impurities and waste products from the bloodstream. Wastes removed by the kidneys are
passed on to the bladder through tubes called ureters, where they are excreted in the form
of urine.
The kidneys also produce three important hormones:
erythropoitein (EPO), which triggers the production of red blood cells in bones; renin,
which regulates blood pressure; and vitamin D, which helps regulate the body's calcium
balance, necessary for healthy bones.
- What Are Kidney Stones?
- For reasons that are not completely understood, some of the
substances in urine, uric acid and calcium in particular, may crystallize within the
kidney, forming rock-like particles (calculi) called stones. The medical term for this
condition is nephrolithiasis or renal stone disease. Kidney stones may be as small as a
grain of sand or larger than a golf ball. Depending on their composition, they may be
smooth, round, jagged, spiky or asymmetrical. Most stones are yellow to brown in color,
although variations in chemical composition can produce stones that are tan, gold or
black.
Some stones stay within the kidney, where they often produce
no symptoms. Other stones may break loose and travel down the urinary tract. The smallest,
smoothest stones may pass out of the body with little resistance and minimal discomfort to
the patient. Larger, irregularly shaped stones cause intense pain. They also can become
lodged in the ureter, bladder or urethra, the tube that carries urine from the bladder out
of the body.
In addition to causing severe pain, a lodged stone can block
the flow of urine, causing wastes to back up into the kidneys. Such a condition must be
corrected swiftly, either by surgically removing the blocking stone or by non surgical
medical procedures that break it up and allow it to pass naturally out of the body. If
not, serious kidney damage and related medical problems can result.
- Who Gets Kidney Stones?
- An estimated 600,000 persons in the United States develop
kidney stones each year; of them about 100,000 are admitted to hospitals for treatment.
Men are roughly four to five times more likely to develop the disease than women. It is
generally estimated that 10% of men and 5% of women age 30 to 50 in this country will
suffer from kidney stones. Most people who develop kidney stones experience their first
episode between the ages of 20 and 30.
The disease usually continues throughout life, particularly
in men. Up to two-thirds of all men who have passed one stone will experience a
recurrence, with an average of nine years between episodes.
Kidney stones appear to be four to five times more common in
whites than in African Americans.
- What are the causes
and risks of kidney stones?
- No one is certain why people do or don't develop kidney
stones. Although the disease appears to run in families, it is unclear whether this is
associated with inherited physiological factors or merely the result of shared family
dietary preferences that may predispose a person to stone formation.
Some researchers suggest the mineral composition of drinking
water plays a factor, although they disagree as to whether "hard" (containing
excessive calcium sulfate) or "soft" (excessive sodium carbonate) water is to
blame. Alcohol consumption also may play a role in stone formation, particularly among
drinkers who suffer from gout, a painful inflammation of the joints resulting from an
imbalance in the body's uric acid metabolism.
Some experts, noting that residents of the southeastern
United States have the highest incidence of kidney stones in the country, claim
temperature and body dehydration are key factors. Others say the South's popular dietary
staples of green vegetables, brewed tea and fatty foods are the cause.
Most scientists agree that eating specific foods by itself
won't causes kidney stones. However, many experts acknowledge that certain diets,
particularly those characterized by high-protein, salt, milk and animal fat intake, may
promote stone formation in people who are already susceptible to the disease.
Super saturation
The most prevalent theory -- super saturation crystallization -- holds that
dehydration causes an imbalance in the liquids and dissolved solids in the urine. The
kidneys must maintain a proper amount of water in the body as they remove harmful waste
materials. If dehydration occurs, the urine may become overloaded (supersaturated) with
substances that will not dissolve in water. These chemicals and trace elements combine to
form crystals which slowly build up, layer upon layer until a stone is formed. Studies
suggest that drinking plenty of water may prevent kidney stones.
Inhibitors
Normally, urine contains chemicals that prevent or inhibit the formation of
crystals. Substances known to act as inhibitors include pyrophosphate, citrate, magnesium,
zinc and macromolecules. Another prevalent theory holds that some persons do not possess
these inhibitors in sufficient quantity to prevent crystallization, or that their
inhibitors somehow fail to produce the necessary chemical reactions that prevent the
formation of crystals. The formation of such crystals is the first stage in the
development of a kidney stone.
Matrix
Another chemical component of urine, a noncrystalline mucoprotein called matrix,
is thought by many experts to play a role in stone formation, although the precise nature
of that role is still unclear. In persons who do not develop stones, matrix seems to act
as an inhibitor. However, among patients who suffer from stones, matrix appears to be an
initiator, and may even provide the chemical framework upon which crystals develop.
- Types of Stones
- Not all kidney stones are alike. Different types of chemical
imbalances in the urine produce stones with different chemical compositions and varying
shapes. The four most common types of stones are those made of calcium salts, struvite,
uric acid and cystine.
1. Calcium Stones
About 70% to 80% of all kidney stones are composed of hard crystals of either
calcium oxalate or insoluble phosphate salt, or a combination of both. Calcium stones are
the most common type experienced by people of Anglo-Saxon descent. They occur in people
who have hypercalciuria, a condition characterized by excessive calcium in the urine.
Calcium is a normal part of a well-balanced diet, responsible for maintaining the health
of teeth and bones. In most people, excess calcium is flushed out by the kidneys and
excreted in the urine. People with hypercalciuria build up excess calcium in their
kidneys, where it joins with other waste products to form a stone.
In about 40% of people who develop calcium stones, this
buildup is caused by an inherited metabolic disorder whose cause is unknown. In rare
cases, a tumor on the parathyroid gland may trigger an overproduction of parathyroid
hormone, the chemical that regulates calcium metabolism. Certain drugs, such as the
diuretic furosemide, antacids and steroids, can produce hypercalciuria. It also can be
brought on by certain intestinal diseases, excessive amounts of vitamin A or D, or a diet
too high in purine, typically associated with meat, fish and poultry consumption.
Calcium oxalate stones also are commonly associated with
having too little vitamin B or too much vitamin C in one's diet.
2. Uric Acid Stones
Uric acid is a normal byproduct made by the body as it breaks down protein. It is normally
flushed out by the kidneys in urine. However, some people, particularly men, build up
excessive uric acid concentrations in their kidneys or joints. In the joints, this can
lead to gout, an inherited disorder of uric acid metabolism with painful arthritic
symptoms. If this buildup occurs in the kidneys as well, it often results in the formation
of uric acid stones.
An estimated 5% to 13% of patients with kidney stone disease,
particularly men, develop uric acid stones. Genetics may play a role in uric acid stone
propensity: Persons of Mediterranean descent, particularly those of Portuguese extraction,
appear to have high incidences of uric acid stones, while those of Anglo-Saxon descent
seem to be less at risk. Patients prone to developing uric acid stones typically are
advised to reduce their consumption of high-protein foods, especially meat.
3. Struvite Stones
Also known as "infection stones," struvite stones account for up to 20%
of all kidney stones. They are made up of crystallized magnesium and ammonia, common
byproducts of excessive acidity in the urine caused by the bacterial breakdown of urea.
This often occurs in persons who experience infections of the urinary system. For this
reason they are most common in women, who suffer more urinary tract infections than men.
Struvite stones typically develop in a jagged or branch-shaped structure called a
"staghorn."
4. Cystine Stones
Cystine is one of the body's chemical building blocks, an amino acid that helps
make up nerves, muscles and other body tissues. A rare genetic defect called cystinuria
can cause excessive cystine buildup in the urine, leading to the development of cystine
stones in the kidneys. Cystine stones are relatively rare, occurring in about 1% to 2% of
persons who experience kidney stone disease. Because it is genetically inherited, the
condition often runs in families.
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