Kidney stones are abnormal, hard, chemical deposits that form inside your kidneys. Kidney stones are a common problem, with about 10% of Americans expected to pass at least one kidney stone in their lifetime. Stones come in various shapes, colors, and sizes; they can be as small as a grain of sand, the size of peas or marbles, or even as large as your fist.
Kidney stones are not related to gallstones. The two types of stones occur in different parts of the body, and having one does not affect your risk of developing the other.
The kidneys are two bean-shaped organs about the size of an adult’s fist, located below the ribs and toward your back. They filter and clean the blood and manufacture urine Figure 01.
Figure 01. Anatomy of the kidneys
Some of the substances that are filtered by the kidneys tend to form crystals, while others help prevent crystal formation. Normally the kidneys can maintain a balance, but if the system becomes disrupted, the balance may tip toward crystal formation. Crystals will begin to stick together and, over a period of months or years, will form a stone. Substances that can form crystals include calcium, oxalate (oxalic acid), uric acid, and cystine. Those that prevent crystal formation include citrate and magnesium. Most kidney stones consist primarily of calcium.
While small stones often pass out of the body in the urine without causing any pain or discomfort, larger stones can become trapped in your urinary tract and may cause extreme pain, blocked urine flow, urinary infections, or blood in the urine.
Kidney stones may be found in the kidney, the tube that drains the urine from the kidney to the bladder (the ureter), or the bladder. Urinary stones may develop inside the kidneys when minerals and other substances in the urine form crystals that build up over time to form a small, stone-like mass. Stones may stay in the kidney or break loose and travel down the ureter toward the bladder.
A stone that remains in your kidney usually doesn’t cause any problems unless it becomes so large that it blocks the flow of urine or becomes infected. Small stones that become trapped in the ureter can cause urinary tract infection, obstruction of urinary flow or kidney damage. If a large stone becomes stuck in the narrow ureter, it can cause severe pain.
Most stones will pass from the body spontaneously, but several treatment options are available for those that will not pass. Treatment measures, as well as measures to prevent the formation of new stones, depend on the size, location, and type of kidney stone you have. Your family physician may treat this condition or refer you to a urologist, a physician who specializes in the surgical treatment of urinary diseases, or to a nephrologist, who specializes in the medical management of kidney diseases.
The formation of kidney stones is complex, but there are at least two mechanisms that play a role in their development: excess amounts of the substances in the urine that may promote crystal formation and the physical and chemical changes that cause the crystals to turn into stones.
These changes may be caused by diet, drugs, heredity, climate, or other medical conditions. In a small number of cases, the cause of kidney stones cannot be determined.
While certain foods that are high in protein, such as meat, may encourage formation of kidney stones, this mainly occurs in people who are susceptible to this condition. If you are not prone to kidney stones because of genetic factors, medical conditions, or other reasons, it is unlikely that eating any specific food will cause you to develop kidney stones.
There are four major types of kidney stones, and each has a different cause. Stones are classified according to their chemical composition.
Approximately 75% to 85% of all kidney stones are calcium stones. These stones are made up of calcium and oxalate or calcium and phosphate and may result from having too much of either of those substances in your urine. High calcium concentrations can result from ingestion of large quantities of vitamin C, excess animal protein or excess salt. Not enough dietary calcium or potassium can increase the risk of calcium stones. Low urine volume and low fluid intake along with such medical problems as gout, diabetes, obesity and the other endocrine problems can increase the risk of calcium stones.
Struvite stones usually result from urinary infections caused by a particular bacterium and are found mainly in women. They account for 10% to 15% of all cases of kidney stones. Struvite stones can also develop in patients who have long-term bladder catheters.
Uric acid stones are caused by having high levels of uric acid in your urine, which may happen if you eat a lot of meat, if you have gout, or if you are receiving chemotherapy. In addition, uric acid stones may form if the acid level in the urine is high. Uric acid is a byproduct of protein metabolism. Uric acid stones are much less common than calcium stones, accounting for only 5% to 10% of all kidney stones.
Cystine stones are the rarest type of kidney stones, affecting 1% to 3% of patients. They are made up of cystine, an amino acid. The buildup of cystine in the urine that leads to cystine stones is caused by a rare genetic disorder.
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