What Are Kidney Stones?

There are several different types of kidney stones, but each of them can cause a lot of pain.

Kidney stones are small chunks of solid material that can form in your kidneys, a pair of organs that filter your blood.

The "stones," which are usually yellow and brown, vary in size and shape.

For instance, some may be jagged and as small as a grain of sand, while others may be lumpy and the size of golf balls.

A stone may stay in the kidney or travel down the urinary tract — the body's waste and excess-water drainage system — and get stuck, causing severe pain in the belly or side of the back.

Other symptoms may include nausea, chills, and blood in the urine.

Prevalence and Demographics of Kidney Stones

Kidney stones are one of the most common disorders of the urinary tract, resulting in more than a million visits to health care providers and 300,000 emergency room visits each year in the United States, according to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK).

Kidney stones affect both men and women, though struvite stones are more common in women and uric acid stones are more common in men.

Overall, however, the prevalence of kidney stones is higher in men than women.

Kidney stones are also more common in obese people than non-obese people, and less common in non-Hispanic African and Mexican-Americans than in non-Hispanic Caucasians, according to the European Urology study.

What Are the Kidneys?

Part of the urinary system, your two kidneys are fist-sized, bean-shaped organs, located just below the rib cage, one on each side of the spine.

They have a number of important functions, mainly filtering the blood to remove waste and excess water, resulting in the formation of urine, which is stored in the bladder and emptied from the body through the urethra.

The kidneys also:

• Balance the body's levels of electrolytes, including sodium, potassium, and phosphate, to maintain the body's balance of acids and bases
• Produce hormones involved in regulating blood pressure, producing red blood cells, and maintaining bone strength
• Prevent the build up of waste and fluid in the body

Development of Kidney Stones

Kidney stones develop when the concentration of normal kidney substances (especially calcium, oxalate, and phosphorus) increases substantially.

This process — sometimes known as nephrolithiasis — can be due to various factors, including low fluid intake, diet, or medications such as diuretics and calcium-based antacids.

A number of issues can increase a person's risk of developing kidney stones, including:

• A family history of kidney stones
• Medical conditions that affect the levels of urinary substances
• Urinary tract blockage
• Digestive problems
• Recurrent urinary tract infections

Types of Kidney Stones

There are four main types of kidney stones: calcium, uric acid, struvite, and cystine stones.

Calcium stones, of which there are two forms — calcium oxalate and calcium phosphate — are the most common type of kidney stone.

In most cases, calcium oxalate stones form from high levels of calcium and oxalate in urine.

But if there are high levels of urine calcium and the urine is alkaline (has a high pH), calcium phosphate stones may form instead.

Uric acid stones develop from overly acidic (low pH) urine.

This can result from a diet high in purines, substances that are broken down to form uric acid and are found in high concentrations in animal protein.

Struvite stones, sometimes called infection stones, are made of magnesium, ammonium, and phosphate, and typically form in alkaline urine.

They develop from upper urinary tract infections, including kidney infections, when bacteria produce urease, an enzyme that helps convert urea (a compound in urine) into ammonia and other products.

Cystine stones result from a genetic disorder that causes cystine, an amino acid, to leak into the urine from the kidneys.