Kidney stones are hard, solid rocks that form in the urinary tract. They are one of the most painful ailments. In many cases, the stones are very small and can pass out of the body without any problems. But if a stone (even a small one) becomes lodged and blocks the flow of urine, excruciating pain may result, and prompt medical intervention may be needed.
The process of urination begins in the kidneys. The kidneys process fluids and dissolve waste matter to produce urine. The two kidneys are located deep behind the abdomen below the ribs and toward the middle of the back.
- Each kidney contains over a million nephrons. These are the tiny filtration units of the kidney.
- Each nephron is composed of a tiny group of blood vessels (a glomerulus) enclosed in a funnel-like structure called a Bowman's capsule.
- Each glomerulus filters waste products, water, and salts out of the blood fluid (plasma) that has entered the kidney.
- About 1% of the blood fluid is converted into urine. The rest is returned into circulation to prevent dehydration. Urine's is primarily made of acids, urea, and creatine (nitrogen compounds).
- Urine passes from the Bowman's capsule into tiny tubules that lead to large collecting tubes in the center of the kidney. As the urine passes through this network it becomes increasingly concentrated.
- Urine then flows from the kidney through thin tubes called ureters into the bladder.
- The bladder's elastic walls expand to store the incoming urine until it is eliminated from the body via a tube called the urethra.
Click the icon to see an image of the urinary tract.
Types of Kidney Stones
Occasionally, various salts build up on the inner surfaces of the kidney and form crystals. Eventually these crystals become large enough to form stones in the kidney, a condition called nephrolithiasis. Kidney stones (renal calculi) may also form in the ureter or the bladder. The salts that form these stones are made up of combinations of minerals and other chemicals, some of which are derived from a person's diet.
Click the icon to see an image of the kidney stones.
Calcium Stones. About 70 - 80% of all kidney stones are made of calcium, usually combined with oxalate, or oxalic acid. Oxalate is found in a number of common vegetables, fruits, and grains. About 6% of calcium stones are composed of calcium phosphate (called brushite).
Uric Acid Stones. About 7% of stones are made up of uric acid, which is formed from a breakdown in purine, a nitrogen compound found in protein. Uric acid is produced in the liver and enters the bloodstream, where most passes into the kidneys and is eliminated in urine. Often, uric acid stones occur with calcium stones.
Struvite Stones. Struvite stones are made of magnesium ammonium phosphate. They are almost always associated with certain urinary tract infections. Worldwide, they make up 30% of all kidney stones. In the United States, however, less than 15% of all stones are struvite. Most struvite stones occur in women. The rate of these stones may be declining in America, perhaps because of better control of urinary tract infections.
Cystine Stones. About 2% of stones in adults and up to 8% of kidney stones in children are caused by a build-up of the amino acid cystine, a building block of protein. The tendency to form these stones is inherited. Cystine stones are marked by rapid growth and recurrence, which, if not treated promptly, can eventually lead to kidney failure.
Xanthine Stones. Others are composed of xanthine, a nitrogen compound. These stones are extremely uncommon and usually occur as a result of a rare genetic disorder.
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