The kidneys are two bean-shaped organs located below your chest, one on each side of the spine. Their function is to filter waste chemical compounds from the body into the urine. They remove extra water and wastes from the blood, producing urine. Additionally, the kidneys produce hormones and maintain a stable balance of salts in the blood.

Under certain conditions, the chemicals in the urine may form crystals, which combine to form urinary stones. These stones may block the flow of urine and cause severe pain, infection, and affect the function of the kidney. Urinary stones are usually found either in the kidney, the ureters (tubes that direct urine from the kidneys to the bladder), or in the bladder and the urethra.

A kidney stone is composed of crystals formed by chemicals that separate from the urine and build up in the kidney. The most common type of stone contains calcium oxalate or calcium phosphate. A less common type of stone, caused by infection in the urinary tract, is called a struvite stone. Other rarer stones include uric acid stones and cystine stones.

Narrow tubes called ureters carry urine from the kidneys to the bladder, an oval-shaped reservoir for urine in the abdomen.

Kidney stones or urinary stones (also known as urolithiasis or renal calculi) are solid deposits that occur within the urinary system. They occur when urine becomes too concentrated with minerals, which then precipitates into multiple small mineral crystals that can eventually accumulate to form stones. These stones can lead to complications especially if they cause obstruction within the urinary tract. This might then lead to severe back pain, blood in the urine, or even severe urinary tract infections.

There are several types of kidney stones, but the four most predominant include:

Accounts for ~10% of stones. These form when there are high concentrations of uric acid within the urine, which is commonly associated with gout, and can also occur when the urine is too acidic, or with a diet high in purines (red meat, organs, and seafood).

These stones usually occur as a result of urinary tract infections with bacteria that break up urea, resulting in an alkaline urine that leads to precipitation of magnesium, ammonium and phosphate, the stone’s constituent salts.

Less common, and occurs in patients with cystinuria, a genetic condition that causes the kidney to excrete excessive amounts of certain amino acids that eventually crystallise into stones.