The Urinary Tract
The kidneys maintain the necessary balance of electrolytes and fluids in the bloodstream by filtering all of the blood in the body through microscopic structures called nephrons. Nephrons allow water and some waste products to enter the urine, while retaining cells and large molecules in the blood (NIDDK 2014). The kidneys also help maintain a slightly alkaline blood pH (between 7.37 and 7.43) by excreting excess acid or base (Koeppen 2009; Lewis 2013).
The final product of this filtration process, urine, is collected from the nephrons and eventually drains into narrow tubes called ureters that connect the kidneys to the bladder. Finally, urine leaves the bladder and the body via another narrow tube called the urethra (NIH 2014).
How Kidney Stones Form
Urine becomes more concentrated as it moves through the kidneys and as water is reabsorbed into the bloodstream (Sands 2009). Minerals and organic molecules that are dissolved in the urine can crystallize if the urine becomes highly concentrated. These crystals can stick together and form complexes with other molecules, particularly proteins. These crystalline-protein complexes are the main components of kidney stones (Aggarwal 2013; UMMC 2014).
There are five major types of kidney stones (Aliotta 2015; Preminger 2015; Antonelli 2015):
- Calcium oxalate. From 60‒80% of all kidney stones are entirely calcium oxalate or predominantly calcium oxalate with a smaller percentage of calcium phosphate.
- Calcium phosphate. Pure calcium phosphate stones are less common, accounting for 10‒20% of all kidney stones.
- Uric acid. Uric acid stones account for up to 20% of all kidney stones.
- Struvite. Kidney stones are made of struvite, which is magnesium ammonium phosphate, in 1‒7% of cases. Struvite stones are associated with chronic urinary tract infections involving certain types of bacteria, and are up to three times more common in women.
- Cystine. Stones made of the amino acid cystine are rare in adults and more common in children, representing 1‒3% of all kidney stone cases.
Very small stones can pass through the entire urinary tract without causing symptoms, but larger stones can become lodged in the ureters, bladder, or urethra (NLM 2015; Johns Hopkins 2015). Stones that become lodged in the urinary tract can cause severe pain and block urine flow (UMMC 2013; Singh 2011). Kidney stones are frequently passed without complications, using only conservative treatment such as pain relief and hydration (Aliotta 2015; Antonelli 2015).
Kidney Stones and Chronic Kidney Disease
Kidney stones increase the risk of chronic kidney disease and end-stage renal disease (Frassetto 2011), independently of risk factors shared by stone formers and those with chronic kidney disease such as high blood pressure and diabetes. It is estimated that the risk of chronic kidney disease is twice as high in stone formers compared with non-stone formers (Keddis 2013).
This relationship may be related to stone size and number. In one study, greater cumulative stone size was associated with higher chronic kidney disease risk, suggesting people who either form large stones or many smaller stones are at greater risk (Ahmadi 2015).
Some researchers have proposed that the presence of crystals and stones in kidney tissues can trigger chronic inflammatory processes and scarring that directly contribute to chronic kidney dysfunction and disease (Keddis 2013; Aggarwal 2013).
Given the link between history of kidney stones and chronic kidney disease, individuals who are stone formers should also review the protocol on Kidney Health, which describes many strategies for keeping the kidneys healthy.