Kidney Function and Solubility
According to the National Institute of Diabetes and Digestive and Kidney Disorders (NIDDK), the main role of the urinary system is to maintain the body's electrolyte balance and to filter out waste products. This means that the kidneys filter the blood and concentrate this filtrate into urine, which is a mixture of different substances (mostly electrically charged molecules called ions) dissolved in water. For any sort of ion there is a maximum concentration at which it can stay dissolved (also known as its solubility). If too much of an ion is in the urine, it will no longer be dissolved and instead will form a solid.
Kidney Stone Types
Different types of kidney stones can be caused by many things. As the Mayo Clinic notes, sometimes infections of the urinary tract can lead to kidney stones being formed out of a substance called struvite. Calcium stones, on the other hand, can be the result of high calcium levels in the blood or elevated levels of another substance called oxalate, which can combine with calcium to form calcium oxalate stones. Uric acid stones can result from dehydration as the kidneys then have less fluid with which to dissolve that ion. Finally, some people have a genetic disorder that causes their kidneys to secrete abnormally high amounts of an amino acid called cystine, which can also form kidney stones.
Kidney Stone Enlargement
In many cases, the kidney stones that do form are small enough that they are able to pass through the ureters (the tubes that connect the kidneys to the bladder) and are expelled along with the rest of the urine. However, if the stone cannot pass through the ureter or gets caught in the kidney, it serves as a "seed" for future stones. This means that other dissolved substances will preferentially solidify on the already formed stone, causing it to grow. When this happens, it can cause damage to the surrounding tissue or can cause a urine blockage.
