Crash course in physiology: in order to absorb fat (which isn’t water soluble – if you don’t believe me, just dump some oil in water), your body has to produce bile salts, which are cholesterol-based. Your liver produces bile, but because you may need more of it for a given meal than the liver can produce in one sitting, your gallbladder hangs off your liver and acts as a receptacle for the extra bile (just in case you have a supersized fast food meal or something. Which I don’t recommend, for the record).
English: Opened gall bladder containing numerous gallstones Deutsch: Geöffnete Gallenblase mit zahlreichen Gallensteinen (Photo credit: Wikipedia)
Bile ought to be liquid, but too much of any given constituent in the bile will make it precipitate out of solution and form stones. (If you add sugar to water by the tablespoon, for instance, eventually you’ll add so much that the water is saturated, and the sugar will just collect at the bottom of the glass in solid form again. Same thing.)
Gallstones come in several varieties. By far the most common type are cholesterol stones. The other relatively common type are those made of bilirubin (breakdown products of red blood cells), and these form when red blood cells are being destroyed (hemolytic anemia) or when the liver isn’t doing its job (as in cases of cirrhosis). Sometimes gallstones can even form as a result of local bacterial infections.
Traditionally recognized risk factors include rapid weight loss, pregnancy (this slows down gallbladder emptying), liver or bile duct infections, diabetes, hemolytic anemia (again, that’s destruction of red blood cells), estrogen (including hormonal birth control), certain ethnic descents (Hispanic, Native American, and fair-skinned Europeans), and age – gallstones are more common over 40.
Unless they’re causing an obstruction, gallstones are frequently not associated with any symptoms, and may be found incidentally on imaging.