Skip to content
Explore atlas
PH-FD15

How stones form

Why substances dissolved in urine, bile and saliva can turn into stones: supersaturation, crystal nucleation, growth and retention, the main types of stone, and what a stone does to the duct or organ that holds it.

By the end, you can

  • Explain how a fluid stays clear although it carries salts near or above their solubility.
  • Describe the four steps of stone formation: supersaturation, nucleation, growth and retention.
  • Relate the main urinary stones (calcium, uric acid, struvite, cystine) to urine volume, pH, solutes and infection.
  • Explain how cholesterol and pigment gallstones form.
  • Relate stones in different sites to obstruction, infection, pressure damage and chronic irritation.
  • Recognise the main stone types by composition and appearance and the tests that identify them, and explain how fluids, thiazides, urine alkalinisation, allopurinol and stone removal act on the steps of stone formation.

What it is

A stone (calculus) is a hard mass that forms inside a duct, hollow organ or cavity from substances normally dissolved in a body fluid such as urine, bile, saliva or pancreatic juice; stone disease is called lithiasis. A stone forms when the fluid becomes supersaturated, crystals nucleate, often on a nidus, grow and aggregate in an organic matrix, and are retained long enough to enlarge.

3 types to tell apart
Urinary stones
Calcium oxalate and phosphate (most), uric acid, struvite (infection) and cystine (inherited); formed in the kidney, sometimes in the bladder.
Gallstones
Cholesterol stones (most in Europe and North America), black pigment stones (haemolysis, cirrhosis) and brown pigment stones (infected bile ducts).
Other sites
Salivary glands (mostly submandibular), pancreatic ducts, prostate, appendix (appendicolith) and teeth (dental calculus).

Normal function

Fluids that carry more than they should hold

The excrete calcium, oxalate, phosphate and uric acid in a small volume of water, so urine often holds them at concentrations above what water alone could keep dissolved. Urine stays clear because it also carries inhibitors of crystallisation, such as citrate, magnesium, pyrophosphate and urinary proteins, and because it keeps flowing from the down the to the , washing out any small crystals.

Bile holds cholesterol, which is insoluble in water, in mixed micelles with bile salts and phospholipid. The gallbladder concentrates bile between meals and empties it after them, so bile rarely stands still for long.

Every fluid that forms stones depends on the same balance: how much solute it carries, how much water dissolves it, its pH, its inhibitors, and how freely it flows.

Point to it on the model