The kidney acts to filter the blood, and removes excess water and wastes. If these wastes become too concentrated or altered in some way, they can precipitate out and form a crystal that slowly grows into a kidney stone (renal calculus or nephrolithiasis). Most stones are flushed down the ureter (the tube that leads to the bladder from the kidney) and are passed out with the urine while still very small. A small number of stones slowly grow in size until they are the size of a grape or larger and completely fill the urine collection chamber of the kidney where they cause repeated kidney infections and pain. Medium sized stones from one to five millimetres in length enter the thin very sensitive ureter, and as the stone is pushed along the tube by the pressure of urine behind, it scrapes the tube wall to cause intense pain (renal or ureteric colic) that can thus come and go for several days every time the stone moves. Kidney stones are more common in men than women, and in hot climates than cold due to the higher incidence of dehydration.
Kidney stones are collected by sieving urine after symptoms of kidney stone present, or analysis of gravel in urine. Analysis of the stone may allow the cause of its formation to be determined. The following substances may form a kidney stone:-
- CALCIUM OXALATE – 40% sole ingredient, 85% partial ingredient. Causes include excess soft drinks, oxalate foods (eg. silverbeet, rhubarb, chocolate, nuts) but often no identifiable cause.
- CALCIUM PHOSPHATE – 2% sole ingredient, 35% partial ingredient. Causes include over active parathyroid gland in the neck (hyperparathyroidism), excess calcium in the blood (hypercalcaemia), kidney disease (distal renal tubular acidosis), or they may be no specific cause.
- URIC ACID – 10% sole ingredient, 30% partial ingredient. Causes include kidney failure and excess urate in blood (hyperuricaemia), or excessively acid urine (aciduria).
- CYSTINE – Rare. Usually an inherited characteristic (familial amino aciduria).
- MAGNESIUM AMMONIUM PHOSPHATE – 2% of stones. Caused by long-term urinary infection.