by Steve Christensen

Created on: April 29, 2008

I've seen my share of kidney stones. Whenever I have examined one as it nestled in a coffee filter, or lay in the bottom of a baby food jar, or rested upon a urine strainer, I have marveled that such a small fragment of grit can cause so much misery. One of my patients - a woman who had endured both the travails of childbirth and the agony of a kidney stone - told me that she'd opt for childbirth any day.

"At least you get to take something home from the hospital," she quipped.

In order to comprehend the symptoms that are caused by passing a kidney stone, it helps to have a basic understanding of renal anatomy. Normally, each of us has two kidneys, one on each side of our mid-back, just beneath the lower ribs. The kidneys are responsible for filtering salts, toxins, and byproducts of metabolism from our bloodstream. During this process, they reabsorb a significant amount of water and other substances that the body can reuse. The waste product (urine) gathers in a "funnel" that is attached to each kidney; from there it drains through a narrow, elastic tube (the ureter) and eventually empties into the urinary bladder, which is located low in the pelvis, just behind the pubic bone. Along the ureter's course toward the pelvis, it passes behind, over, or through several regions where it gets slightly pinched. These constrictions are important in the evolution of symptoms that occur as a stone makes its way from the kidney to the bladder.

Because urine is a concentrated solution, and because there are tiny impurities suspended in this solution, conditions occasionally arise that lead to the formation of a stone, much like a hailstone forms in a cloud or - for those of us who made the stuff as kids - rock candy forms around a string in a solution of sugar water. Most kidney stones form in the funnel at the top of the ureter; some can remain there for a long time without causing any symptoms. However, once a stone drops into the ureter and begins its journey toward the bladder, most kidney stone sufferers wish they could trade places with just about anyone or anything.

The hallmark of a kidney stone (also called nephrolithiasis) is pain: acute, severe, disabling pain. This discomfort often comes in waves, as the ureter contracts in spasms due to the irritation of the stone rubbing against its walls. Depending on the size of the stone, at each point where the ureter is narrowed patients may experience a crescendo and peak in their pain, after which the misery abates somewhat

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