"Q - Why Do Kidney Stones Hurt So Much?
A - A kidney stone attack is described as the most painful experience possible to live through. It's more painful than gunshot wounds, major surgery, broken bones, burns and even childbirth. The pain is unrelated to the size of the stone and is not caused by the stone "moving" or scratching as many people believe. In fact, the pain is caused by the dilating or stretching of the urinary system caused by the blockage the stone produces when it gets stuck in the ureter. (The ureter is the tube that drains urine from the kidneys into the urinary bladder). When the urine that the kidney produces cannot pass the blockage, the ureter and urinary system stretches. This stretching is what causes the intense pain. (The same process causes the pain from intestinal gas that we all get from time to time). This also explains why the stones don't cause pain when they are inside the kidney. Since they don't produce any blockage, stretching or dilating of the urinary system, they don't usually produce any pain until they pass out of the kidney and get stuck. The degree of pain is unrelated to the size of the stone which is why it is possible to have excruciating pain from a stone smaller than a grain of rice. "
http://www.kidneystonesbook.net/author.html
"It's the 50th time your wife has told "The Story." In graphic detail she relates the agonizing moments of her ten-hour labor. It's a harrowing tale. Her badge of honor. Childbirth, she says, is an experience in pain that mere men could never relate to.
Not so fast.
Imagine, if you must, a pea-sized rock traveling ever so slowly through your kidney and then down the tube connecting your kidney to your bladder. Each time the pebble makes any progress, the tube - called the urethra - gets gouged, resulting in a sharp, excruciating, widespread ache. All the while, your urine is backing up.
Not in a sweat yet? Then picture the stone slowly pushing its way down your urethra - yes, the tube that goes through you-know-what - until it is finally, and this could be a whole month later, shot from your body during urination. Now that's pain - pain that can cause nausea and vomiting.
This instrument of torture is called a kidney stone, and about 15% of men will get one at some point in their lives. They are, unquestionably, the kingpins of internal ache. It's the worst pain a man can have. Women who've had them say they'd rather go through natural childbirth again than have another kidney stone. It feels like a knife that's being twisted inside you.
Kidney stones hurt, but they're often not serious. As long as there isn't an infection, you can wait weeks or even months for a small stone to pass on its own."
http://menstuff.org/issues/byissue/kidneys.html