Kidney stones form in the renal pelvis, where the plumbing diameter is pretty big. If they come loose, they have to pass through the ureter, where the plumbing space is pretty small. Probability of passage is related to size of stone, as described here . Kidney stones are shaped like broken glass, and poke holes in the ureter, causing some of the pain. The real pain-pain worse than most heart attacks, 10 on the scale of 10-is caused by the stone blocking the ureter, which spasms and causes renal colic. Conservative treatment involves pain management and iv fluids, which relax the patient and maybe the ureter, and help flush the stone through. If the stone doesn't flush the resulting pain can be incapacitating, and resulting damage to the kidney can be severe. All the stuff suggested to decrease the likelihood of stone formation is good information. The IV fluids and parenteral analgesics and antinausea drugs needed to do this probably don't belong in a first aid kit, tho I have seen them there. An obstructed ureter is a surgical emergency, and toughing it out is not an option. Cat scans can identify the number and size of the stones that are sitting in your kidney, and an obliging doc might prescribe one for you to help assess the risk before a major trip. Otherwise, this would be a good time for medevac. A kidney is a terrible thing to waste.