Why is lying on a cold floor bad for your kidneys?

Please be specific.

if you really get cold through and through, then your system is prone to geting infections. This happens in medieval stone dungeons.

you also lose heat from your back being on the floor. if you cover the area around your kidneys you lose less heat. its the heat loss that is bad for you. it seems that you are protecting your kidneys but you are just preventing heat loss.

People noticed that their kidneys are working more when they are lying on a cold floor, so they figure that the kidneys are being stressed. This is probably a misconception.

if the person is cold for days on end and startved like in a dungeon they get sick and a kidney infection is one possible infection that can kill them.

also when you are in the cold your blood vessels in you skin clamp down and more blood gets into the "central circulation" that feeds the body organs like the kidneys. the more blood going to the kidneys the more you pee.

liying down also gets more blood flowing to your kidneys and being cold gets your adrenaline up and your heart pumps harder for a while and your kidneys get even more blood.