The Hebrew word sometimes translated as reins is elsewhere translated as kidneys. This is another usage of the word in modern language, and it's the origin of the word renal.
Kidneys are Reins on our Heart
June 16th
The kidneys, referred to in the Bible as reins, regulate our physiology. They filter out the spirits which God sends to us as messengers. They’re a way in which God has put a bridle on us, by which we can be steered and controlled.