When you’re dehydrated, you have less water in your blood so the blood becomes more concentrated and blood volume decreases. Your kidney is used to filtering all the blood, so it can feel when you have less blood to filter, and it releases hormones that help increase your blood pressure and decrease the amount of water in your pee. A special part of your brain samples your blood and can sense both the kidney hormones and the increased blood concentration. This brain part sends signals to the area responsible for thirst, turning the thirsty feeling on.