Hypovolemia is when a person doesn't have enough fluid in their body. Think of your body like a water balloon - when it's full of water, it's big and bouncy. But if you take some of the water out, the balloon gets smaller and can't bounce as well.
Your body needs fluid to help your blood move around and bring oxygen and nutrients to all your organs and muscles. If you don't have enough fluid, your body can't work as well, and you might feel tired, dizzy or thirsty. It's like your body is running on empty.
There are lots of things that can cause hypovolemia, like not drinking enough water, sweating a lot, vomiting or diarrhea. Sometimes, people who have lost a lot of blood because of an injury or illness can have hypovolemia too.
It's important to drink enough water and fluids to keep your body healthy and happy. And if you ever feel dizzy or tired, make sure to let an adult know so they can help you feel better!