The body does a great job balancing itself when it is healthy. When a body part gets weaker, your system tries to compensate for it. In this lesson, liquid is used because the body has water in many forms including blood, lymph, digestive juices, saliva, etc.
The body is made of mostly water and has several ways to store, circulate, and regulate it.
The heart pumps blood, detects volume of liquid, and sends hormones for adjustment.
The kidneys filter blood, detects oxygenation, takes and makes hormones, and dumps out extra liquid. That liquid then gets eliminated as urine.
SPACES
The liquid is stored and moved in different types of spaces.
First space: Fluid is where it is supposed to be, like inside the cells, blood vessels, organs, and tissues.
Second space: Extra fluid in and around cells. Too much is also known as edema or swelling. You can do a quick check by pressing on a normally bony area to see if it leaves a dent. There are many causes, some are normal and some are caused by imbalance.
Third space: Fluid trapped in between tissues or organs. Examples are fluid around the lungs, abdomen, or even blisters. People with heart failure experience “wet lungs” (pleural effusion) with difficulty breathing until they can urinate out the extra water.
I hope this helps you to understand why a doctor might give you diuretics or give a fluid restriction. The better you understand, the better you can advocate for yourself.
Chronic Living Blog 
I am always interested in your ideas of topics. Please share them!