Any fluid inside the chest cavity decreases the amount of space that your lungs have to expand. In order to fill with air when you breathe in, your lungs must be able to fully expand. When you breathe, your lungs should expand smoothly and painlessly within the pleural cavity inside your chest. When your lungs can’t expand properly, you have trouble getting the oxygen you need. This causes you to become short of breath and it may even hurt for you to breathe.