Interstitial Fluid, AKA tissue fluid, works in conjunction with lymph and plasma (the liquid part of your blood) to maintain your body’s internal pressure and make sure your organs and other fluids properly interact. Most interstitial fluids have a specific job and a specific name, like peritoneal fluid, which lubricates everything inside the abdomen, or pleural fluid, which coats the lungs to allow them to do that whole oxygen exchange thing so you can live. And you were giving air all the credit.
Read about the rest and what they do at mental_floss. Link