Extract

Well, the spaces between the cells ... this milieu interieur … are filled with fluid. And this could be said to be the internal environment ...the immediate surroundings of the cells. The bathing fluid has a number of names.  
It's sometimes called intercellular fluid ...sometimes it´s called  interstitial fluid and sometimes it´s called tissue fluid. And this fluid provides the cells with the medium in which they live ...
In other words this is the organism's internal environment ...and it´s this internal environment, this interstitial fluid or call it what you will that has to be kept constant if the cells are going to continue with their vital functions ...
Similarly, intercellular fluids are found in invertebrates although the pumping diagrams, flow diagrams, pumping mechanisms may be more or less different.
Plants, too, are, in fact have intercellular fluids - as   I´ve already mentioned but here´s much more reliance on osmosis and diffusion.
source:  http://www.uefap.co.uk/listen/exercise/biology/biolfram.htm