Interstitial fluid is a solution which bathes and surrounds the cells of multicellular animals. It is the main component of the extracellular fluid, which also includes plasma and transcellular fluid.
On average, a person has about 11 litres (2.4 imperial gallons) of interstitial fluid providing the cells of the body with nutrients and a means of waste removal. (from wikipedia)

I believe I have stated before how much I love Google. Well I'll say it again. I LOVE Google!! I could not for the life of me remember what the difference between interstitial fluid and extracellular was. There was a big discussion at work and it was decided that I would go search google for the answer. well surprise they're the exact same thing! grrr. All this because I'm reading the text book Anatomy and Physiology for Midwives for fun. It's actually an awesome book. All full of uteruses (uteri?) and placenta and cellular functions. woo hoo! okay so I'm a dork .... no telling!