interstitial fluid
This term is a technical biological descriptor used primarily in medical, anatomical, and physiological contexts. It describes the specific environment where cellular exchange occurs, distinguishing it from intracellular fluid (inside cells) and plasma (inside blood vessels).
Because it refers to a substance or a medium, it is treated as an uncountable mass noun. It is almost never pluralized, as it describes a general biological component rather than individual units.
Meanings
The liquid that fills the narrow spaces between cells in multicellular organisms, acting as a medium for the exchange of nutrients and waste between blood capillaries and tissues.
The movement of oxygen from the blood into the interstitial fluid allows it to reach the surrounding cells.