Extracellular space refers to the part of a multicellular organism outside the cells, usually taken to be outside the plasma membranes, and occupied by fluid.
According to the Gene Ontology, the extracellular space is a cellular component defined as: "That part of a multicellular organism outside the cells proper, usually taken to be outside the plasma membranes, and occupied by fluid.
Gene products from a multi-cellular organism that are secreted from a cell into the interstitial fluid or blood can therefore be annotated to this term".
[1] The composition of the extracellular space includes metabolites, ions, various proteins and non-protein substances (e.g. DNA, RNA, lipids, microbial products etc.
For example, hormones, growth factors, cytokines and chemokines act by travelling the extracellular space towards biochemical receptors on cells.