The interstitium surrounds the capillaries. It consists of cells that are embedded in a matrix of extracellular biomacromolecules known as the extracellular matrix (ECM; see Section 2.4). Depending on the tissue, the cells can account for a large fraction of interstitial volume or can be relatively few in number. An important task of the resident cells, or at least of some subpopulation of the resident cells, is to synthesize and secrete the extracellular biopolymers that make up the ECM.

Interstitial fluid flow

We have seen in Section 4.4 that there is a continuous transport of fluid from capillary blood into the lymphatic system, via the interstitium. The net flow rate from capillaries to lymphatics is determined by the driving potential (the difference in p - π between the capillaries and the lymphatics), as well as the hydrodynamic resistance of the intervening interstitium. Interstitial flow resistance is therefore very important in fluid homeostasis, or maintaining proper tissue hydration.

Intuition suggests that interstitial flow resistance should depend on tissue structure, or more specifically on:

• the local concentrations of ECM components

• the local number of cells per unit tissue volume

• the manner in which cells and ECM components are packed together.

Cells and ECM components are packed together in a random manner. Because of this random character, it is challenging to describe detailed flow patterns on a cellular or subcellular length scale.