Interstitial fluid

Interstitial fluid is the extracellular fluid between the body's cells, outside the circulating blood plasma. It provides the immediate fluid environment through which many dissolved substances move between capillaries and cells. Fluid entering lymphatic vessels becomes lymph, so exchange with blood and drainage through the lymphatic system jointly influence its volume.^[ref]

A simplified volume balance iswhere $J_v$ is net filtration into the interstitial compartment and $J_L$ is lymphatic outflow. In an illustrative compartment, inflow and outflow of $2\mathrm{mL}\mathrm{mi}{\mathrm{n}}^{-1}$ leave volume steady even though fluid continues moving. If inflow becomes $3$ while outflow stays $2$, the compartment gains $1\mathrm{mL}$ each minute. A steady volume therefore does not imply an absence of exchange.

The barrier's microscopic structure also matters. In 2004 experiments, researchers perfused rat microvessels with albumin solution and adjusted albumin around the vessel. A substantial effective oncotic pressure still opposed filtration even when bulk tissue and vessel concentrations matched. Imaging and modeling supported a protected concentration region near the endothelial glycocalyx. The result shows why bulk interstitial fluid composition alone does not fully specify the forces across the vessel wall.^[ref]