= 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>.<Anatomy and Physiology 2e by J. Gordon Betts and others>[sections26.1, fluid compartments, and21.1, lymphatic drainage]{ref}
A simplified volume balance is
$$
\frac{dV_i}{dt}=J_v-J_L,
$$
where $J_v$ is net filtration into the interstitial compartment and $J_L$ is lymphatic outflow. In an illustrative compartment, inflow and outflow of $2\,\mathrm{mL\,min^{-1}}$ 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.<Oncotic pressures opposing filtration across non-fenestrated rat microvessels by R. H. Adamson and others>[abstract, matched albumin concentrations and microvascular barrier measurements]{ref}
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