= Isolated tubule perfusion
<Isolated tubule perfusion> is an experimental technique in which a dissected <renal tubule> segment is supplied internally with a controlled solution while a separate bath contacts its outside. Fine <micropipettes> deliver and collect tubular fluid. Keeping luminal and external conditions separate lets investigators test transport across the <epithelium> of one identified segment.
For example, isolated rabbit <thin descending limb> segments were perfused in a 1970 study while bath <osmolality> was changed. Concentration of the collected fluid increased mainly through water leaving, rather than substantial salt entering. The apparatus converted a question about whole-kidney concentration into a direct test of one segment's <membrane permeability>.<Sodium chloride and water transport in the descending limb of Henle by Juha P. Kokko>[Abstract and Methods: isolated-segment perfusion and controlled bath gradients]{ref}
A simple <mass balance> estimates net water transport. If inflow and collected outflow are $Q_{\mathrm{in}}$ and $Q_{\mathrm{out}}$ for a segment of length $L$, then outward volume transport per unit length is
$$
J_v=\frac{Q_{\mathrm{in}}-Q_{\mathrm{out}}}{L},
$$
assuming no leak or accumulation. Hypothetical flows of $20$ and $18\,\mathrm{nL/min}$ through a $2\,\mathrm{mm}$ segment give $J_v=1\,\mathrm{nL\,min^{-1}\,mm^{-1}}$. A solute requires a separate balance involving concentration as well as flow.
Isolation provides experimental control while removing the segment from its normal <blood> supply, neighboring cells and systemic signals. It therefore complements <renal micropuncture> in an intact kidney: neither preparation alone supplies every part of the physiological explanation.
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