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An air-displacement pipette uses a piston to change the pressure and volume of an air cushion above a liquid sample. On piston withdrawal, reduced pressure allows ambient pressure to drive liquid into the tip; subsequent piston motion expels it. The piston itself remains separated from the sample.^[ref]

The air cushion explains why a mechanical setting alone does not establish the delivered volume. Its response can depend on temperature and sample properties, including volatility and density. A positive-displacement pipette uses direct piston contact instead of that intervening gas volume.^[ref]

For an original repeated-delivery example, compare sets of $100\mu \mathrm{L}$ settings with measured mean deliveries of $98\mu \mathrm{L}$ and $100\mu \mathrm{L}$. Knowing those means alone does not identify which set has less scatter. Both the mean error and a dispersion measure such as standard deviation are needed to describe performance. A microlitre is an exact unit relationship; an individual micropipette delivery is an experimental measurement.

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.^[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 isassuming no leak or accumulation. Hypothetical flows of $20$ and $18nL/min$ through a $2\mathrm{mm}$ segment give $J_v=1\mathrm{nL}\mathrm{mi}{\mathrm{n}}^{-1}\mathrm{m}{\mathrm{m}}^{-1}$. 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.

A laboratory micropipette is a pipette for transferring small measured quantities of liquid, commonly expressed in microlitres. In piston-operated models, the displacement mechanism matters. An air-displacement pipette leaves an air cushion between piston and sample, whereas a positive-displacement pipette places the piston directly against the liquid.^[ref]

Weighing a delivered sample provides a concrete check. In a simplified calculation that neglects air buoyancy and evaporation, liquid volume follows from mass $m$ and density $\rho$:For an assumed $\rho =1.000mg/\mu \mathrm{L}$, a delivered mass of $99.5\mathrm{mg}$ corresponds to $99.5\mu \mathrm{L}$. Relative to a nominal $100\mu \mathrm{L}$ setting, the deviation is $-0.5\%$. These are illustrative values, not calibration data.

A repeated series can distinguish a displaced mean from wide scatter: measurement accuracy and standard deviation describe different aspects of performance. Actual gravimetric checks account for temperature-dependent water density, air buoyancy and evaporation losses. The manufacturer's method uses a conversion factor and an evaporation correction, showing why a displayed volume alone does not establish the quantity delivered.^[ref]

Renal micropuncture is an experimental method for accessing tiny fluid compartments in a living kidney with a fine micropipette, usually under microscopy. It can sample a particular renal tubule or vessel instead of measuring only the final mixed urine. The sampling location turns a whole-organ observation into a spatial test of renal function.

In 1959, Carl W. Gottschalk and Margaret Mylle collected nanolitre-scale samples in oil-filled glass micropipettes and measured osmolality using freezing-point depression. Comparing loop bends, collecting ducts and renal vasa recta at corresponding medullary levels tested the countercurrent multiplication hypothesis directly. Position mattered as much as the concentration measurement.^[ref]

An ideal freely filtered marker provides a quantitative example. Let its molar concentration in blood plasma be $C_p$, its tubular concentration at a sampled site be $C_t$, the initial single-nephron filtration flow be $Q_f$, and the remaining tubular flow be $Q_t$. At steady state, with no tubular gain or loss of marker, mass balance givesIf the hypothetical measured ratio is $C_t/C_p=4$, one quarter of the initial fluid flow remains and three quarters has been reabsorbed upstream. The marker became concentrated without needing to be added along the tubule.

This interpretation depends on the marker assumptions and faithful sampling. Leakage, changing flow, evaporation or an incorrectly identified sampling site can change the answer. A final urine concentration alone cannot establish where along the nephron the inferred transport occurred.