Pressure-flow-diameter relationships in isolated perfused thin limb of Henle.

To measure directly the relationships between flow rate, tubule diameter, and flow resistance in thin descending limbs of Henle, isolated tubule segments from the rabbit were studied by in vitro microperfusion. Small increases in pressure and flow cause rapid enlargement of the tubule. Flow resistance is inversely related to tubule diameter and, by its effect on transmural pressure, indirectly limits the extent of tubule dilation. In a range of transmural pressures comparable to that in vivo, the tubule is capable of radial dilations as great as 35% and does not reach its structural limit of distensibility. Flow resistance may be approximately by a Poiseuille equation for cylindrical tubules. The effective luminal diameter is approximately a constant fraction of outer tubule diameter and is defined approximately by the innermost projection of semirigid epithelial nuclei.