To distend the bladders, we used varying pressure head, by varying the height of the liquid reservoir coupled to the bladder via the urethra [Fig. 2(a)]. To ensure that the polarimetric signal was coming from the illuminated spot on the bladder wall, we had to minimize the reflection and scattering from sidewalls and distal portions of the bladder wall; to do so, the liquid in the reservoir-bladder system was a mixture of crimson blue dye dissolved in ethanol (to prevent contributions from bladder regions other than the illuminated region) and oil (to prevent the dye from penetrating into the bladder wall and thus altering the wall tissue optics). This mixture has been optimized empirically, ensuring before the bladder wall begins to absorb the dye. For this reason and also to make sure that we are testing the bladders in their fresh condition, all imaging measurements were performed within 1 h. The density of the liquid mixture was measured to be . The pressure at the top of the bladder (at the urethra) was then determined according to , where is the oil-dye density, is the reservoir height and is the acceleration due to gravity. Three bladders (referred to as 1, 2, and 3 in the text) were distended at three different reservoir heights of 15, 30, and 45 cm, corresponding to pressures of 1.0, 2.2, and 3.3 kPa; this range of pressures in within the relevant physiologic range.7 With this method, we could image bladder 1 distended at 1.0 kPa, bladder 2 at 2.2 kPa and bladder 3 at 3.3 kPa. This was done instead of distending a single bladder to three different pressures, to avoid any possible hysteresis effects.