Surface tension properties of human urine: relationship with bile salt concentration

The surface tension of urine has been simply and rapidly measured using two methods, the DuNouy ring detachment method and the Wilhelmy blade immersion method. The methods agree, the correlation coefficient, r, was 0.992. The effect of ageing of the surface, of storage, of temperature, pH, dilution and albumin on urine surface tension are described. Volume corrected 24 hour urine surface tensions from 6 volunteers over 12 or more days showed variations between individuals of mean surface tensions of 6 mN/m (S.D. +/- 2.16 mN/m). This suggests individual variation in excretion of surface active agents, probably bile salts. This was confirmed by further studies which showed that: 1. Surface tension of urines and rediluted extracts were not significantly different when amphiphilic and hydrophobic solutes including bile salts were extracted from urine and subsequently rediluted in water of the same volume. 2. Bile salt concentration in urine measured by RIA and by enzymatic spectrofluorometric methods correlated well with urine surface tension, r = -0.91, and r = -0.60. 3. Molecular surface area for urinary surfactant was 79 A2 similar to pure conjugated bile salt solutions calculated from dilution studies. We conclude that the main determinant of urine surface tension is bile salt concentration.