During the course of a study of the biological changes in old age, the renal excretion of surface active substances was found to be considerably below the levels seen in younger age groups. These findings were considered important since (1) they were observed in individuals with no evidence of significant renal impairment, (2) the same decrease was observed in all subjects examined, and (3) information on this subject is lacking in the absence of a suitable analytic method.

The capillary device has made it possible to determine surface tension within a few seconds, as part of routine urinalysis, without need for complicated apparatus or involved formulas.

A group of 23 inmates of an old age institution were studied. Ages of these 14 men and 9 women ranged from 70 to 87 years. All appeared in good general condition and showed no evidence of significant renal pathology. Morning urine specimens were obtained from each patient for several weeks. These were examined within several hours after voiding.

Distribution of urinary surface tension values in the group of old people studied

Fig. 260. Distribution of urinary surface tension values in the group of old people studied.

Surface tension values for the entire group averaged 70.9 dynes/cm. The average value for the women was 71.5 and, for the males, 70.4 dynes/cm. When surface tension values were determined in serial samples from individual subjects, variations of only 2 to 6 dynes/cm. were found. None of the samples from the women showed a surface tension below 69 dynes/cm. All but 3 of the specimens from the men had surface tension values of 68 dynes/cm. or higher. (Fig. 260) Specific gravity varied from 1.006 to 1.030, with an average of 1.017 for the group.

In control groups, the average surface tension is 67 to 68 dynes/cm. Diurnal and day to day variations of 5 to 11 dynes/cm., with values ranging between 72 to 62 dynes/cm., are found characteristically in healthy subjects.

It is evident that the pattern of excretion of surface active material in old age is quite different. This is indicated by (1) the high average surface tension value of 71 dynes/cm. for the group, which is at least 3 dynes/cm. higher than for the controls; (2) the limited day to day range of variations, which in no case was greater than 6 dynes/cm.; and (3) the striking disparity in the distribution of values in healthy young adults and in old age. (Fig. 261) Eighty eight percent of the urine specimens from old individuals had surface tension values of 70 dynes/cm. or higher, and only 1 percent were below 66 dynes/cm. In contrast, 23 percent of the specimens from the control group had a surface tension of 70 dynes/cm. or higher, and 23 percent were below 66 dynes/cm. These findings indicate that the amount of surface active substances excreted in the urine of old people of 70 to 87 years is greatly reduced as compared with that in healthy young adults. The fact that specific gravity values for the old age group were not significantly different from the controls indicates that this change cannot be explained on the basis of a failure of the kidneys to concentrate by reabsorbing water.

The average surface tension value for the females was 1 dyne/cm. higher than for males. The same sex difference has been noted in studies of healthy young adults and in groups of patients with a variety of diseases. It is of interest to find the difference persisting in these old people.

In the light of current concepts of the possible role of surface active "colloidal" substances in the urine, it is interesting to speculate on the possible significance of the observed decreased excretion in terms of renal physiopathology. Various authors have contended that the crystalloids of the urine are maintained in solution by the colloids, which prevent agglomeration and precipitation of salts. (238, 239) Some such mechanism is undoubtedly present in the complex physiological solution finally excreted by the kidneys, since most of the urinary salts are present in proportions far exceeding their ordinary limits of solubility in aqueous solution. Low levels of colloidal activity and high surface tension values have recently been correlated with an increased tendency to stone formation in the urinary tract. (240) It is possible that this reduction in surface active substances in the urine of old people, combined with the obstruction at the vesical neck due to prostatic hypertrophy, may lead to retention of precipitated material in the bladder, accounting for the increased incidence of vesical calculi during the later decades. In women, incidence of bladder stone is relatively low, even though the quantity of excreted surface active substances is small, because there is no anatomical obstruction to urinary flow.

Distribution of surface tension values in healthy adults and in old age, shows the high proportion of values of 70 dyne/cm and above for the last group

Fig. 261. Distribution of surface tension values in healthy adults and in old age, shows the high proportion of values of 70 dyne/cm and above for the last group.

Administration of hyaluronidase raises the level of colloid activity (241) and lowers the surface tension of the urine to a slight degree. This enzyme has been utilized in the treatment of chronic recurrent stone formers with apparent effectiveness. (242) The reduced excretion of tensio active substances in the urine may be related to the decreased enzymatic activity within the tissues.