Human uterine fluid, examined in undiluted samples for osmolarity and the concentrations of inorganic ions, albumin, glucose, and urea.

Human uterine fluid and serum, when analyzed, had the same osmolarity. Both the total cation concentration and that of albumin were lower in uterine fluid than in serum. Decreased cation concentration is probably a regulatory mechanism maintaining electroneutrality in uterine fluid when the protein concentration is decreased, since protein molecules have multiple negative charges. Compared with their serum values, concentrations of potassium were high, and those of sodium and calcium were low. Concentrations of potassium and calcium in uterine fluid were found to vary cyclically, both having lower values at midcycle than in the proliferative and luteal phases. The concentrations of chloride, urea, glucose, and fructose in uterine fluid did not differ significantly from those in serum. A possible influence of the ionic composition of uterine fluid on sperm migration at midcycle and on the implantation process in the luteal phase was also considered.