Everything you wanted to know about the bladder epithelium but were afraid to ask.

The mammalian urinary bladder epithelium (urothelium) performs the important function of storing urine for extended periods, while maintaining the urine composition similar to that delivered by the kidneys. The urothelium possesses four properties to perform this function. First, it offers a minimum epithelial surface area-to-urine volume; this reduces the surface area for passive movement of substances between lumen and blood. Second, the passive permeability of the apical membrane and tight junctions is very low to electrolytes and nonelectrolytes. Third, the urothelium has a hormonally regulated sodium absorptive system; thus passive movement of sodium from blood to urine is countered by active sodium reabsorption. Last, the permeability properties of the apical membrane and tight junctions of the urothelium are not altered by most substances found in the urine or blood. The importance of the barrier function of the urothelium is illustrated by infectious cystitis. The loss of the barrier function results in the movement of urinary constituents into the lamina propria and underlying muscle layers, resulting in suprapubic and lower back pain and frequent, urgent, and painful voiding.