Excretion in mammals: role of the renal pelvis in the modification of the urinary concentration and composition.

The mammalian renal pelvis has previously been regarded as having no physiological function other than that of a receptacle for the urine before it moves through the ureter. It has long been known that the renal pelvis in mammals shows elaborate extensions that bring the urine in intimate contact with outer and inner medullary tissue. The epithelium covering these areas is of a type that indicates that transport of solutes and water can take place across it. Physiological studies have shown that urea and water readily move across the renal papillary epithelium. The pronounced ability of ruminants to conserve urea when placed on a low protein diet coincides with a highly developed renal pelvis with specialized fornices and secondary pouches. It is suggested that urea conservation in low protein animals takes place partly in the renal pelvic extensions where urea is returned to the blood. A hypothesis is also proposed for the concentrating mechanism in which recycling of urea from the pelvic urine across the papillary epithelium plays an important role.