Volumetric analysis of glomerular size in kidneys of mammals living in desert, semidesert or water-rich environment in the Sudan.

• Adaptations to desert life have attracted much interest in the past forty years." In addition to the physiological aspects of this subject, there are morphological signs of adaptation to an arid environment in the kidneys of desert animals when compared to the kidneys of animals that require free access to water for their normal life. One of the distinguishing features of the former is the long renal papilla which extends beyond the pelvis of the ureter and projects into the proximal portion of the ureter." The renal cortex forms a relatively thin layer around the thick medulla. The long renal papilla is composed of deeply situated juxtamedullary nephrons with long thin segments of the loops of Henle. These elongated, deeply placed nephrons make such kidneys so efficient that desert mammals use only onefourth of the amount of water needed by water-requiring mammals in order to excrete the same amount of urea. According to the generally accepted countercurrent hypothesis the concentration of the urine takes place in the renal papilla. Here the loops of Henle act as a countercurrent multiplier and the vasa recta as a countercurrent diffusion exchanger, producing a steadily increasing osmotic gradient from cortex to papilla. The longer the loops of Henle and accompanying vasa recta, the more effective is the countercurrent