Corticopapillary Osmotic Gradient
- The Corticopapillary Osmotic Gradient refers to the gradient of osmolarity that exists in the renal interstitial fluid between the renal cortex and the papillae of the renal medulla. The corticopapillary osmotic gradient is critical for the generation of a concentrated urine as described in the Regulation of Urine Osmolarity. Here we describe how the corticopapillary osmotic gradient is generated, regulated, and maintained.
- The primary osmolytes responsible for the corticopapillary osmotic gradient are sodium and urea and distinct processes are responsible for generating their respective corticopapillary gradients. The countercurrent mechanism generates the sodium gradient while process of urea recycling generates that of urea.
- While the cortical terminus of the corticopapillary osmotic gradient is always roughly 300 mOsm/L, the papillary terminus of the gradient can range from 600 mOsm/L to 1200 mOsm/L. The size of the gradient is modulated according to the physiological demands of the body and is maximized when a highly concentrated urine is required. Control of the gradient's size is achieved by Antidiuretic Hormone (ADH) which, as described in regulation of urine osmolarity, is secreted when ECF osmolarity increases. The presence of ADH increases the size of the corticopapillary osmotic gradient and thus allows generation of a highly concentrated urine results in net resorption of free water to the ECF, thus diluting the ECF osmolarity. Mechanistically, ADH enhances the size of the gradient by enhancing the countercurrent mechanism and urea recycling as discussed on their respective pages.
- Like all tissues, the renal medulla must receive a blood supply; however, this presents a physiologic challenge as unidirectional flow of blood through this region would rapidly wash away the corticopapillary osmotic gradient. This would occur because the solutes comprising the hyperosmotic fluid at the papillae would passively diffuse into the perfusing blood and be washed away. Furthermore, water from the perfusing blood would osmotically diffuse into the hyperosmotic fluid in the papillae, thus diluting the highly concentrated solutes. The solution to this potential problem is elegantly achieved by the unique hairpin-like structure of the vasa recta. Diffusion of solutes or water into or out of blood as it travels down the descending limb of the vasa recta simply reverses direction as the blood travels up the ascending limb of the vasa recta. Consequently, oxygenated blood can perfuse the renal medulla and papillae without washing away the corticopapillary osmotic gradient.