Explain the basic principles that allow the renal countercurrent system to achieve a concentrating effect.

Outline:

·        Separate regulation of solute and water excretion

·        Countercurrent multiplier: role of Henle’s loop

·        Countercurrent exchanger: role of vasa recta

·        Regulation of urine concentration by ADH

Essay:

            The kidney tubules control the concentration of urine by varying the amounts of water and NaCI reabsorbed in the distal regions of the nephron. Under normal circumstances, the excretion of water is regulated separately from the excretion of solutes. For this separate regulation to occur, the kidneys must excrete urine that is either hypo-osmotic or hyper-osmotic with respect to the body fluids. To produce dilute urine, the kidneys must reabsorb solute without allowing water to follow osmotically. To produce concentrated urine, the nephrone must reabsorb water while leaving salt behind in the lumen. When the epithelium of the of the collecting duct is permeable to water, water flows out of the lumen and into the extracellular fluid, where it is carried off in the blood. The permeability of the collecting duct to water is regulated by ADH.

            The process of urine concentration requires coordination between the loop of Henle and the collecting duct. The loop of Henle is anatomically and functionally arranged to form a countercurrent multiplier, which plays a critical role in the formation of a medullary osmotic gradient for urinary concentration and dilution. Henle’s loop consists of two parallel limbs with tubular fluid flowing in opposite directions (countercurrent flow). Fluid flows into the medulla in the descending limb, and out of the medulla in the ascending limb. The ascending limb is impermeable to water and reabsorbs solute from the tubular fluid. Thus, fluid within the ascending limb becomes diluted. The solute removed from the ascending limb tubular fluid accumulates in the surrounding interstitial fluid and raises its osmolality. Because the descending limb is highly permeable to water, the increased osmolality of the medullary interstitium causes water to be absorbed and thereby concentrates the tubular fluid.

The countercurrent flow within the descending and ascending limbs of Henle’s loop multiplies the osmotic gradient between the tubular fluid in the descending and ascending limbs of Henle’s loop.

            The medullary osmotic gradient set up by the countercurrent multiplier is maintained by a countercurrent exchanger, the vasa recta. The vasa recta are peritubular capillaries which dip into the medulla and then go back up to the cortex, forming hairpin loops. Functionally, blood flow in the limbs of the vasa recta is in opposite direction from fluid flow in the loop of Henle. As blood flows into the medulla, it loses water and picks up solutes transported out of the ascending limb of loop of Henle, carrying them further into the medulla. As blood in vasa recta flows out of medulla, its high osmolality attracts the water that is being lost from the tubule. Without the “loop-back” arrangement of the Henle’s loop and vasa recta, solutes would be lost from the intersititum if blood were to exit directly from the deep medulla. The vasa recta prevents water from diluting the concentrated interstitial fluid and helps to maintain the interstitial osmotic gradient which is 300 mOsM at the cortex to 1200 mOsM at the medullary papilla.

            The ability of the kidneys to regulate the osmolality of the final urine depends on the high osmolality in the medullary interstitial fluid that creates an osmotic gradient for water reabsorption and the permeability of the distal tubule and collecting duct to water, regulated by ADH. Fluid leaving the loop of Henle and entering the distal tubule is always hypoosmotic, around 100 mOsM. In the absence of ADH, the collecting duct remains impermeable to water and a dilute urine is produced. When ADH is present, it binds to a receptor in the basolateral membrane of the tubular cells and initiates the insertion of water channels in the apical side of the tubular epithelium. Water is reabsorbed back into the tubular cells via osmosis and a concentrated urine is produced.