How the Mammalian Kidney Conserves Water: A Closer Look
The osmolarity of human blood is about 300 mosm/L, but the kidney can excrete a urine up to four times as concentrated-about 1200 mosm/L. The loop of Henle and the collecting duct cooperate to maintain the gradient of osmolarity in the interstitial tissue of the kidney that makes it possible to concentrate the urine. The two solutes responsible for this osmolarity gradient are NaCl, which is deposited in the kidney medulla by the loop of Henle, and urea, which leaks across the epithelium of the collecting duct in the inner medulla.
To better understand the physiology of the mammalian kidney as a water-conserving organ, let us retrace the flow of filtrate through the renal tubule, this time focusing on how the juxtamedullary nephrons maintain an osmolarity gradient in the kidney and use that gradient to excrete a hyperosmotic urine (Figure 40.13). Filtrate passing from Bowman's capsule to the proximal convoluted tubule has an osmolarity of about 300 mosm/L, the same as blood.
As the filtrate flows through the proximal convoluted tubule, located in the kidney cortex, a large amount of water and salt is reabsorbed; thus, the volume of filtrate decreases substantially at this stage, but the osmolarity remains about the same. Now the filtrate begins its serpentine trip, down into the medulla within the descending limb of the loop of Henle, back up to the cortex in the ascending limb, and then down to the medulla one more time within the collecting duct.
As the filtrate flows from cortex to medulla in the descending limb, water exits from the renal tubule by osmosis and the osmolarity of the filtrate increases as solutes, including NaCl, become more concentrated. Increasing gradually from cortex to medulla, the salt concentration of the filtrate peaks at the elbow of the loop of Henle.
This maximizes the diffusion of salt out of the tubule as the filtrate rounds the curve and enters the ascending limb, which, remember, is leaky to salt but not to water. Thus, the two limbs of the loop of Henle cooperate in maintaining the gradient of osmolarity in the interstitial fluid of the kidney: The descending limb produces a progressively saltier filtrate, and the ascending limb exploits this concentration of NaCl to help maintain a high osmolarity in the interstitial fluid of the kidney medulla.
Notice that the loop has some of the qualities of a countercurrent system, similar in principle to the countercurrent mechanism that maximizes oxygen absorption by the gills of fishes (see Chapter 38). Although the two limbs of the loop of Henle are not in direct physical contact, they are close enough together to affect one another's chemical exchanges with a common interstitial fluid. The loop of Henle can concentrate salt in the inner medulla only because traffic in the descending limb counters the osmolarity gradient produced by the ascending limb in the interstitial fluid.
What prevents the capillaries of the kidney medulla from dissipating the osmolarity gradient by carrying away the NaCl that leaks from the ascending limb into the interstitial fluid? Notice in Figure 40.10 that the vasa recta is a countercurrent system, with descending and ascending capillaries carrying blood in opposite directions through the kidney's osmolarity gradient.
As the descending vessel conveys blood toward the inner medulla, water is lost from the blood and NaCl diffuses into the blood. These fluxes are simply reversed as blood flows back toward the cortex in the ascending vessel, with water reentering the blood and salt diffusing out of the blood. Thus, the vasa recta can supply oxygen and other important substances carried by blood without interfering with the osmolarity gradient that makes it possible for the kidney to excrete a hyperosmotic urine.
By the time the filtrate completes its circuitous excursion in the loop of Henle, it is not hyperosmotic to body fluids at all, but is actually slightly hypoos-motic. This is because the thick segment of the ascending limb actively pumps NaCl out of the tubule, making the filtrate more and more dilute. Now the filtrate descends once again toward the medulla, this time in the collecting duct, which, remember, is permeable to water but not to salt. Flowing from cortex to medulla, the filtrate loses water by osmosis as it encounters interstitial fluid of increasing osmolarity.
This concentrates urea in the filtrate, and some of this urea leaks out of the lower portion of the collecting duct, making a major contribution to the high interstitial osmolarity of the inner medulla. (This urea is recycled by its secretion into the loop of Henle, but continual leaking of urea from the collecting duct maintains a high interstitial concentration of this solute.)
The urea that remains in the collecting duct is excreted with a min- imal loss of water from the body because osmosis causes the filtrate in the collecting duct to equal the osmolar-ity of the interstitial fluid, which can be as high as 1200 mosm/L in the inner medulla. Notice that urine, its most concentrated, is actually isosmotic to the interstitial fluid of the inner medulla, but that makes it hyperosmotic to blood and interstitial fluid elsewhere the body. The juxtamedullary nephron, with its urine-concentrating features, is a key adaptation to terres-tial life, enabling mammals to get rid of nitrogenous waste without squandering water.