50 Countercurrent Flow

Countercurrent Flow

The osmotic gradient needed to produce concentrated urine depends on two chief factors. First, individual areas of the nephron loop differ in permeability and reabsorption characteristics. Second, fluid flows in opposite directions through adjacent tubes in different parts of the urinary system. This process is called countercurrent flow of fluid. It occurs down and up the descending and ascending limbs of the nephron loop. Blood flowing along the ascending and descending portions of the vasa recta also follows countercurrent flow. Two countercurrent mechanisms operate in the kidneys: countercurrent multiplication and countercurrent exchange.

Countercurrent Exchange

In countercurrent exchange, countercurrent flow enables the passive exchange of water and solutes between blood in the vasa recta and the medullary interstitial fluid. The osmolarity of blood entering the vasa recta is around 300 mOsm/liter. In its descending limb, urea and sodium and chloride ions diffuse from the increasingly concentrated medullary interstitial fluid into the blood, while water diffuses from the blood into the interstitial fluid. Along the ascending limb of the vasa recta, the concentration of the interstitial fluid steadily decreases. At this point, urea and sodium and chloride ions diffuse from the blood back into the interstitial fluid, and water diffuses in the opposite direction. Because the osmolarity of blood leaving the vasa recta is only a little higher than that of blood leaving it, it can provide oxygen and nutrients to the renal medulla without washing out the osmotic gradient. To summarize, the osmotic gradient in the renal medulla is created by countercurrent multiplication in the nephron loop, and it is maintained by countercurrent exchange in the vasa recta.

Countercurrent Multiplication

The nephron loop is responsible for the countercurrent multiplication mechanism that establishes an osmotic gradient within the medullary interstitium. This gradient is necessary for the collecting duct to create an osmotic gradient.

Three factors are involved in countercurrent multiplication. First, recall that the ascending limb is responsible for the active transport of sodium, chloride and potassium out of the tubular fluid and into the interstitium. Because this limb is relatively water impermeable, the solutes move into the interstitial fluid without additional water, increasing the osmotic pressure of the interstitium.

Second, the descending limb, which is in very close proximity to the ascending limb, is solute-impermeable and water-permeable. Because the ascending and descending limb share the same interstitial fluid, water moves along the osmotic gradient from the descending limb into the interstium, concentrating the tubular filtrate in the descending limb. Thus, the osmolarity of the medullary interstitial fluid along this limb steadily increases, and water leaves the filtrate. The osmolarity of the filtrate is highest (1,200 mOsm) where the nephron loop bends, and the filtrate moves from the descending limb to the ascending limb.

The third factor in the countercurrent multiplication is the shift of the fluid along the length of the tubule so that fluid that participated in water loss in the descending limb, will now participate in solute loss in the ascending limb. When the filtrate in the descending limb turns the corner and enters the ascending limb, the concentrations of sodium and chloride ions are very high in the filtrate The molecules are now actively pumped from the tubule into the interstitial fluid to maintain the high osmotic pressure in the interstitium. It turns out that this system “traps” high salt concentrations deep in the medulla as the ions are most actively pumped out there.

Losing salt but not water makes the filtrate in the ascending limb increasingly more dilute. By the time it reaches the distal tubule, it is at 100 mOsm and therefore has a lower osmotic pressure than the blood plasma and interstitial fluids in the renal cortex. This is what allows for the generation of a dilute urine.

Another contributor to the high osmolarity of the the medullary interstitium is the recycling of urea. Urea from the interstitial fluid diffuses into the filtrate in the thin limbs of the nephron loop. Because the thick limb and collecting duct are urea-impermeable, by the time the filtrate reaches the collecting duct, water reabsorption has created highly concentrated urea that diffuses back into the medullary interstitial fluid. The resulting pool of urea is a major contributor to the high osmolarity in this region. This urea is then recycled back into the thin limbs of the loop, and the cycle starts again. The presence of ADH amplifies urea recycling, which, in turn, amplifies the osmotic gradient and enables the formation of more concentrated urine.