Countercurrents exist when fluids flow in opposite directions in parallel and adjacent tubes. There are 2 countercurrent systems and an osmotic equilibrating device:
- Countercurrent multiplier (Loop of Henle): Establishes gradient of osmolarity from cortex (300mOsm/L) to the papilla (1200mOsm/L) aided by Urea recycling
- Countercurrent exchanger (Vasa recta): Maintains the corticopapillary osmotic gradient established by Countercurrent multiplier
- Osmotic equilibrating device (Collecting duct): Depending on the plasma level of ADH, collecting duct urine is allowed to equilibrate with the hyperosmotic medullary gradient resulting from countercurrent system
Remember 2 exceptions on which the countercurrent multiplier is based:
- Descending limb of loop of Henle doesn’t reabsorb solute but does reabsorb water (Concentrates Urine)
- Ascending limb of loop of Henle doesn’t reabsorb water but does reabsorb solute actively (Dilutes Urine and the urine leaving ascending limb of loop of henle is hypo-osmotic ~100 mOsm/L)
- As NaCl is reabsorbed from the thick ascending limb by the Na+ K+ 2Cl- cotransport it creates a gradient in the interstitium (maximum 200 mOsm/L at a time because paracellular diffusion of ions back into eventually counterbalances transport of ions out of lumen when 200mOsm/L concentration gradient is achieved)
- Urine in the descending limb now equilibrate osmotically with the interstitium and water leaves
- Flow of urine now moves hyperosmotic urine into the ascending limb and the NaCl transport creates another gradient
- The loop configuration creates a counter-current multiplier for the effect of the Na+ pump to create the cortico-medullary gradient (300-1200 mOsm/Kg)
Remember 3 things:
- Vasa recta is freely permeable to both solute and water throughout the length. Water diffuses along the osmotic gradient and NaCl diffuses along its concentration gradient.
- Blood entering the descending limb of vasa recta is ~ 300mOsm/L and Blood leaving the ascending limb of vasa recta is ~ 325mOsm/L. Only slight increase in the solute content of the blood going out of the medulla shows that the medullary concentration gradient is maintained as most of the solute is left in the interstitium.
- Urine osmolarity is inversely related to medullary (vasa recta) blood flow. Faster the blood flows, there is less time for equilibration and increased solute leave blood leading to decreased medullary concentration gradient.
- As the blood descends through the descending limb of vasa recta, water diffuses out and NaCl diffuses in to equilibrate with the increasing osmolarity of medullary interstitial fluid (ISF) from top to bottom established by countercurrent multiplier.
- As the blood ascends through the ascending limb of vasa recta, water diffuses in and NaCl diffuses out to equilibrate with the decreasing osmolarity of medullary interstitial fluid (ISF) from bottom to top.
- The process continues and the equilibrium is never reached.
Role of Urea recycling in Medullary Concentration Gradient
Absorption of urea in the collecting tubules, under the influence of ADH, and secreation in the loop of henle contributes ~ 50% of the medullary concentration gradient.
Osmotic Equilibrating Device:
1. When ADH plasma levels are increased during negative water balance:
The collecting ducts become highly permeable to water and water moves out of the collecting duct into the hyperosmotic medullary interstitium down its chemical gradient until the collecting duct lumen and corresponding medullary interstitium have equal water concentrations. So much water leaves by the end of the collecting duct that urine volume is low (perhaps 500 ml/day) and the urine osmolality is high (~ 1200 mOsm/L). The kidneys have saved volume.
2. When ADH plasma levels are decreased during positive water balance:
Water is trapped in the collecting ducts and some solute removal still occurs in the collecting ducts; therefore a very large volume of dilute urine (upto 100 mOsm/L) is formed.
Obligatory urine volume:
If maximal urine concentrating ability is 1200 mOsm/L, the minimal volume of urine that must be excreted is: Concentration of solute to be excreted per day / Maximal urine concentration ability.
To excrete 600 mOsm of solute each day, the obligatory urine volume is 0.5 L/day (600/1200).
Why drinking sea water leads to dehydration?
Ans: This is due to limited ability of human kidney to concentrate the urine to maximal concentration of 1200 mOsm/L. Osmolarity of sea water is ~ 1200 mOsm/L. Hencer for each litre of sea water drunk, 1L of water is required to excrete 1200 mOsm of sodium. But still dehydration occurs. This is because of requirement to excrete other substances as well. At maximal concentration ability, urea contributes 600 mOsm/L. Hence meximum concentration of NaCl that can be excreted by kidney is 600 mOsm/L. Hence, for every 1L of sea water drunk, 2L of fluid loss occurs.