Counter Current Mechanism
The production of concentrated urine is achieved by osmotic equilibration of the collecting duct luminal fluid with the hypertonic medullary interstitium. Urinary concentration is achieved by countercurrent multiplication in the inner medulla. The single effect in the outer medulla is active NaCI absorption from the thick ascending limb. While the single effect in the inner medulla is not definitively established, the majority of experimental data favors passive NaCI absorption from the thin ascending limb. Continued experimental studies in inner medullary nephron segments will be needed to elucidate fully the process of urinary concentration.
Step 1: Assume that the loop of Henle is filled with a concentration of 300mOsm/L the same as that leaving the proximal tubules.
Step 2: The active ion pump of the thick ascending limb on the loop of Henle reduces the concentration inside the tubule and raises the interstitial concentration.
Step 3: The tubular fluid in the descending limb and the interstitial fluid quickly reach osmotic equilibrium because of osmosis of water out of the descending limb.
Step 4: The Additional flow of the fluid into the loop of Henle from the proximal tubule, which causes the hyperosmotic fluid previously formed in the descending limb to flow into the ascending limb.
Step 5: Additional ions pumped into the interstitium with water remaining in the tubular fluid, until a 200-mOsm/L osmotic gradient is established.
Step 6: Again, the fluid in the descending limb reaches equilibrium with the hyperosmotic medullary interstitial fluid and as the hyperosmotic tubular fluid from the descending limb flows into the ascending limb, still more solute is continuously pumped out of the tubules and deposited into the medullary interstitium.
Step 7: These steps are repeated over and over, with net effect of adding more and more solute to the medulla in excess of water, with sufficient time, this process gradually traps solutes in the medulla and multiplies the concentration gradient established by the active pumping of ions out of the thick ascending limb , eventually raising the interstitial fluid osmolarity to 1200- 1400 mOsm/L .
The existence of a steep osmotic gradient in the renal medullary interstitium is the most critical in the formation of concentrated urine. The architectural organization of the renal tubules and blood vessels in the medulla constitutes counterflow systems which are essential for both generating and
maintaining a high osmotic pressure of the renal medulla.
While it has been generally accepted that active NaCl transport in the thick ascending limb of Henle’s loop plays the most fundamental role in the operation of the countercurrent multiplication
system in the renal medulla, it is still a matter of considerable dispute whether the thin ascending limb (tAL) also has an active salt transport system to provide a “single effect” necessary for the operation of the countercurrent multiplication system
Role of Urea
The concept of urea recycling has been mentioned in various studies. Urea is absorbed from the inner medullary collecting duct and secreted into the thin ascending limb, after which it remains within the tubule lumen until it returns to the inner medullary collecting duct. Maximal concentrating ability is decreased in protein-deprived animals and restored by urea.
Thus, the passive mechanism, which critically depends on an adequate delivery of urea to the inner medulla, provides an explanation for the well-described importance of urea to concentrating ability. Several recent studies have improved our understanding of urea absorption across the inner medullary collecting duct.
Urea transport occurs by a vasopressin-stimulated facilitated transport process in terminal inner medullary collecting ducts. As the urea concentration in the lumen of terminal inner medullary collecting ducts exceeds that in vasa recta, urea is rapidly absorbed into the inner medullary interstitium, down its concentration gradient.
Quick points about Counter Current Mechanism:
- The counter current mechanism takes place in Juxtamedullary nephron.
- The function of the countercurrent multiplier is to produce the hyperosmotic Medullary Interstitium.
- The ADH promotes water reabsorption through the walls of the distal convoluted tubule and collecting duct.
- Urea reabsorbed from collecting duct to medullary interstitum produces the hyperosmotic Medullary interstitium.
- Reabsorption of urea will occur in the presence of ADH.
- Excretion of large volume or small volume of urine won’t affect the rate of solute excretion.
- Nephrogenic diabetes insipidus patients will have no response from the kidney to ADH.
- The function of the Countercurrent exchanger “vasa recta” is to maintain hyperosmolar medulla.
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