Countercurrent Exchanger

Blood flow must be provided to the renal medulla to supply the metabolic needs of the cells in this part of the kidney. Special features of the blood flow in vasa recta that contribute to the preservation of the high solute concentrations

Countercurrent Exchange in the Vasa Recta Preserves Hyperosmolarity of the Renal Medulla 
Blood flow must be provided to the renal medulla to supply the metabolic needs of the cells in this part of the kidney. Without a special medullary blood flow system, the solutes pumped into the renal medulla by the countercurrent multiplier system would be rapidly dissipated.
Special features of the renal medullary blood flow that contribute to the preservation of the high solute concentrations:
1. The sluggish blood flow (accounting for less than 5 per cent of the total renal blood flow) is sufficient to supply the metabolic needs of the tissues but helps to minimize solute loss from the medullary interstitium.
2. The vasa recta serve as countercurrent exchangers, minimizing washout of solutes from the medullary interstitium.

The countercurrent exchange mechanism 
As blood descends into the medulla toward the papillae, it becomes progressively more concentrated, partly by solute entry from the interstitium and partly by loss of water into the interstitium.
By the time the blood reaches the tips of the vasa recta, it has a concentration of about 1200 mOsm/L, the same as that of the medullary interstitium.
As blood ascends back toward the cortex, it becomes progressively less concentrated as solutes diffuse back out into the medullary interstitium and as water moves into the vasa recta.
* Thus, although there is a large amount of fluid and solute exchange across the vasa recta, there is little net dilution of the concentration of the interstitial fluid at each level of the renal medulla because of the U shape of the vasa recta capillaries, which act as countercurrent exchangers. Thus, the vasa recta do not create the medullary hyperosmolarity, but they do prevent it from being dissipated.
 

Countercurrent Multiplier

Osmotic gradient in the medulla is useful in producing concentrated urine. The osmolarity gradually increases from 300 mOsm/L in the outer medulla to about 1200 mOsm/L in the inner medulla. How is this gradient established?

Steps Involved in Causing Hyperosmotic Renal Medullary Interstitium.Step-1First, assume that the loop of Henle is filled with fluid with a concentration of 300 mOsm/L, the same as that leaving the proximal tubule.

Step-2
The active pump of the thick ascending limb on the loop of Henle is turned on, reducing the concentration inside the tubule and raising the interstitial concentration; this pump establishes a 200-mOsm/L concentration gradient between the tubular fluid and the interstitial fluid.

Step-3
The tubular fluid in the descending limb of the loop of Henle and the interstitial fluid quickly reach osmotic equilibrium because of osmosis of water out of the descending limb. The interstitial osmolarity is maintained at 400 mOsm/L because of continued transport of ions out of the thick ascending loop of Henle.

Step-4
Additional flow of fluid into the loop of Henle from the proximal tubule causes the hyperosmotic fluid previously formed in the descending limb to flow into the ascending limb.

Step-5
Once this fluid is in the ascending limb, additional ions are pumped into the interstitium, with water remaining behind, until a 200-mOsm/L osmotic gradient is established, with the interstitial fluid osmolarity rising to 500 mOsm/L.


Step-6
Then, once 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 of the loop of Henle 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 the 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 loop of Henle, eventually raising the interstitial fluid osmolarity to 1200 to 1400 mOsm/L as shown in step 7. Thus, the repetitive reabsorption of sodium chloride by the thick ascending loop of Henle and continued inflow of new sodium chloride from the proximal tubule into the loop of Henle is called the countercurrent multiplier. The sodium chloride reabsorbed from the ascending loop of Henle keeps adding to the newly arrived sodium chloride, thus "multiplying" its concentration in the medullary interstitium.

Overview
Click on the following image to see a have a larger view of all the steps at once: