Question

Explain how the movement of NaCl and water in the ascending and descending limbs of the loop of Henle work as a countercurrent system. You may use a diagram to help explain this.

Answer 1

COUNTERCURRENT MECHANISM COUNTERCURRENT FLOW
A countercurrent system is a system of ‘U’­shaped tubules (tubes) in which, the flow of fluid is in opposite direction in two limbs of the ‘U’­shaped tubules.
Divisions of Countercurrent System
Countercurrent system has two divisions:
1. Countercurrent multiplier formed by loop of Henle 2. Countercurrent exchanger formed by vasa recta.
COUNTERCURRENT MULTIPLIER Loop of Henle
Loop of Henle functions as countercurrent multiplier. It is responsible for development of hyperosmolarity of medullary interstitial fluid and medullary gradient.
Role of Loop of Henle in Development of Medullary Gradient
Loop of Henle of juxtamedullary nephrons plays a major role as countercurrent multiplier because loop of these nephrons is long and extends upto the deeper parts of medulla.
Main reason for the hyperosmolarity of medullary interstitial fluid is the active reabsorption of sodium chloride and other solutes from ascending limb of Henle loop into the medullary interstitium. These solutes accumulate in the medullary interstitium and increase the osmolarity.
Now, due to the concentration gradient, the sodium and chlorine ions diffuse from medullary interstitium into the descending limb of Henle loop and reach the ascending limb again via hairpin bend.
Thus, the sodium and chlorine ions are repeatedly re­ circulated between the descending limb and ascending limb of Henle loop through medullary interstitial fluid leaving a small portion to be excreted in the urine.
Apart from this there is regular addition of more and more new sodium and chlorine ions into descending limb by constant filtration. Thus, the reabsorption of sodium chloride from ascending limb and addition of new sodium chlorine ions into the filtrate increase or multiply the osmolarity of medullary interstitial fluid and medullary gradient. Hence, it is called countercurrent multiplier.
Other Factors Responsible for Hyperosmolarity of Medullary Interstitial Fluid
In addition to countercurrent multiplier action provided by the loop of Henle, two more factors are involved in hyperosmolarity of medullary interstitial fluid.
i. Reabsorption of sodium from collecting duct
Reabsorption of sodium from medullary part of collect­ ing duct into the medullary interstitium, adds to the osmolarity of inner medulla.
ii. Recirculation of urea
Fifty percent of urea filtered in glomeruli is reabsorbed in proximal convoluted tubule. Almost an equal amount of urea is secreted in the loop of Henle. So the fluid in distal convoluted tubule has as much urea as amount filtered.
Collecting duct is impermeable to urea. However, due to the water reabsorption from distal convoluted tubule and collecting duct in the presence of ADH, urea concentration increases in collecting duct. Now due to concentration gradient, urea diffuses from inner medullary part of collecting duct into medullary interstitium.
Due to continuous diffusion, the concentration of urea increases in the inner medulla resulting in hyperosmolarity of interstitium in inner medulla.
Again, by concentration gradient, urea enters the ascending limb. From here, it passes through distal convoluted tubule and reaches the collecting duct. Urea enters the medullary interstitium from collecting duct. By this way urea recirculates repeatedly and helps to maintain the hyperosmolarity of inner medullary interstitium. Only a small amount of urea is excreted in urine.
Urea recirculation accounts for 50% of hyper­ osmolarity in inner medulla. Diffusion of urea from collecting duct into medullary interstitium is carried out by urea transporters, UT­A1 and UT­A3, which are activated by ADH.