Mechanism of Concentration of Filtrate – Class 11 | Chapter – 19 | Biology Short Notes Series PDF

Mechanism of Concentration of Filtrate: The kidney produces urine, and during the process of excretion, urine gets out of the body in concentrated form so that the maximum amount of water gets absorbed in the body. Mammals have the ability to produce concentrated urine, and Henle’s loop and vasa recta play an important role in the process. This process of forming concentrated urine is known as the mechanism of concentration of the filtrate.

Mammals have the ability to produce a concentrated or hypertonic  urine. The different phases through which the urine becomes hypertonic in relation to body fluids have been studied by Wirz and  associates (1951) and later on by Bray (1960).It is a complex process and related to the anatomical distribution of tubules along with Na+ ion concentration at different depths from thecortex towards the medulla of kidney.

Mechanism of Concentration of Filtrate

A countercurrent system is a system of U‘U Shaped tubular system in which the fluid flows in the opposite direction in two limbs of the U‘U Shaped tubules.

Divisions of Countercurrent System
The countercurrent system has two divisions:

  1. Countercurrent multiplier which is formed by a loop of Henle.
  2. Countercurrent exchanger is formed by the vasa recta.

Countercurrent Multiplier

Loop of Henle
Loop of Henle functions as a countercurrent multiplier. Active reabsorption of sodium chloride and other solutes from the ascending limb of the Henle loop into the medullary interstitium results in the development of hyperosmolarity. These solutes accumulate and increase the osmolarity.
Moreover, the continuous addition of sodium and chloride ions increases the osmolarity of fluid. Hence, it is called a countercurrent mechanism.

Countercurrent Exchanger

Vasa Recta
Vasa recta functions as a countercurrent exchanger. It acts as a countercurrent exchanger because of its position. It is a U‘U Shaped tubule having a descending limb, hairpin bend, and an ascending limb.
The Vasa recta run parallel to the loop of Henle. Its descending limb is parallel to the ascending limb of the Henle loop, and its ascending limb is parallel to the descending limb of the Henle loop.

  1. The sodium chloride reabsorbed from the ascending limb of the Henle loop enters the medullary interstitium.
  2. Then it enters the descending limb of the vasa recta.
  3. Simultaneously water diffuses from the descending limb of the vasa recta into the medullary interstitium.
  4. The blood flows very slowly through the vasa recta. Till the time the blood reaches the ascending limb of the vasa recta, the concentration of sodium chloride increases.
  5. This causes diffusion of sodium chloride into the medullary interstitium.
  6. Simultaneously, water from the medullary interstitium enters the ascending limb of the vasa recta. And the cycle is repeated.

Thus, the vasa recta maintain the hyperosmolarity by retaining sodium chloride in the medullary interstitium and removing water from it.
Hence this system is called a countercurrent exchanger.

Steps in Mechanism of Concentration of Filtrate

Step 1: Assume that Henle’s loop is filled with a 300mOsm / L concentration equal to that which leaves the proximal tubules.

Step 2: The thick ascending limb active ion pump on Henle ‘s loop reduces the concentration inside the tubule and increases the interstitial concentration.

Step 3: Due to osmosis of water out of the descending limb, the tubular fluid in the lower limb and the interstitial fluid rapidly achieve osmotic equilibrium.

Step 4: Additional fluid flow from the proximal tubule into Henle ‘s loop, which allows the hyperosmotic fluid produced previously in the descending limb to flow into the ascending limb.

Step 5: Additional ions were pumped into the interstitium with water remaining in the tubular fluid until an osmotic gradient of 200 mOsm / L was established.

Step 6: Again, the fluid in the descending limb comes into equilibrium with the hyperosmotic interstitial medullary fluid, and as the hyperosmotic tubular fluid from the descending limb flows into the ascending limb, the more solute is continually drained out of the tubules and deposited in the medullary interstitium.

Step 7: These steps are repeated over and over, with the net effect of introducing more and more water-soluble to the medulla, with ample time, this cycle slowly traps the solutes in the medulla and multiplies the concentration gradient formed by the active pumping of ions from the thick ascending limb, eventually increasing the osmolarity of the interstitial fluid to 1200-1400 mOsm / L.

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By Team Learning Mantras