Countercurrent Mechanism

Countercurrent Multiplier

A condensed form, the urine must be excreted unless one drinks very large amounts of water. Otherwise, the body will lose a lot of water, and the person will suffer from dehydration and the effects of low blood pressure.

"The method used by the kidneys to concentrate urine is called the current method of counteracting it."

Next, we need to consider how a counter-current multiplier works, to understand the counter-current process.


Countercurrent Mechanism

Let us say that there are 2 tubes through which a solution of the same substance. The solution is exchanged freely between the two tubes. These tubes may provide two forms of flow.


Concurrent Flow

Here the solutions for two tubes flow in the same direction. If one of them starts at a 0 per cent concentration at one end, and the other begins at a 100 per cent concentration. As seen in the figure, the concentrations in each tube should be about 50 per cent by the time they enter the other end of the pipe.


Countercurrent Flow

Here the liquids flow through both tubes in opposite directions. Within one tube, the concentration of the solution 0 per cent begins to flow from one end, and the concentration of the solution 100 per cent begins to flow from the opposite end in the other.

By the time solutions reached the end of the tube, a concentration equal to the other tube would have been produced at this stage due to the free flow of substances between the two tubes. The key adaptation for the conservation of water is the countercurrent mechanism that works within the kidney. There are two countercurrent mechanisms in the kidneys. They 're the loop of Henle and the vasa recta.

Henle's loop is a U-shaped part of the nephron. Blood flows in opposite directions in the two limbs of the vessel, giving rise to counter-currents. Vasa recta is an efferent arteriole that forms a capillary network around the tubules within the renal medulla. It runs parallel to the Henley loop and is U-shaped. Blood flows in opposite directions in the two legs of the vasa recta. As a result, blood entering the renal medulla in the descending limb is in near contact with the existing blood in the ascending limb. The osmolarity of the inner medulla increases by the countercurrent mechanism. It helps to preserve the concentration gradient, which in effect helps to promote the flow of water from the collection of tubules. The gradient is the result of NaCl and urea movements.


Countercurrent Mechanism Steps


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.


How is concentrated urine Formed?

The countercurrent multiplier, or counter-current mechanism, is used by the nephrons of the human excretory system to concentrate urine in the kidneys.

 

Countercurrent Mechanism in Henle’s loop

The nephrons involved in concentrated urine formation stretch all the way from the kidney cortex to the medulla and are followed by vasa recta. The movement of filtrate is in opposite directions in the two limbs of the Henle 's circle, and so is the movement of blood cells in vasa recta.


Concentrated urine is produced in the Following manner

  • NaCl shall be transported from the ascending limb of the Henle loop to the descending limb of the recta vasa.

  •  The ascending limb of the recta vasa, in turn, carries NaCl to the interstitium (the tissue between the Henle loop and the recta vasa). A concentration gradient of 300 mm is thus created in the cortex to 1200 mm in the medulla (mOsm or milliosmoles is a unit of osmolarity, i.e. osmotic active substance concentration).

  • Urea contributes to this process by being transported through the descending limb of the Henle loop into the interstitium.

  •  Higher and higher amounts of solutes are found in the interstitium as urine flows down the collection tubule. Then, osmosis causes it to lose water. So this is how the urine is concentrated.


Quick Points about Counter Current Mechanism

  • The countercurrent process takes place in Juxtamedullary Nephron.

  •  Hyperosmotic Medullary Interstitium is produced by the countercurrent multiplier.

  •  ADH facilitates the reabsorption of water through the distally coiled tubular walls and through the collection duct.

FAQ (Frequently Asked Questions)

Q1 – Explain the Countercurrent Multiplication Mechanism?

Ans - Following are the important steps of the countercurrent mechanism:

Thick ascending Loop of Henle Transport

Equilibration of descending thin Loop of Henle

Fluid Flow

Q2 - What is Countercurrent Multiplication?

Ans - Countercurrent kidney multiplication is the process of using energy to generate an osmotic gradient that allows you to reabsorb tubular fluid water and produce concentrated urine. This mechanism prevents you from producing litres and litres of diluted urine every day and is why you don't need to be drinking constantly to stay hydrated.