Counter Current Mechanism: Concentrated Urine Formation

A countercurrent mechanism is a form of mechanism that uses kidneys to concentrate urine from the body. It is used to maintain homeostasis and the balance of water and minerals in the body. 

• Counter current mechanism is also known as Countercurrent multiplier.
• The concentration of urine depends on the extent to which the body has water. 
• Less concentrated urine is excreted when the body is well-hydrated. 
• High-concentrated urine is released when the body is less hydrated. 

So, a countercurrent mechanism is used to concentrate urine in the body of mammals. The word countercurrent implies the flow of solution in the opposite direction. 

• If a person loses more water, the body tissues become dehydrated (lose excess water).
• This, in turn, leads to hypotension (low blood pressure). 

Key Terms: Counter Current Mechanism, Countercurrent Flow, Concurrent Flow, Kidney, Urine, Urine Formation, Solution, Blood Pressure, Human Body, Henle's Loop, Vasa Recta, Countercurrent Mechanism Steps

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Countercurrent Mechanism

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Countercurrent Mechanism​ can easily be understood if we can differentiate the two fluid-flow systems in the lab. Considering there are two tubes with the same solution, two types of flows can be created - concurrent flow and countercurrent flow.

Concurrent Flow 

Concurrent flow means the flow of solution in two tubes is taking place in the same direction. In one tube, the solution starts flowing with 0% concentration, whereas in the other tube, it starts at 100% concentration. 

• When solutions reach the end of the tubes, they will have 50% concentration each. 
• As solutions in two tubes flow in the same direction and finally get the same concentration, the flow becomes concurrent.

Countercurrent Flow 

In countercurrent flow, solutions in two different tubes flow in opposite directions. In one tube, the solution starts flowing with 0% concentration, whereas in the other tube, it starts at 100% concentration. 

• When solutions reach the end of the tubes, they will have more concentration (not 50%). 
• The cause for this phenomenon is the free exchange of solute particles between two tubes. 
• As the solution flow occurs in two opposite directions, the flow becomes countercurrent.

Countercurrent Mechanism

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Countercurrent Mechanism Steps

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The countercurrent mechanism occurs when the filtrate flows in two opposite directions in the two limbs of Henle's loop. The vasa recta, which is closely associated with these two limbs, also functions in two opposite directions. 

• The ascending limb of Henle's loop plays an important role in the maintenance of high osmolarity of medullary interstitial fluid. 
• The descending limb is permeable to water and impermeable to electrolytes. 
• This increases the concentration of the filtrate as it moves down. 

Conversely, the ascending limb is permeable to electrolytes and impermeable to water. This decreases the concentration of filtrate as it moves up.

• Transport of NaCl (without water) in the thin and thick ascending limbs of Henle's loop.
• It results in an interstitial osmolar gradient from 300 mOsmol/L in the cortex to 1200 mOsmol/L in the medulla.
• The medulla is situated at the tip of the renal papilla.
• It becomes hypertonic through renal tubules and vasa recta. 
• The osmotic gradient is mainly caused by NaCl and urea.

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How is concentrated urine formed?

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The steps for formation of concentration of urine are as follows:

Step 1

Solution of 300mOsm / L concentration reaches Henle's loop from the proximal tubule.

Step 2

Solute particles (NaCl) move from ascending limb to interstitium through an active pump. So the concentration of the solution in the thick ascending limb of the Henle's loop is decreased.

• The concentration of the solution in the interstitium is increased.
• Here water does not move out due to absence of aquaporin 1 on the tubular walls.

Step 3

Water moves out of the descending limb by osmosis (Movement of water from low concentrated solution to high concentrated solution through plasma membrane).

• This is possible due to aquaporin 1 on the tubular walls.
• Consequently, solution in the lower limb and the interstitium gain osmotic equilibrium.

Step 4

Continuous flow of solution occurs from the proximal tubule into Henle 's loop. So the hyperosmotic solution continuously moves from the descending limb and moves into the ascending limb.

Step 5

Na⁺ ions are pumped into the interstitium with water till an osmotic gradient of 1200 mOsm / L is achieved.

Step 6

Solution in the descending limb comes into equilibrium with the hyperosmotic interstitial medullary solution. As the hyperosmotic tubular solution moves from the descending limb into the ascending limb, more ions are continually pumped out of tubules.

• They are accumulated in the medullary interstitium.

Step 7

The cycle runs several times resulting in more concentration of solution in interstitium maintained upto 1200 mOsm / L.

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Countercurrent Mechanism in Henle's Loop & Vasa Recta

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Henle's loop, or nephron loop, lies between the proximal convoluted tubule and the distal convoluted tubule. It is a U-shaped tube. The loop consists of parallel and closely running ascending limbs and descending limbs.

• The fluids flow in opposite directions (counter-current).
• Its primary function is creating a concentration gradient in the medulla of the human kidney. 
• This is the part where the countercurrent multiplier system works. 
• Henle's loop uses electrolyte pumps to run this mechanism. 

The loop is surrounded by a tuft of efferent arterioles called vasa recta. Vasa recta forms a network of minute blood capillaries around the nephron tubules within the renal medulla. 

• The renal filtrate flows into the two limbs of the Henle loop in opposite directions. 
• At the same time, the flow of blood in the vasa recta will be in opposite directions. 
• So Henle's loop and vasa recta play an important role together in the countercurrent mechanism.

Countercurrent Mechanism in Henle's Loop & Vasa Recta

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Kidneys

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Our body consists of a pair of bean-shaped kidneys on either side of the backbone, below the ribs and behind the belly. The internal structure consists of the cortex, medulla and renal pelvis. 

• The microscopic functional units of the kidneys are known as nephrons. 
• It is estimated that there are 1 to 1.5 million (10 - 15 lakhs) nephrons in each kidney. 
• These microscopic structures are chiefly responsible for the formation of urine.

Kidney

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Things to Remember

• The countercurrent mechanism is dependent on the amount of water present in the body.
• The mechanism is used to maintain equilibrium in the medulla of the kidneys.
• Kidneys are the chief excretory organs in mammals.
• Homeostasis can be defined as the maintenance of equilibrium in the human body.
• Dehydration is the loss of excess water, whereas rehydration is the compensation of lost water.
• The countercurrent mechanism is to maintain equilibrium in the medulla of the kidneys.
• Microscopic functional units of kidneys are nephrons.
• The countercurrent mechanism occurs in the limbs of Henle's loop and vasa recta.
• ADH or vasopressin is helpful in maintaining the concentration of urine.
• Concentrated urine formation is helpful in the retention of body fluids and minerals and the maintenance of blood pressure.

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