The counter current mechanism

A salty medulla

A mammal that can make very concentrated urine saves water. The kangaroo rat of the desert is a standard example, and it can live without drinking.

Mammals can make a concentrated urine. The human kidney makes urine nearly four times more concentrated than the filtrate. The parts that make this possible are the loop of Henle and the vasa recta.

First, a word on the unit. Osmolarity tells how many dissolved particles are in a litre of a solution. It is measured in milliosmoles per litre, mOsmol/L. A more concentrated solution has a higher osmolarity.

Two flows in opposite directions

Look at the loop of Henle. The filtrate flows down the descending limb and up the ascending limb. The two flows are in opposite directions, so they form a counter current. The blood in the two limbs of the vasa recta also flows in a counter current pattern. The limbs of the loop and the vasa recta lie close to each other.

This arrangement keeps the fluid of the medulla, the interstitial fluid, more and more concentrated with depth. The osmolarity goes from 300 mOsmol/L in the cortex to about 1200 mOsmol/L in the inner medulla. NaCl and urea make up most of this.

How the gradient is built

Follow the steps.

1. The descending limb is permeable to water but almost impermeable to electrolytes. As the filtrate goes down, water leaves it, and it becomes more and more concentrated.
2. The ascending limb is impermeable to water but lets electrolytes out, actively or passively. As the filtrate goes up, it becomes diluted. The fluid that leaves the loop is less concentrated than the blood plasma.
3. The NaCl that leaves the ascending limb enters the interstitial fluid. It is picked up by the descending limb of the vasa recta, and carried deeper. It is returned to the interstitium by the ascending limb of the vasa recta.
4. A little urea enters the thin segment of the ascending limb of the loop. The collecting duct carries it back to the interstitium.
5. So the salt goes round between the loop, the interstitial fluid and the vasa recta. It stays in the medulla, and the blood does not carry it away.

This arrangement is the counter current mechanism. It keeps up a concentration gradient in the medullary interstitium.

Why a gradient makes a concentrated urine

The collecting duct runs down through this gradient. The fluid in it is dilute at the top and meets a saltier fluid as it goes deeper. Water moves out of the duct into the interstitium by osmosis, so the fluid in the duct becomes more concentrated. The water is taken back by the vasa recta. The urine leaves at about the concentration of the deepest part of the medulla.

The solute does not leave the duct. Only water does. So the amount of solute in the duct stays the same, and the volume falls:

If fluid at 300 mOsmol/L becomes urine at 1200 mOsmol/L, its volume becomes a quarter. The gradient limits the concentration of the urine. A kidney with longer loops can reach a saltier region, so it can make a more concentrated urine.

You should find that the longer the loop, the higher the osmolarity at its tip. A short loop that reaches only two levels gives a tip of 600 mOsmol/L, about twice the cortex. A long loop that reaches six levels gives 1200 mOsmol/L, which is four times the cortex.

Try it in Python

The program lists the osmolarity at each level of the model and the highest concentration of the urine.

Practise with new numbers

Common mistakes

• Saying that the descending limb is impermeable to water. It is permeable to water and almost impermeable to electrolytes.
• Saying that the ascending limb lets water out. It is impermeable to water. It lets electrolytes out.
• Saying that the gradient runs from 1200 in the cortex to 300 in the medulla. It is the other way round.
• Forgetting the vasa recta. It keeps the salt in the medulla and carries away the water.
• Saying that the urine is more concentrated than the deepest medulla. It cannot be more concentrated than the fluid that it meets.

Remember these

• Counter current: the filtrate in the two limbs of the loop, and the blood in the two limbs of the vasa recta, flow in opposite directions.
• Gradient: 300 mOsmol/L in the cortex to about 1200 mOsmol/L in the inner medulla. It is caused mainly by NaCl and urea.
• Descending limb: water out. Ascending limb: electrolytes out. The collecting duct lets some urea into the medulla.
• The urine can be nearly four times more concentrated than the filtrate.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What does counter current mean in the loop of Henle?

2. Which limb of the loop is permeable to water, and what happens to the filtrate in it?

3. What leaves the ascending limb, and what happens to the filtrate there?

4. Give the osmolarity in the cortex and in the inner medulla.

5. Fluid at 300 mOsmol/L enters a collecting duct, and the urine leaves at 1200 mOsmol/L. By what factor does its volume fall?

References

• NCERT (2023). Biology, Textbook for Class XI, chapter 16, Excretory Products and Their Elimination.
• Hall J. E. and Hall M. E. (2021). Guyton and Hall Textbook of Medical Physiology, 14th edition, Elsevier, the chapter on urine concentration and dilution.
• Reece J. B. et al. (2020). Campbell Biology, 12th edition, Pearson, the chapter on osmoregulation and excretion.