Counter current mechanism

Counter Current Mechanism  Notes with Questions & Answers 

Q1. What is meant by counter current mechanism?

It’s a process in the kidney that helps in the exchange of substances like water and salts between two fluids moving in opposite directions. This mechanism helps in conserving water and making urine concentrated.

Q2. Where does this mechanism take place?

It mainly takes place between the loop of Henle and vasa recta (blood capillaries) present in the medulla of the kidney.

Q3. Why is it called “counter current”?

Because the filtrate in the loop of Henle and the blood in vasa recta flow in opposite directions, forming a counter current system.

🔹 Structure and Function of Loop of Henle

Q4. What are the two parts of the loop of Henle?

1. Descending limb

2. Ascending limb

Q5. What happens in the descending limb?

The descending limb is permeable to water but not to salts. So water moves out of it into the medulla, and the filtrate becomes more concentrated.

Q6. What happens in the ascending limb?

The ascending limb is impermeable to water but actively transports salts (Na⁺, K⁺, Cl⁻) out of the filtrate. As salts move out, the filtrate becomes dilute

Q7. What kind of transport occurs in the ascending limb?

Active transport happens here, carried out by special co-transporter proteins that push Na⁺, K⁺, and Cl⁻ ions out.

Q8. Why does the medulla become hypertonic?

Because salts are pumped out from the ascending limb into the medulla, increasing its salt concentration. This salty environment helps in water reabsorption later.

🔹 Hormonal Control

Q9. Which hormone controls sodium reabsorption?

Aldosterone, released from the adrenal cortex, increases sodium reabsorption and also helps in water retention.

Q10. What effect does aldosterone have on urine?

It makes the body reabsorb more sodium and water, so less urine is produced.

Q11. Which hormone helps in water reabsorption from the collecting duct?

Anti-diuretic hormone (ADH), secreted from the posterior pituitary gland.

Q12. What does ADH actually do?

ADH makes the collecting duct walls more permeable to water, allowing more water to move back into the blood. As a result, urine becomes concentrated.

Q13. What happens if ADH is not released?

If ADH is absent, water cannot be reabsorbed properly, and the urine becomes very dilute. This happens in a condition called Diabetes insipidus.

🔹 Counter Current Multiplier

Q14. What is the counter current multiplier?

The loop of Henle acts as a multiplier that builds up a concentration gradient in the medulla by moving salts and water in opposite directions.

Q15. Which part of the loop works as a salt pump?

The ascending limb — it pumps out Na⁺, K⁺, and Cl⁻ into the medulla.

Q16. Which part mainly reabsorbs water?

The descending limb, as it’s permeable to water but not to salts.

Q17. Why is it called a “multiplier”?

Because small concentration differences between the two limbs keep multiplying along the length of the loop, resulting in a strong gradient in the medulla.

🔹 Counter Current Exchanger

Q18. What is the counter current exchanger?

The vasa recta (capillaries around the loop) act as a counter current exchanger. They maintain the concentration gradient created by the loop of Henle.

Q19. How does blood flow in the vasa recta?

Blood flows in the opposite direction to the filtrate in the loop of Henle.

Q20. What happens to the blood as it goes down the vasa recta?

As it moves down, blood loses water and gains salts from the medulla.

Q21. What happens to the blood as it goes up the vasa recta?

As it rises, blood gains back water and loses salts, becoming dilute again before leaving the medulla.

Q22. Why is this process necessary?

Because it prevents the medulla from losing its concentration and ensures that water is effectively reabsorbed back into the blood.

🔹 Role of Urea

Q23. What is the function of urea in this mechanism?

Urea diffuses from the collecting duct into the medulla, making the medulla even more concentrated. This helps in reabsorbing more water from the collecting duct.

🔹 Filtrate Composition and Flow

Q24. What substances are present in the filtrate after filtration?

Water, salts (NaCl), bicarbonates, urea, glucose, amino acids, and some drugs or toxins.

Q25. What are the three main steps of urine formation?

1. Filtration

2. Reabsorption

3. Secretion 

Q26. Where does filtration happen?

In the Bowman’s capsule of the nephron.

Q27. Where does most of the reabsorption take place?

In the proximal convoluted tubule (PCT).

Q28. Which substances are reabsorbed in the PCT?

Water, Na⁺, glucose, amino acids, and bicarbonates.

Q29. Which substances are secreted into the tubule?

H⁺, K⁺, ammonia, drugs, and toxins.

Q30. What is the main function of the distal convoluted tubule (DCT)?

It helps in maintaining the balance of Na⁺, K⁺, and H⁺ ions under the control of hormones.

Q31. What happens in the collecting duct?

Final water reabsorption happens here. ADH controls how much water gets reabsorbed before the filtrate becomes urine.

🔹 Concentration and Dilution of Urine

Q32. Why is the medulla region hypertonic?

Because of the high concentration of salts and urea stored there.

Q33. What kind of urine is formed when you’re dehydrated?

Concentrated urine, because ADH levels increase, reabsorbing more water.

Q34. What happens when water intake is high?

Less ADH is released → less water is reabsorbed → dilute urine is produced.

🔹 Key Definitions

Q35. Counter Current Multiplier

It’s the system in which the loop of Henle builds up the medullary concentration gradient by moving salts and water in opposite directions.

Q36. Counter Current Exchanger

The vasa recta maintain that gradient by exchanging salts and water without disturbing it.

Q37. Hypertonic Solution

A solution that has more solutes (salts) than another, so water moves into it.

Q38. Hypotonic Solution

A solution with fewer solutes, so water moves out of it.

🔹 Diagram Understanding (Fig. 15.9)

Q39. What happens in the proximal tubule?

Reabsorption of nutrients and ions, and secretion of drugs and hydrogen ions.

Q40. What happens in the distal tubule?

Reabsorption of sodium and secretion of potassium and hydrogen ions under hormonal control.

Q41. What happens to urea in the collecting duct?

Urea moves out into the medulla and helps maintain its high osmotic concentration.

🔹 Final Outcomes

Q42. What is the main goal of the counter current mechanism?

To conserve water and produce concentrated urine when the body needs it.

Q43. What happens if this system doesn’t work properly?

The body will lose excess water and produce a lot of dilute urine, leading to dehydration.

Q44. How does the kidney control water balance?

By adjusting water reabsorption with the help of ADH and aldosterone hormones.

Q45. Which structure maintains the concentration gradient in the medulla?

Vasa recta — by counter current exchange.

Q46. Which structure creates the gradient?

Loop of Henle — by counter current multiplication.

Q47. Why is blood flow in vasa recta slow?

So that there’s enough time for exchange of salts and water through diffusion.

Q48. Which nephrons help in forming concentrated urine?

Juxtamedullary nephrons, because they have long loops of Henle that reach deep into the medulla.

Q49. What are the main contents of urine?

Water, urea, salts, and some waste products.

Q50. Why is the counter current mechanism so important?

Because it lets our kidneys save water, remove wastes, and keep the body’s water and salt balance stable — which is essential for survival.

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