Kidneys: Countercurrent Multiplication

Dr. Huising, Fall 2024, Lecture 21

Long Loop of Henle (juxtamedullary nephrons)

• Filtrate is isotonic as it enters the loop of Henle, the same Na+ concentration as the ECF/ plasma

• Establish the vertical osmotic gradient in the medulla of the kidneys by countercurrent multiplication

• An osmolarity gradient is
  maintained in the renal medulla
  from isotonic to severely
  hypertonic (1200 mOsm/liter)

Isotonic

• At normal fluid balance and solute concentration

• ECF osmolarity is 300 mOsm/ liter

Hypotonic

• Excess H₂O relative to solute

• ECF osmolarity is <300 mOsm/liter

• Dilute urine

Hypertonic

• Too litle H₂O relative to solute

• ECF osmolarity >300 mOsm/liter

• Concentrated urine

Descending Limb of the loop of Henle

• Does not extrude/ impermeable to Na+ (no na+ leak channels)

• Highly permeable to H₂O (has AQP1)

Ascending Limb of the loop of Henle

• Actively transports Na+ out of the tubule into interstitial fluid

• Impermeable to H₂O

What are the steps in countercurrent multiplication?

1. Na+ is reabsorbed from the lowest point int he ascending limb of the loop of Henle, creating a hypertonic ECF at that level (200 mOsm/liter difference)

2. Water from the same level in the descending limb is reabsorbed as it follows the path of Na+, the osmolarity will rise

3. The water in the medulla is carried away by peritubular capillaries

4. The hypertonic solution from the descending limb moves to the ascending limb and the Na+ reabsorbed at that vertical level will continue to reabsorb until there is a 200 mOsm/liter difference

5. These steps repeat until the entire length has a vertical osmotic gradient

The strength of the vertical osmotic gradient depends on…

• The length of loop of Henle

• The concentration difference between the filtrate and ECF

What is the purpose of countercurrent multiplication?

• Produce a hypotonic urine that can be excreted if the ECF within the body has too much water

• Establishes a vertical osmotic gradient that can be used by the collecting ducts to concentrate urine if the ECF within the body doesn’t have enough H₂O

How does vasopressin control H₂O reabsorption in the collecting tubules?

• In the face of a water deficit, vasopressin leads to the insertion of AQP2 in the luminal membrane causing H₂O to be reabsorbed into the ECF and the urine to have a high Na+ concentration

Vasa Recta (capillaries)

• The hairpin loop of the vasa recta by passive countercurrent exchange preserves the vertical osmotic gradient while supplying blood ot the medulla