Lecture 6: Countercurrent Multiplier System

Increase H2O reabsorption Results

urine is hyperosmotic, concentrated

Decrease H2O reabsorption results

urine is hypo-osmotic, dilute

Max [urine] Osmolarity #

1400 mOsmol/L

Plasma Osmolarity #

300 mOsmol/L

Min amount of urine excreted per day #

0.5L/day

If we were stranded on an island (dehydrated), why would we still excrete this minimum amount of urine?

Your body still works to flush certain minerals out of the body

Where does urine concentration take place?

tubules & collecting duct

Why does urine concentration occur?

Medullary interstitial fluid is very hyperosmotic

How is there hyperosmolarity?

1. Countercurrent anatomy at Juxtamedullary Loop of Henle
2. Reabsorption of NaCl in ascending limb
3. Impermeability of water in ascending limb
4. Urea entrapment in medulla
5. Vasa recta loops minimize changes

Vasa recta

hairpin loops
changed with the tubule
helps to maintain hyperosomolatiy of medullary IF

Proximal Convoluted Tubule

Reabsorbs most water and solutes at the same rate: filtrate is non-cellular (no protein)
same as plasma - 300 mOsmol/L

Osmolarity of filtrate entering descend limb is...

tubular fluid entering descending limb 300 Osmol/L

Ascending Limb

1. NaCl reabsorption into medullary interstitial fluid
- thick limb: active mediated transport
- thin limb: simple diffusion
summary - treat as active
2. Impermeable to water.
- water does not follow sodium
- Becomes hyperosmotic

Descending limb

1. no NaCl reabsorption
2. Highly permeable to water.
- diffusion of water into medullary

How do we maintain hyperosmolarity in the renal medulla?

Diffusion of urea from the collecting duct

Countercurrent mechanism

Horizontally: isosmotic
Vertically: multiplier
at lowest: 1400 mOsmol in medullary

What is most important in the whole system?

1. Active NaCl transport
2. Urea entrapment. (solute)
- some urea follows flow of water
- increase osmolarity

Why is the tubular fluid entering the DCT hypoosmotic?

because of the active pumping occurring in the ascending limb of the loop of Henle.

Entering the distal convoluted tubule...

- TF is hypo osmotic
- additional reabsorption of water and solutes
- reabsorption of water due to ADH

Entering the collecting duct...

- TF is hypo osmotic
- dependent on aquaporins and ADH
- Inc. ADH, urine is hyperosmotic

ADH role in water absorption

ADH induces expression of water transport proteins in the late distal tubule and collecting duct to increase water reabsorption.

Urea Recycling