Lecture 19 - Urine Concentration and Dilution

Question 1

How does the kidney regulate extracellular osmolarity by urine secretion?

L19 S4-9

Answer 1

Excretion of dilute urine:

• up to 20 L/day at concentrations as low as 50 mOsm/L
• achieved by reabsorption of solutes in the ascending limp of Henle and late distal convoluted tubule which are made impermeable to water

Excretion of concentrated urine:

• no less than 0.5 L/day at a concentration of 1200 mOsm/L
• achieved in presence of ADH an high renal medullary interstitial pressure

Question 2

What is the obligatory urine volume and what controls it?

L19 S9

Answer 2

• 0.5 L/day

- required to eliminate waste in the urine

Question 3

Describe how the countercurrent multiplier system increases concentration in the medullary region.

L18 S17 (just use the book)

Answer 3

1. Ion pump in thick ascending limb of Henle (ALH) increases interstitial concentration (concentration difference of 200mOsm/L between interstitium and ALH)
2. Water diffuses out of descending limb of Henle (DLH) to balance 100mOsm/L difference between interstitium, DLH concentration become more concentrated
3. Fluid from DLH flows to ALH (through medullary region) causing it to increase in concentration

These steps repeat causing increased concentrations in the medullary region

Question 4

Explain how urea is used to concentrate urine.

L19 S19-20

Answer 4

-urea leaves the medullary collecting duct and water follows; this concentrates urine

Question 5

Describe the osmoreceptor-ADH feedback mechanism.

L19 S28-29

Answer 5

• high extracellular osmolarity leads to shrinking of osmoreceptors in hypothalamus generating an action potential
• ADH is stimulated to be released from the posterior pituitary
• ADH increases the permeability off water in distal tubules and collecting ducts
• water is reabsorbed
• extracellular osmolarity decreases