1. Unit Five: The Body Fluids and Kidneys
Chapter 28: Urine Concentration and Dilution; Regulation of ECF Osmolarity and Sodium Concentration
Guyton and Hall, Textbook of Medical Physiology, 12th edition

2. Kidneys Excrete Excess Water by Forming Dilute Urine
ADH (Vasopressin) Controls Urine Concentration
Renal Mechanisms for Excreting Dilute Urine
Fig Water diuresis in a human after ingestion of 1 liter of water

3. Kidneys Excrete Excess Water by Forming Dilute Urine
Fig Formation of dilute urine when ADH levels are very low

4. Kidneys Excrete Excess Water (cont.)
Tubular fluid remains isosmotic in the proximal
tubule
Tubular fluid is diluted in the ascending loop of Henle
Tubular fluid in the distal and collecting tubules is
further diluted in the absence of ADH

5. Kidneys Conserve Water by Excreting Concentrated Urine
Water Deficit—kidney excretes solutes but reabsorbs
water therefore decreasing the volume formed
Urine Specific Gravity
Fig Relationship between specific gravity
and osmolarity of the urine

6. Kidneys Conserve Water by Excreting Concentrated Urine
Requirements for Excreting a Concentrated Urine
High levels of ADH
High osmolarity of the renal medullary interstitial
fluid

7. Kidneys Conserve Water by Excreting Concentrated Urine
Countercurrent Mechanism Produces a Hyper-
osmotic Renal Medullary Interstitium
Buildup of solute concentration in the medulla
1. Active transport of Na and cotransport
of K, Cl, and other ions from the loop of Henle
2. Active transport of ions from the collecting
ducts
3. Facilitated diffusion of urea from collecting ducts
4. Diffusion of water from the tubules

8. Active NaCl Transport Water Permeability NaCl Urea ++ + +ADH ++ADH
Table Summary of tubule characteristics—urine concentration
Active NaCl
Transport
Water
Permeability
NaCl
Urea
Prox. Tubule ++ +
Thin descending
Thin
Ascending
Thick
Dist. Tubule
+ADH
Cortical Coll. Tubule
Inner med.
Coll. Duct
++ADH

9. Conserving Water (cont.)
Steps Involved in Causing Hyperosmotic Renal
Medullary Interstitium
Fig Countercurrent multiplier system in the loop of Henle for producing
a hyperosmotic renal medulla (values are in milliosmoles per liter

10. Conserving Water (cont.)
Role of Distal Tubule and Collecting Ducts in
Excreting Concentrated Urine
Fig Formation of a concentrated urine when ADH levels are high.

11. Conserving Water (cont.)
Urea Contributes to Hyperosmotic Renal Medullary
Interstitium and Formation of Concentrated Urine
Recirculation of Urea from Collecting Duct to Loop of
Henle Contributes to Hyperosmotic Renal Medulla
In general the rate of urea excretion is determined by
The concentration of urea in the plasma
The glomerular filtration rate

12. Fig. 28.6 Recirculation of urea absorbed from the medullary collecting duct
into the interstitial fluid

13. Conserving Water (cont.)
Countercurrent Exchange in the Vasa Recta
Preserves Hyperosmolarity of the Renal Medulla
Two features that contribute to the preservation of
high solute concentrations
The medullary blood flow is low
The vasa recta serve as countercurrent exchangers
Increased Medullary Blood Flow reduces Urine
Concentrating Ability

14. Fig. 28.7 Countercurrent exchange in the vasa recta

15. Conserving Water (cont.)
Summary of Urine concentrating Mechanism and
Changes in Osmolarity in Different Segments
of the Tubules
Fig. 28.8

16. Control of ECF Osmolarity and Sodium Concentration
Estimating Plasma Osmolarity from Plasma
Sodium Concentration
Na ions in the ECF and associated anions are
the principal determinants of fluid movement
across the cell membrane

17. Osmoreceptor-ADH Feedback System
Fig Osmoreceptor-ADH feedback
mechanism for regulating ECF
osmolarity in response to a
water deficit

18. Osmoreceptor-ADH Feedback System
ADH Synthesis in the Hypothalamus and Release
from the Posterior Pituitary
Fig

19. Osmoreceptor-ADH Feedback System
Stimulation of ADH Release
Arterial baroreceptor reflexes
Cardiopulmonary reflexes
Decreased arterial pressure
Decreased blood volume

20. Osmoreceptor-ADH Feedback System
Either a decrease in effective blood volume or an
increase in ECF osmolarity stimulates ADH secretion
Fig The effect of increased plasma
osmolarity or decreased blood
volume on the level of plasma
ADH

21. Importance of Thirst in Controlling ECF Osmolarity and Na Concentration
Table Regulation of ADH Secretion
Increase ADH
Decrease ADH
plasma osmolarity
blood volume
blood pressure
Nausea
Hypoxia
Drugs:
Morphine
Alcohol
Nicotine
Clonidine (antihypertensive)
Cyclophosphamide
Haloperidol (dopamine blocker)

22. Thirst (cont.)
Stimuli for Thirst
Increased ECF osmolarity which causes intracellular
dehydration in the thirst centers
Decrease in ECF volume and arterial pressure
Production of angiotensin II
Dryness of the mouth and mucous membranes
GI and pharyngeal stimuli
Threshold for Drinking – when the Na concentration
increases 2 mEq/L above normal, the thirst
mechanism is activated