1. Urine Concentration and Dilution; Regulation of Extracellular Fluid Osmolarity and Sodium Concentration. Chapter 28 Lecture-7 7/4/2015
Yanal A. Shafagoj MD. PhD

2. One of the most important tests is to look if the kidney can concentrate urine which is considered to be a tubular function
- Urine specific gravity test ( it tests the ability of the kidneys to concentrate urine)
High specific gravity means concentrated urine
Low specific gravity means diluted urine
This is the last function to be retained following acute renal failure

3. Minimal urine concentration
Concentration and Dilution of the Urine
Maximal urine concentration
= mOsm / L
(specific gravity ~ 1.030)
Minimal urine concentration
= mOsm / L
(specific gravity ~ 1.003)

4. Relationship between urine osmolarity and specific gravity
Influenced by
glucose in urine
protein in urine
antibiotics
radiocontrast
media
S.G versus osmolality:
One mole of any solution contains the same number of particles (Avogadro’s number: 6.02 X 1023). The freezing point of any solution with a concentration of one Mole/kg water is –1.86 C. This solution is called osmolal solution. The change in temperature is called molal freezing point constant (Kf) this is equal one Osmole. To calculate the osmolality of a solution you use an osmometer. It calculate the freezing point of the solution and compare it to Kf. For instant the freezing point for plasma equals –0.53 C. Therefore the osmolality of plasma equals –0.53/-1.86 X 1000=285 mOsm.
S.G depends on the mass of the particles present in a given weight of urine (mass/mass) and is a traditional measure of urine concentration (very simple test).
diluted urine (compared to plasma)
isotonic urine (the same as plasma).
concentrated urine (compared to plasma)
The urine osmolality is determined by the number of particles in the solution. In contrast, the urine specific gravity, which is a measure of the weight of the solution compared to that of an equal volume of distilled water, is determined by both the number and size of particles in the solution.
In most cases, the urine specific gravity varies in a relatively predictable way with the osmolality, with the specific gravity rising by .001 for every 35 to 40 mosmol/kg increase in osmolality. Thus, a urine osmolality of 280 mosmol/kg (which is isosmotic to plasma) is usually associated with a specific gravity of or
To predict osmolality from S.G (e.g ) as follows:
Take the last 2 digits (for example: 06) and multiply by 40
06 X 40 = 240 mOsm/L
Normal urine osmolarity is  600 mOsm/lit (SG 1.015) (Fig. 28-1).
Osmolality of the urine varies from as low as 30 mOsm/L to as high as 1400 (30-fold difference). (30 mOsm/L concentration means moreBut if all these are absent, we can predict osmolality from S.G (e.g ) as follows:
Osmolality of the urine varies from as low as 30 mOsm/L to 1400 (30-fold difference). (30 mOsm/L concentration means at least 20 liters of urine per day to remove 600 mOsm/day)
This relationship, however, is altered when there are appreciable quantities of larger molecules in the urine, such as glucose, radiocontrast media, or some antibiotic s. In these settings, the specific gravity can reach to (falsely suggesting a very concentrated urine), despite a urine osmolality that may be only 300 mosmol/kg.

5. Making concentrated urine means conservation of body water.
- Making diluted urine mean eliminating excess water.
Observation:
Only those species that have juxtamedullary nephrons (long loop of Henle) can make concentrated urine (desert rodents). While beaver with short loop of Henle make less concentrated urine

6. To make diluted urine is somehow easy to understand
To make diluted urine is somehow easy to understand. The kidneys needs just to actively transport NaCl at the collecting duct and end up with diluted urine=hypo-osmolar urine.
To make concentrated urine was an enigma for 100 years (Active transport of water is not known). The micropuncture technique came up with the answer. Using the micropuncture technique scientists measured the osmolarity of inner medullary interstitium and found to be hyperosmolar.

7. Introduction
Before we start the lecture, we ask the following question: Why we must drink water?
Normally, we must excrete 700 mOsm\day as a minimum amount (in a complete bed rest individual), or 1000 mOsm/D at normal daily activity.
The kidney can make a concentrated urine up to 1400 mOsm/l
Therefore, 0.5 l is the minimum daily obligatory volume of urine (MDOVU): (700/1400) or 714 ml (1000/1400) during normal daily activity
MDOVU is:
Infants < 1 ml/kg/h
Children < 0.5 ml/kg/h
Adults <0.3 ml/kg/h
In general MDOVU= 300ml\m2 BSA/D

8. Water Balance Output Intake GIT 100 ml Metabolism 200 ml 1. Skin
GIT
100 ml
Metabolism
200 ml 1.
Skin
600 ml
Drinking
1600 ml 2.
Respiratory
300 ml
Food
700 ml 3.
Urine
1500 4.
2500

9. In order to form Concentrated Urine, two conditions must be met:
1. Hyperosmolar interstitium around the medullary collecting ducts
2. ADH to permit H2O permeability.

10. How to generate hyperosmolar interstitium?
By the so called “single effect”. In thick ascending loop of Henle, Na+-K+ -2Cl – transporter without H2O reabsorption, making interstitium hyperosmolar with 700 mOsm/L
- The rest is made by urea
- Na+ that is reabsorbed must not be removed from the area, and therefore blood flow in Vasa Recta must be low (2% of renal blood flow). High blood flow does not allow this hyperosmolar media to build up.

11. ADH Both ADH and oxytocin are composed of 9 aa (small peptides)
ADH: 5/6th from supraoptic neuclei and 1/6th from paraventricular nuclei.
ADH V2 receptor (AQP V2) cAMP  intracellular vesicles aquaporins  in luminal side (takes 5-10 min).
AQP V3 and V4 do not respond to ADH.
ADH receptors are found in last one third of DT and further.
Functions of ADH:
1.  H2O reabsorption from last one third of DT and further.
2. Enhances active NaCl reabsorption from last one third of DT and further.
3. Enhances urea reabsorption from IMCD.
The last two factors make medulla hyperosmolar.

12. Control of Extracellular Osmolarity (NaCl Concentration)
ADH
Thirst ]
ADH -Thirst Osmoreceptor System
Mechanism:
increased extracellular osmolarity (NaCl)
stimulates ADH release, which increases
H2O reabsorption, and stimulates thirst
(intake of water)

13. Water diuresis in a human after ingestion of 1 liter of water.

14. Formation of a dilute urine
Continue electrolyte reabsorption
Decrease water reabsorption
Mechanism:
Decreased ADH release and reduced water permeability in distal and collecting
tubules

15. Continue electrolyte reabsorption Increase water reabsorption
Formation of a Concentrated Urine
Continue electrolyte reabsorption
Increase water reabsorption
Mechanism :
Increased ADH release which increases water
permeability in distal and collecting tubules
High osmolarity of renal medulla
Countercurrent flow of tubular fluid

16. Formation of a Concentrated Urine when
antidiuretic hormone (ADH) are high.

17. Obligatory Urine Volume
Again: The minimum urine volume in which
the excreted solute can be dissolved and excreted
Example:
If the max. urine osmolarity is 1400 mOsm/L,
and 700 mOsm of solute must be excreted each
day to maintain electrolyte balance, the
obligatory urine volume is:
700 mOsm/d
1400 mOsm/L
= L/day

18. Obligatory Urine Volume
In renal disease the obligatory urine volume may be
increased due to impaired urine concentrating ability
Example:
If the max. urine osmolarity = 300 mOsm/L,
If 700 mOsm of solute must be excreted each
day to maintain electrolyte balance
obligatory urine volume = ?
700 mOsm/d
= 2.3L/day
300 mOsm/L

19. Active transport of Na+, Cl-, K+ and other ions from thick
Factors That Contribute to Buildup of Solute in Renal Medulla - Countercurrent Multiplier
Active transport of Na+, Cl-, K+ and other ions from thick
ascending loop of Henle into medullary interstitium
Active transport of ions from medullary collecting ducts
into interstitium
Passive diffusion of urea from medullary collecting ducts
Diffusion of only small amounts of water into medullary
interstitium

20. Summary of Tubule Characteristics
Permeability
Tubule
Active NaCl
Segment
Transport
H2O NaCl Urea
Proximal
Thin Desc
Thin Ascen
Thick Ascen
Distal ADH
Cortical Coll ADH
Inner Medullary + +ADH
Coll.

21. Countercurrent multiplier system in the loop of Henle.
Figure 28-4
Page 394

22. Net Effects of Countercurrent Multiplier
1. More solute than water is added to the renal medulla.
i.e solutes are “trapped” in the renal medulla
2. Fluid in the ascending loop is diluted
3. Most of the water reabsorption occurs in the cortex
(i.e. in the proximal tubule and in the distal
convoluted tubule) rather than in the medulla
4. Horizontal gradient of solute concentration established
by the active pumping of NaCl is “multiplied”
by countercurrent flow of fluid.

23. Recirculation of urea absorbed from medullary collecting duct into interstitial fluid.

24. Urea recycling
Urea toxic at high levels, but can be useful in small amounts.
Urea recycling causes buildup of high [urea] in inner medulla.
This helps create the osmotic gradient at loop of Henle so H2O can be reabsorbed.

25. Urea Recirculation Urea is passively reabsorbed in proximal tubule
(~ 50% of filtered load is reabsorbed)
In the presence of ADH, water is reabsorbed in
distal and collecting tubules, concentrating
urea in these parts of the nephron
The inner medullary collecting tubule is highly
permeable to urea, which diffuses into the
medullary interstitium
ADH increases urea permeability of medullary
collecting tubule by activating urea
transporters (UT-1)

26. The Vasa Recta Preserve Hyperosmolarity of Renal Medulla
The vasa recta serve as countercurrent exchangers
Vasa recta blood flow is low (only 1-2 % of total renal blood flow)
Figure 28-7

27. Changes in osmolarity of the tubular fluid
Figure 28-8
Page 352

28. Summary of water reabsorption and osmolarity in different parts of the tubule
Proximal Tubule: 65 % reabsorption, isosmotic
Desc. loop: 15 % reasorption, osmolarity increases
Asc. loop: 0 % reabsorption, osmolarity decreases
Early distal: 0 % reabsorption, osmolarity decreases
Late distal and coll. tubules: ADH dependent
water reabsorption and tubular osmolarity
Medullary coll. ducts: ADH dependent water reabsorption and tubular osmolarity

31. If: Uosm > Posm, CH2O = -
“Free” Water Clearance (CH2O)
(rate of solute-free water excretion)
CH2O = V -
Uosm x V
Posm
where:
Uosm = urine osmolarity
V = urine flow rate
P = plasma osmolarity
Cosm is osmolar clearance
Quantifying Renal Urine Concentration and Dilution:
FREE WATER AND OSMOLAR CLEARANCE:
Osmolar Clearance =Cosm
FREE WATER CLEARANCE
CH2O = V - Cosm
Normally is equal to (1 – 2.1) = -1.1 ml/min
If the number is positive, then we excrete water more than solute
(solute-free water excretion).
If: Uosm < Posm, CH2O = +
If: Uosm > Posm, CH2O = -

32. Question Given the following data, calculate “ free water” clearance :
urine flow rate = 6.0 ml/min
urine osmolarity = 150 mOsm /L
plasma osmolarity = 300 mOsm / L
Is free water clearance in this example positive
or negative ?

33. Answer Uosm x V Posm CH2O = V - = 6.0 - ( 150 x 6 ) 300 = 6.0 - 3.0
= ml / min (positive)

34. Disorders of Urine Concentrating Ability
Failure to produce ADH :
“Central” diabetes insipidus
Failure to respond to ADH:
“nephrogenic” diabetes insipidus
- impaired loop NaCl reabs. (loop diuretics)
- drug induced renal damage: lithium, analgesics
- malnutrition (decreased urea concentration)
- kidney disease: pyelonephritis, hydronephrosis,
chronic renal failure

35. Maximum urine osmolarity
Development of Isosthenuria With Nephron Loss in Chronic Renal Failure (inability to concentrate or dilute the urine)
300
600
1200
Maximum urine osmolarity
Osmolarity
(mOsm / L)
Plasma Osmolarity
Minimum urine osmolarity
Nephrons (% normal)

36. Total Renal Excretion and Excretion Per Nephron in Renal Failure
75 % loss of nephrons
Normal
Number of nephrons 2,000,000
Total GFR (ml/min 125
GFR per nephron (nl/min)
Total Urine flow rate (ml/min)
Volume excreted
per nephron (nl/min)
500,000 40 80
1.5
3.0

37. Osmoreceptor– antidiuretic hormone (ADH) feedback mechanism for regulating extracellular fluid osmolarity.

38. ADH synthesis in the magnocellular neurons of hypothalamus, release by the posterior pituitary, and action on the kidneys

39. Stimuli for ADH Secretion
Increased osmolarity
Decreased blood volume (cardiopulmonary reflexes)
Decreased blood pressure (arterial baroreceptors)
Other stimuli :
- input from cerebral cortex (e.g. fear)
- angiotensin II
- nausea
- nicotine
- morphine

40. ADH is released upon:
1. Increased osmolarity (Osmoreceptors in hypothalamus). An increase of 1% (2-3 mOsm/l) is enough to stimulate ADH secretion.
2. Decreased blood pressure (arterial baroreceptor reflex (high-pressure regions such as aortic and carotid sinus, and low-pressure regions such as cardiac atria and cardiopulmonary reflexes),
3. Decreased blood volume. We need a decrease of blood volume by 10% to stimulate ADH release.

41. The effect of increased plasma osmolarity or decreased blood volume.
Figure 28-11

42. Factors That Decrease ADH Secretion
Decreased osmolarity
Increased blood volume (cardiopulmonary reflexes)
Increased blood pressure (arterial baroreceptors)
Other factors :
- alcohol
- clonidine (antihypertensive drug)
- haloperidol (antipsychotic, Tourette’s)

43. Stimuli for Thirst Increased osmolarity Decreased blood volume
(cardiopulmonary reflexes)
Decreased blood pressure
( arterial baroreceptors)
Increased angiotensin II
Other stimuli:
- dryness of mouth

44. Factors That Decrease Thirst
Decreased osmolarity
Increased blood volume
(cardiopulmonary reflexes)
Increased blood pressure
( arterial baroreceptors)
Decreased angiotensin II
Other stimuli:
-Gastric distention

45. Sodium Intake ( mEq/day)
Effect of Changes in Sodium Intake on Plasma
Sodium After Blocking ADH-Thirst System
136
140
144
148
152
ADH-Thirst
blocked
Plasma
Sodium
Conc.
(mEq/L)
normal
Sodium Intake ( mEq/day)

46. Effect of Changes in Sodium Intake on Plasma
Sodium After Blocking Aldosterone or Ang II System
136
140
144
148
152
Plasma
Sodium
Conc.
(mEq/L)
Ang II Blocked
normal
aldosterone system blocked
Sodium Intake ( mEq/day)