UNIT V
Chapter 28:
Urine Concentration and Dilution;
Regulation of Extracellular Fluid Osmolarity
and Sodium Concentration
Slides by John E. Hall, PhD
Copyright 2011 by Saunders, an imprint of Elsevier Inc.
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)
Copyright 2011 by Saunders, an imprint of Elsevier Inc.
Osmoreceptor
antidiuretic hormone (ADH)
feedback mechanism for
regulating extracellular
fluid osmolarity.
Figure 28-9
Copyright 2011 by Saunders, an imprint of Elsevier Inc.
ADH synthesis in the
magnocellular neurons
of hypothalamus,
release by the posterior
pituitary, and action on
the kidneys
Figure 28-10
Copyright 2011 by Saunders, an imprint of Elsevier Inc.
Concentration and Dilution
of the Urine
Maximal urine concentration
= 1200 - 1400 mOsm / L
(specific gravity ~ 1.030)
Minimal urine concentration
= 50 - 70 mOsm / L
(specific gravity ~ 1.003)
Copyright 2011 by Saunders, an imprint of Elsevier Inc.
Water diuresis
in a human
after ingestion
of 1 liter of
water.
Figure 28-1
Copyright 2011 by Saunders, an imprint of Elsevier Inc.
Formation of a dilute urine
Continue electrolyte
reabsorption
Decrease water reabsorption
Mechanism:
Decreased ADH release
and reduced water
permeability in distal and
collecting
tubules
Figure 28-2
Copyright 2011 by Saunders, an imprint of Elsevier Inc.
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
Copyright 2011 by Saunders, an imprint of Elsevier Inc.
Formation of a Concentrated Urine when
antidiuretic hormone (ADH) are high.
Figure 28-4
Copyright 2011 by Saunders, an imprint of Elsevier Inc.
Obligatory Urine Volume
The minimum urine volume in which the excreted
solute can be dissolved and excreted
Example:
If the max. urine osmolarity is 1200 mOsm/L,
and 600 mOsm of solute must be excreted each
day to maintain electrolyte balance, the
obligatory urine volume is:
600 mOsm/d = 0.5 L/day
1200 mOsm/L
Copyright 2011 by Saunders, an imprint of Elsevier Inc.
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 600 mOsm of solute must be excreted each
day to maintain electrolyte balance
obligatory urine volume = ?
600 mOsm/d = 2.0 L/day
300 mOsm/L
Copyright 2011 by Saunders, an imprint of Elsevier Inc.
Summary of Tubule Characteristics
Tubule
Segment
Active NaCl
Transport
Proximal
Thin Desc.
Thin Ascen.
Thick Ascen.
Distal
Cortical Coll.
Inner Medullary
Coll.
++
0
0
+++
+
+
+
Permeability
H2O
NaCl
Urea
+++
+++
0
0
+ADH
+ADH
+ADH
+
+
+
0
0
0
0
+
+
+
0
0
0
+++
Copyright 2011 by Saunders, an imprint of Elsevier Inc.
Changes in osmolarity of the
tubular fluid
Figure 28-8
Copyright 2011 by Saunders, an imprint of Elsevier Inc.
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
Copyright 2011 by Saunders, an imprint of Elsevier Inc.
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
Copyright 2011 by Saunders, an imprint of Elsevier Inc.
Development of Isosthenuria With Nephron Loss in
Chronic Renal Failure (inability to concentrate or dilute the urine)
1200
Maximum urine osmolarity
Osmolarity 600
(mOsm / L)
300
Plasma Osmolarity
Minimum urine osmolarity
0
100
75
50
Nephrons (% normal)
25
Copyright 2011 by Saunders, an imprint of Elsevier Inc.
Total Renal Excretion and Excretion
Per Nephron in Renal Failure
Normal
Number of nephrons
2,000,000
Total GFR (ml/min
125
GFR per nephron (nl/min)
62.5
Total Urine flow rate (ml/min)
1.5
Volume excreted
0.75
per nephron (nl/min)
75 % loss of
nephrons
500,000
40
80
1.5
3.0
Copyright 2011 by Saunders, an imprint of Elsevier Inc.
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
Copyright 2011 by Saunders, an imprint of Elsevier Inc.
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, Tourettes)
Copyright 2011 by Saunders, an imprint of Elsevier Inc.
Stimuli for Thirst
Increased osmolarity
Decreased blood volume
(cardiopulmonary reflexes)
Decreased blood pressure
(arterial baroreceptors)
Increased angiotensin II
Other stimuli:
- dryness of mouth
Copyright 2011 by Saunders, an imprint of Elsevier Inc.
Factors That Decrease Thirst
Decreased osmolarity
Increased blood volume
(cardiopulmonary reflexes)
Increased blood pressure
(arterial baroreceptors)
Decreased angiotensin II
Other stimuli:
-Gastric distention
Copyright 2011 by Saunders, an imprint of Elsevier Inc.
