Regulation of water and Electrolyte
Balance (II)
Water, sodium & potassium balance
Lecture Outline
I. 
Water balance
II. 
Sodium balance
III.  Potassium balance
1
I. Water Balance
•  Osmolarity and the movement of water
•  Water reabsorption in the proximal tubule
•  Establishment of the medullary osmotic gradient
•  Role of the medullary osmotic gradient in water
reabsorption in the distal tubule and collecting
duct
Exchange of Water
•  Intake
•  Gastrointestinal tract (2.2 L/day)
•  Metabolism (0.3 L/day)
•  Output
•  Insensible loss & Sweating (0.9 L/day)
•  Gastrointestinal tract (0.1 L/day)
•  Kidneys (1.5 L/day)
Osmosis
Water diffuses down concentration gradient
•  Water moves from low solute to high solute
•  Water reabsorption follows solute reabsorption
2
Factors Affecting Water Balance
Note: Kidneys can only minimize fluid loss
Intake is required in order to add fluids
G & S: Figure 19.3
Osmolarity of Fluids
•  Osmolarity of body fluids = 300 mOsm
•  Kidneys compensate for changes in osmolarity of
extracellular fluid by regulating water reabsorption
Water reabsorption:
•  Passive based on osmotic gradient
•  Proximal tubules
•  70% filtered water is reabsorbed
•  Not regulated
•  Distal tubules and Collecting ducts
•  Most remaining water is reabsorbed
•  20% distal tubules
•  10% collecting ducts
•  Regulated by ADH
3
Water Reabsorption in the Proximal Tubule
G & S: Figure 19.5
Medullary Osmotic Gradient for Water Reabsorption
•  Osmolarity of interstitial fluid of renal medulla
varies with depth
•  Lower osmolarity near cortex
•  Greater osmolarity near renal pelvis
•  Gradient critical to water reabsorption
4
The Medullary Osmotic Gradient
Osmotic gradient established by counter-current multiplier
in the Loop of Henle
G & S: Figure 19.6
Counter-Current Multiplier
Peritubular fluid
1
2
3
4
Tubular fluid
Fluid
300
Cortex
Medulla
300
300
No osmotic
gradient
(mOsm)
300
300
Na+
Cl–
300
K+
300
300
300
300
300
300
350
300
300
400
400
400
400
200
400
200
400
300
Iso-osmotic state in
descending limb;
osmotic difference
between descending
and ascending limbs
150
300
400
350
350 150
H 2O
400
500 300
H 2O
400 500 300
350
500
Na+
Cl–
400
K+
200
400
400
More fluid
enters tubule,
pushing fluid
through by
bulk flow
Active
transport of
Na+, Cl–, K+
ions into
medullary
interstitial
fluid increases
osmolarity
7
300
150
H 2O
300
200
400 400 400
200
300
350
300
400
300
6
200
300
300
200
400 400 200
Water moves
out of descending
limb by osmosis
5
300
300
200
300 400 200
Active transport
of Na+, Cl–, K+ ions
into medullary
interstitial fluid
increases
osmolarity
300
300
H 2O
400
200
H 2O
400 200
300
300
300
400
300
300
300 300 300
Fluid
enters
tubule
300
350
150
500
300
Osmotic
gradient
established
(mOsm)
300
100
300
300
100
500
500
300
900
900
700
1200 1200 1100
1400
500 500 300
1400
Water moves
out of descending
limb by osmosis
Iso-osmotic state in descending
limb; osmotic difference
between descending and
ascending limbs
More water
enters tubule
and process
continues
System
is in steady
state
G & S: Figure 19.7
5
Result of Counter-Current Multiplier
•  Fluid in proximal tubule = 300 mOsm
•  Fluid in descending limb—osmolarity increases as it
descends
• 
Osmolarity = interstitial fluid
• 
Osmolarity > ascending limb
•  Fluid in ascending limb—osmolarity decreases as it ascends
• 
Osmolarity < interstitial fluid, descending limb
•  Fluid in distal tubule = 100 mOsm
•  Cortical interstitial fluid = 300 mOsm
•  Medullary interstitial fluid
• 
Increases from cortex to renal pelvis
Vasa Recta: Countercurrent Exchanger
G & S: Figure 19.8
6
Water Reabsorption in Distal Tubules
and Collecting Ducts
•  Dependent on osmotic gradient established
by counter-current multiplier
•  Dependent on epithelium permeability to water
•  Water permeability dependent on water channels
•  Aquaporin-3: present in basolateral membrane
always
•  Aquaporin-2: present in apical membrane only
when ADH present in blood
Effects of ADH on Principal Cells
G & S: Figure 19.10
7
Regulation of ADH Release
•  ADH = posterior pituitary hormone
•  Released from neurosecretory cells originating in
hypothalamus
•  Primary stimulus for release
•  Increased osmolarity (osmoreceptors)
•  Other stimuli
•  Increase blood pressure (baroreceptors)
•  Increased blood volume (volume receptors)
Responses: Increase in ECF Osmolarity / Decrease in BP
G & S: Figure 19.11 & 12
8
II. Sodium Balance
• 
• 
• 
• 
• 
• 
• 
• 
Hypernatremia = high plasma sodium
Hyponatremia = low plasma sodium
Sodium—primary solute in ECF
•  Critical for normal osmotic pressure
•  Critical to function of excitable cells
Renal Handling of Sodium
Freely filtered
Reabsorbed in proximal tubule, distal tubule, and collecting duct
No secretion
Reabsorption regulated by aldosterone and ANP
Reabsorption regulated at principal cells of distal tubule and
collecting duct
Proximal Tubule/distal tubule Sodium Reabsorption
Coupled to the
reabsorption of
other solutes
Coupled to the secretion
of K+ and H+
G & S: Figure 19.13
9
Effects of Aldosterone on Sodium Reabsorption
•  Aldosterone increases sodium reabsorption
•  Steroid hormone
•  Secreted from adrenal cortex
•  Acts on principal cells of distal tubules and
collecting ducts
•  Increases number of Na+/K+ pumps on basolateral
membrane
•  Increases number of open Na+ and K+ channels on
apical membrane
Effects of Aldosterone
G & S: Figure 19.14
10
Renin-Angiotensin-Aldosterone System
[NA+] tubular fluid ( MAP)
G & S: Figure 19.15
Angiotensin II Effect on MAP
Angiotensin II
Systemic arterioles
Adrenal cortex
Posterior pituitary
Vasoconstriction
Aldosterone secretion
ADH secretion
Hypothalamic neurons
Thirst stimulation
Kidneys
Sodium reabsorption
in late distal tubules
and collecting ducts
Water reabsorption
in late distal tubules
and collecting ducts
Extracellular fluid
osmolarity
Plasma volume
MAP
G & S: Figure 19.16
11
Atrial Natriuretic Peptide
•  Secreted by atrial cells in response to distension
of atrial wall
•  Increases GFR
•  Dilation of afferent arteriole
•  Constriction of efferent arteriole
•  Decreases sodium reabsorption by closing sodium
channels in apical membrane
•  Overall effect: increased sodium excretion
III. Potassium Balance
•  Hyperkalemia = high plasma potassium
•  Hypokalemia = low plasma potassium
•  Potassium crucial to function of excitable cells
Renal Handling of Potassium Ions
•  Glomerulus—freely filtered
•  Proximal tubules—reabsorbed
•  Distal tubules and collecting ducts—reabsorbed and
secreted
•  Potassium secretion in distal tubules and collecting
ducts is regulated
•  Aldosterone regulates principal cells
12
Proximal Tubule: Potassium Reabsorption
G & S: Figure 19.19a
Distal and Collecting Tubule: Potassium Secretion
G & S: Figure 19.19b
13