Chapter 20a
Integrative
Physiology II:
Fluid and Electrolyte
Balance
About this Chapter
•
•
•
•
•
Fluid and electrolyte homeostasis
Water balance
Sodium balance and ECF volume
Potassium balance
Behavioral mechanism in salt and water
balance
• Integrated control of volume and osmolarity
• Acid-base balance
Fluid and Electrolyte Homeostasis
Na+ and water
ECF volume and osmolarity
K+
Cardiac and muscle function
Ca2+
Exocytosis, muscle contractions, and other
functions
H+ and HCO3
–
Body must maintain mass
balance
pH balance
Excretion routes: kidney and lungs
Fluid and Electrolyte Homeostasis
• The body’s
integrated
responses to
changes in blood
volume and blood
pressure
Blood volume
Blood pressure
Volume receptors in atria and
carotid and aortic baroreceptors
trigger homeostatic reflexes
Cardiovascular
system
Cardiac output,
vasoconstriction
Behavior
Kidneys
Thirst causes
water intake
ECF and ICF
volume
KEY
Stimulus
Receptor
Blood
pressure
(a)
Conserve H2O
to minimize
further volume
loss
Effector
Tissue response
Systemic response
Figure 20-1a
Fluid and Electrolyte Homeostasis
Blood volume
Blood pressure
Volume receptors in atria,
endocrine cells in atria, and
carotid and aortic baroreceptors
trigger homeostatic reflexes
Cardiovascular
system
Cardiac output,
vasodilation
Kidneys
Excrete salts
and H2O in urine
ECF and ICF
volume
KEY
Stimulus
Receptor
Effector
Blood
pressure
Tissue response
Systemic response
(b)
Figure 20-1b
Water Balance
Water gain
2.2 L/day
Water loss
Food and drink
Skin
Insensible
water loss
0.9 L/day
Lungs
0.3 L/day
2.5 L/day
Metabolism
Totals
Intake
Metabolic production
+
–
2.2 L/day
0.3 L/day
Urine
1.5 L/day
Feces
0.1 L/day
2.5 L/day
Output
2.5 L/day
=0
Figure 20-2
Water Balance
• The kidneys conserve volume but cannot
replace lost volume
Volume gain
Volume loss
GFR can
be adjusted.
Glomerular
filtration rate
(GFR)
If
volume
falls
too low,
GFR
stops.
Body fluid
volume
Kidneys
recycle
fluid.
Kidneys
conserve
volume.
Regulated H2O
reabsorption
Volume
loss in
the urine
Figure 20-3
Urine Concentration
• Osmolarity changes as filtrate flows through
the nephron
Distal
tubule
Proximal
tubule
1 Isosmotic fluid leaving the
proximal tubule becomes
progressively more concentrated
in the descending limb.
300 mOsM
300 mOsM
300
CORTEX
100
MEDULLA
1
3
2
600 mOsM
Only water
reabsorbed
Ions
reabsorbed
but no
water
Permeability to
water and solutes
is regulated by
hormones.
Variable reabsorption
of water and solutes
900 mOsM
2 Removal of solute in the thick
ascending limb creates
hyposmotic fluid.
3 Hormones control distal nephron
permeability to water and solutes.
4 Urine osmolarity depends on
reabsorption in the collecting
duct.
Loop
of
Henle
1200
1200 mOsM
Collecting
duct
4
50–1200 mOsM
urine excreted
Figure 20-4
Urine Concentration
Distal
tubule
Proximal
tubule
1 Isosmotic fluid leaving the
proximal tubule becomes
progressively more concentrated
in the descending limb.
300 mOsM
300 mOsM
300
CORTEX
100
MEDULLA
1
Ions
reabsorbed
but no
water
600 mOsM
Only water
reabsorbed
Permeability to
water and solutes
is regulated by
hormones.
Variable reabsorption
of water and solutes
900 mOsM
Loop
of
Henle
1200
1200 mOsM
Collecting
duct
50–1200 mOsM
urine excreted
Figure 20-4, step 1
Water Reabsorption
• Vasopressin makes
the collecting duct
permeable to water
Figure 20-5a
Water Reabsorption
Figure 20-5b
Water Reabsorption
• Vasopressin causes insertion of water pores
into the apical membrane
Cross section of
kidney tubule
Collecting
duct
lumen
Medullary
interstitial
fluid
Collecting duct cell
Vasa
recta
1 Vasopressin binds to
membrane receptor.
Filtrate
300 mOsM
600 mOsM
600 mOsM
H2O
H2O
H2O
2 Receptor activates cAMP
second messenger system.
H2O
4
700 mOsM
Storage vesicles
3 Cell inserts AQP2 water
pores into apical membrane.
Second
2
messenger
signal
Exocytosis
of vesicles
3 Aquaporin-2
water pores
1
cAMP
Vasopressin
4 Water is absorbed by
osmosis into the blood.
Vasopressin
receptor
Figure 20-6
Water Reabsorption
Cross section of
kidney tubule
Collecting
duct
lumen
Medullary
interstitial
fluid
Collecting duct cell
Vasa
recta
1 Vasopressin binds to
membrane receptor.
Filtrate
300 mOsM
600 mOsM
600 mOsM
H2O
H2O
H2O
2 Receptor activates cAMP
second messenger system.
H2O
4
700 mOsM
Storage vesicles
3 Cell inserts AQP2 water
pores into apical membrane.
Second
2
messenger
signal
Exocytosis
of vesicles
3 Aquaporin-2
water pores
1
cAMP
Vasopressin
4 Water is absorbed by
osmosis into the blood.
Vasopressin
receptor
Figure 20-6, steps 1–4
Factors Affecting Vasopressin Release
Figure 20-7
Water Balance
• The effect of plasma osmolarity on
vasopressin secretion by the posterior
pituitary
Figure 20-8
Countercurrent Heat Exchanger
Warm
blood
Cold
blood
Warm Warm
blood blood
Heat lost
to external
environment
Limb
(a)
(b)
Figure 20-9
Water Balance
Filtrate entering the
descending limb
300
mOsM
600
Blood in the
vasa recta
300
mOsM
300
mOsM
500
500
600
The ascending limb pumps
out Na+, K+, and Cl–
100
mOsM
600
• Countercurrent
exchange in the
medulla of the
kidney
600
900
900
900
1200
Vasa recta
900
1200
mOsM
1200
mOsM
Loop of Henle
KEY
H2O =
K+ =
Cl– =
Na+ =
(a)
Figure 20-10a
Ion reabsorption
• Active reabsorption
of ions in the thick
ascending limb
creates a dilute
filtrate in the lumen
4 100 mOsm
leaving
the loop
3 Water cannot
follow solute
2 Salt
reabsorption
Cells of ascending
loop of Henle
1 1200 mOsm
entering
ascending
loop of Henle
Interstitial
fluid
(b)
KEY
H2O =
K+ =
Cl– =
Na+ =
Figure 20-10b
Fluid and Electrolyte Balance
• Vasa recta removes water
• Close anatomical association of the loop of
Henle and the vasa recta
• Urea increases the osmolarity of the
medullary interstitium
Sodium Balance
• Homeostatic
responses to
salt ingestion
Figure 20-11
Sodium Balance
Interstitial
fluid
P cell of distal nephron
Blood
1 Aldosterone combines with
a cytoplasmic receptor.
Lumen
of distal
tubule
2
1
3 Translation and
protein synthesis
New
channels
New pumps
4
Proteins modulate
existing channels and pumps
K+ secreted
5
Na+ reabsorbed
Aldosterone
Aldosterone
receptor
3 New protein channels and
pumps are made.
ATP
4 Aldosterone-induced
proteins modify existing
proteins.
K+
K+
Na+
2 Hormone-receptor complex
initiates transcription in
the nucleus.
ATP
K+
5 Result is increased Na+
reabsorption and
K+ secretion.
Na+
Na+
Figure 20-12
Sodium Balance
• The renin-angiotensin-aldosterone system
(RAAS)
Liver
Blood
pressure
constantly
produces
Granular
cells
(kidney)
Angiotensinogen
in the plasma
produce
Renin
ANG I in plasma
Blood vessel
endothelium
contains
ACE
(enzyme)
ANG II in
plasma
Arterioles
Vasoconstrict
Adrenal
cortex
Cardiovascular
control center
in medulla
oblongata
Cardiovascular
response
Hypothalamus
Aldosterone
Vasopressin
Blood
pressure
Thirst
Na+ reabsorption
Volume
and maintain
osmolarity
Figure 20-13
Sodium Balance
• Decreased blood pressure stimulates renin
secretion
Blood
pressure
Cardiovascular
control
center
GFR
direct effect
NaCl
transport
Sympathetic
activity
across
Macula densa
of distal tubule
Paracrines
Granular cells of
afferent arteriole
Renin
secretion
Figure 20-14
Sodium Balance
• Natriuretic peptides promote salt and water
excretion
Increased
blood volume
causes increased
atrial stretch
Myocardial
cells
stretch and
release
Natriuretic peptides
Hypothalamus
Less
vasopressin
Kidney
Increased
GFR
Decreased
renin
Adrenal
cortex
Less aldosterone
Medulla
oblongata
Decreased
blood pressure
NaCl and
H2O excretion
Figure 20-15