Regulation of Body Fluid
Balance
Osmotic Relations Between Intracellular
Fluid, Interstitial Fluid and Plasma
Plasma
Na+
Na+
protein
protein
Intracellular
fluid
H2O
Na+
K+
H2O
K+
K+
Interstitial
fluid
Crucial points
• Animal plasma membranes are so delicate that no
osmotic gradient between ISF and ICF can exist.
• Only impermeant solutes can act as osmotic effectors
• Cytoplasmic protein is the major osmotic effector of the
ICF; its osmotic effect is balanced by the transmembrane
Na+ gradient, otherwise cells would swell.
• Plasma proteins are the major osmoeffectors of plasma
– they counteract the effect of capillary hydrostatic
pressure.
• Na+ is the major osmoeffector of ECF versus ICF. ECF
volume closely tracks total body Na+ content.
Characteristics Of ICF and ECF
Compartments
Intracellular Fluid
Extracellular
Fluid
30 L total volume
15 L total volume
9000 mOsm total
solute
4500 mOsm total
solute
Posm = 300 mOsm
2175 mEq total
Na+
[Na+] = 145
mEq/L
Posm = 300
mOsm
The ECF consists of the ISF compartment and the
plasma compartment
Extracellular
Fluid
15 L total volume
4500 mOsm total
solute
2175 mEq total
Na+
Posm = 300
mOsm
Interstitial Fluid
Plasma
11.25 L total
volume
3.75 L
total
volume
3375 mOsm
total solute
Posm = 300
mOsm
1125
mOsm
total
solute
Posm =
300
mOsm
There are three basic homeostatic
challenges
• Gain or loss of isotonic solution
– Affects only the ECF volume
• Gain or loss of pure water
– Both ICF and ECF compartments change volume
proportionately – osmotic concentration changes in
each are equal
• Gain or loss of pure salt
– Na+ is confined to the ECF compartment – loss results
in volume shift from ECF to ICF; gain results in
volume shift from ICF to ECF.
Regulation of Renal Function
• Intrinsic
• Baroreceptor Reflex
• Three endocrine systems
– ADH system
– Renin-Angiotensin-Aldosterone System
– Atrial Natriuretic Hormone system
Intrinsic regulation
Blood Volume
Arterial Blood Pressure
GFR
Intrinsic regulation + Baroreceptor reflex
Blood Volume
Arterial Blood Pressure
Baroreceptor
Reflex
GFR
Afferent
arteriole
dilates
ADH system
“Peripheral volume receptors” are
stretch receptors located in the right
atrium – increased stretch signals a
plasma volume increase and exerts an
inhibitory effect on ADH secretion
Osmoreceptor cell bodies are in
ventromedial hypothalamus –
sensitive mainly to [Na+]
ADH = arginine vasopressin – an
octapeptide with two major peripheral
effects:
Increased water permeability of
collecting duct
Vasoconstriction (at high levels)
Response of ADH system in gain of pure water
Response of ADH system to loss of pure water
The Renin-Angiotensin-Aldosterone System –
response to loss of pure Na+ or loss of isotonic
solution
• Macula densa (Juxtaglomerular
apparatus) secretes renin (a protease)
when:
– Blood [Na+] falls below normal
– Glomerular blood volume flow decreases
Angiotensin cascade
Angiotensinogen
Renin
Angiotensin I
Angiotensin Converting
Enzyme (ACE) in lung
Angiotensin II
Adrenal Cortex
Distal tubule (also
sweat glands,
salivary glands,
colon, etc.
Aldosterone
Increased Na+ reabsorption
3 Major factors that increase Aldo secretion
Increased Plasma [K+]
Adrenocorticotrophic
Hormone (ACTH)
Angiotensin II
Adrenal Cortex
Aldosterone
Kidney distal tubule
Na+ reabsorption
K+ secretion
H+ secretion
Aldosterone effects
• Steroid hormone that increases
expression of Na+/K+ ATPase in target
epithelia
• Directly regulates total body Na+ Indirectly regulates ECF volume.
• Also involved in K+ regulation – by a direct
effect on the adrenal cortex: increased
plasma [K+] increases aldo secretion
Atrial natriuretic peptide – response
to gain of isotonic solution
• Stretched atria release 22 aa peptide
which
– increases GFR by vasodilating renal afferent
arterioles and constricting efferent arterioles
– Inhibits Aldo secretion and antagonizes
tubular effect of aldosterone
– Inhibits ADH secretion and blocks its action
• Causes marked diuresis (volume loss) and
natriuresis (net loss of Na+ )
Study Goals
• Be able to trace the responses of each of the 3
major renal endocrine systems to each of the 3
simple homeostatic challenges.
• Integrate your understanding of these systems
with what you know about the baroreceptor
reflex and capillary filtration to arrive at a
complete picture of whole-body responses to
blood loss and plasma volume expansion – i.e.
short term and long term regulation of mean
arterial pressure.