Regulation of Body Fluid Volume and Plasma Osmolarity
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Transcript of Regulation of Body Fluid Volume and Plasma Osmolarity
8/10/2019 Regulation of Body Fluid Volume and Plasma Osmolarity
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Regulation of Body fluid volumeand plasma osmolality
Mr. Lok Bandhu Chaudhary
Department of Basic and ClinicalPhysiology
8/10/2019 Regulation of Body Fluid Volume and Plasma Osmolarity
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overview
•
Introduction• Body fluid volume and compartments
• Osmolar substances in extracellular and
intracellular fluids.• Pressure Natriuresis and Pressure diuresis
mechanism.
• Regulation of Extracellular fluid volume.• Regulation of extracellular osmolarity
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Introduction
Regulation of Extracellular fluid volume andplasma osmolarity are closely linked.
In case of ECF volume, the control systemmodulates the urinary excretion of Na+ .
To maintain plasma osmolality, the controlsystem modulates the urinary excretion ofwater.
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Body fluid volume and compartments
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osmolarity
Definition: The osmolal concentration of a
solution is called osmolality. When the
concentration is expressed as osmoles per Kg
of water, it is termed as osmolarity.
Plasma osmolarity(Posm) can be roughly
approximated as;
Posm = 2.1 × plasma sodium concentration
ECF Osmolarity = 280-295 mosmol/Kg
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Osmolar substances in Extracellular
and intracellular body fluids
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Pressure Natriuresis and Pressure
diuresis
• Pressure diuresis: it refers to the effect of increasedpressure to raise urinary volume excretion.
•
Pressure Natriuresis: it refers to the rise in Na+
excretion that occurs with elevated blood pressure.
• Both pressure natriuresis and pressure diuresis
mechanism is independent of changes in activity of; Sympathetic nervous system
Various hormones such as Ang II, ADH or Aldosterone.
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Fig; Acute and chronic effects of arterial Pressure on sodium output by the kidneys
(Pressure Natriuresis)
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Control of Extracellular fluid volume
The body regulates ECF volume by monitoring and
adjusting the total body content of NaCl.
The critical parameter that the body recognizes as anindex of change in Na content is the Effective
circulating volume.
Effective circulating volume is controlled by followingcontrol mechanisms;
•
Sympathetic Nervous system• Renin-Angiotensin-aldosterone system
• Arginin vasopressin(AVP) and
• Atrial Natriuretic peptide(ANP)
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Sympathetic Nervous system control
of Renal excretion
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Effect of changes in GFR on urinary Na
excretion
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Renin-angiotensin-aldosterone system
Fig: Renin-angiotensin-aldosterone axis
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Functions of Angiotensin II
1. Stimulation of aldosterone release from
glomerulosa cells in the adrenal cortex.
2. Vasoconstriction of renal and other systemic
vessels.
3. Enhanced Na-H exchange.
4. Renal hypertrophy.
5. Stimulated thirst and AVP release.
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Role of Angiotensin II in controlling
Renal excretion
Fig; effect of excessive ang II formation and effect of blocking ang II formation
On the renal-pressure natriuresis curve
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Role of Aldosterone and Angiotensin II
• Aldosterone increases
sodium reabsorption,
especially in the cortical
collecting tubules.• Ang II and aldosterone
have little effect on
sodium concentration,
except under extremeconditions(Addison’s
disease).Fig; Effect of large changes in sodium intake on ECF Na
concentration in dogs under normal conditions
and after aldosterone feedback system had been blocked
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Role of Atrial Natriuretic peptide(ANP)
in controlling Renal excretion
• ANP is a peptide released by thecardiac atrial muscle fibres.
• ANP increases GFR and
decreases Na reabsorption bythe collecting ducts.
• Other Natriuretic peptides are;
a. BNP andb. CNP
db k l f ff i i l i
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Feedback control of Effective circulating
volume
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Control of water content
(Extracellular Osmolarity)
• Water accounts for half or more of body weight( ̴60% in men and 50% in women).
•
Osmolarity deviations of ±15% lead to severedisturbances of CNS function.
• Two elements control the whole body osmolarity
are;a. The kidneys: control excretion and
b. Thirst mechanisms: control the oral intake ofwater.
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Control of AVP synthesis and release
Fig; Arginin vasopressin synthesis and release by osmoreceptors
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Control of thirst
• Fluid intake is regulated by the thirstmechanism, which together withosmoreceptor-ADH mechnismmaintains precise control of ECFosmolarity and Na concentration.
• The osmoreceptors that triggerthirst are located in twocircumventricular organs, the OVLTand SFO.
• When Na concentration increasesonly about 2 mEq/L above normal,the thirst mechanism is activated,causing desire to drink water.
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Feedback control of Extracellular
Osmolality
Fig; Feedback systems involved in the control of Osmolality
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Fig: Effect of sodium intake on ECF concentration under normal
conditions and after blockade of ADH and thirst systems
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References
1. Textbook of Medical Physiology, Guyton andHall, 11th edition
2. Ganong’s Review of medical Physiology, 24th
edition3. Medical Physiology; Walter F. Boron, Emile L.
Boulpaep, 2nd edition
4. Understanding Medical Physiology; R L Bijlani,
3rd edition
5. Vander’s Human Physiology, 11th edition