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



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



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.



Body fluid volume and compartments



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



Osmolar substances in Extracellular

and intracellular body fluids



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.



Fig; Acute and chronic effects of arterial Pressure on sodium output by the kidneys

(Pressure Natriuresis)



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)



Sympathetic Nervous system control

of Renal excretion



Effect of changes in GFR on urinary Na

excretion



Renin-angiotensin-aldosterone system

Fig: Renin-angiotensin-aldosterone axis



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.



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



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



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



Feedback control of Effective circulating

volume



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.



Control of AVP synthesis and release

Fig; Arginin vasopressin synthesis and release by osmoreceptors



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.



Feedback control of Extracellular

Osmolality

Fig; Feedback systems involved in the control of Osmolality



Fig: Effect of sodium intake on ECF concentration under normal

conditions and after blockade of ADH and thirst systems



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

regulation of body fluid volume and plasma osmolarity

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