Regulation of Fluid and Electrolyte Balance

Prof. K. Sivapalan

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Regulation of Fluid and Electrolyte Balance

• Consumption of water can increase volume but reduce the osmolality of the fluids.

• Consumption of salt and other substances alter the osmolality and through that the volume of fluid compartments.

• The constancy of the composition and volume of the fluid compartments is maintained by many inter-related physiological mechanisms.

June 2013

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Regulation of Osmolality

• Osmoreceptors are located in anterior Hypothalamus.

• Stimulated by increase in osmolality of plasma and stimulate thirst- drinking water [?amount to balance osmolality].

• Anti diuretic Hormone secreation by posterior piotuitary is increased

• ADH acts in Kidney to reduce water excretion in urine.

June 2013

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Renal Excretion of Water and Electrolytes

• About 180 liters are flitered.

• About 7/8 reabsorbed in PCT

• Rest in the other portions of the tubule.

• About 18 liters of water absorbed in collecting dut under ADH

June 2013

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Effects of Drinking Water

June 2013

Drinking water dilutes plasma.

But due to inhibition of ADH secretion urine flow increases within 30 minutes without increase in solute excretion.

In about 2-2.5 hours excess water is eliminated.

Reverse occurs in dehydration

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Thirst Mechanism

• Increase in plasma osmolarity [even small- 2 mOsm of NaCl] provokes thirst and drinking.

• Salt appetite is increased consumption of salt in salt depletion.

June 2013

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Regulation of Volume

• Body fluid volume reflects on blood volume.

• Large veins and right atrium contain “volume receptors” in their walls.

• Distention increases and collapse reduces impulses sent to Hypothalamus to regulate ADH secretion.

• After reduction of 10 % of the blood.

June 2013

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Control of Sodium Balance

• Sodium is the main osmotic substance in plasma.

• Sodium balance [and the osmolarity of the plasma] is controlled by renin- angeotensin- aldesteron mechanism.

• Renin converts Angeotensinogen to Angeotensin I. In lung paranchyma, it is converted to Angeotensin II.

June 2013

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Aldosterone

• Angeotensin II stimulates secretion of aldosterone from Adrenal Cortex.

• Aldosterone increases sodium absorption in renal tubules, preventing excretion.

June 2013

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Over Hydration- Oedema

• Oedema is a condition seen as swelling

• Occurs when interstitial fluid is more than usual- increased filtration or reduced re-absorption.

• The cells are far apart and tissue pressure increases depending on the tension of the covering- skin, fascia or bone (brain).

• Reduced re-absorption will affect the tissues by not providing the nutrients.

• Increased tissue pressure can damage the cells.

June 2013

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Causes of Oedema

• Intracellular:-

– Depression of Metabolism or lack of nutrients resulting in failure of sodium pump.

– Inflammation resulting from increased cell membrane permeability

• Extracellular: increased capillary filtration [pitting] or lymphatic failure [non pitting]

June 2013

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Increased Tissue Fluid

• Increased capillary filtration coefficient [capillary permiability].

• Increased capillary hydrostatic pressure.

• Decreased plasma colloid osmotic pressure.

June 2013

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Increased capillary pressure• Excessive kidney retention of salt and water

– Acute or chronic kidney failure, Mineralocorticoid excess

• High venous pressure and venous constriction

– Heart failure, Venous obstruction, Failure of venous pumps

• Paralysis of muscles

– Immobilization of parts of the body, Failure of venous valves

• Decreased arteriolar resistance

– Excessive body heat, Insufficiency of sympathetic nervous system

June 2013

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Decreased plasma proteins

• Loss of proteins in urine (nephrotic syndrome)

• Loss of protein from denuded skin areas

– Burns

– Wounds

• Failure to produce proteins

– Liver disease (e.g., cirrhosis)

– Serious protein or caloric malnutritionJune 2013

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Increased capillary permeability

• Immune reactions that cause release of histamine and other immune products

• Toxins

• Bacterial infections

• Vitamin deficiency, especially vitamin C

• Prolonged ischemia

• Burns

June 2013

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Blockage of lymph return

• Cancer

• Infections (e.g., filaria nematodes)

• Surgery

• Congenital absence or abnormality of lymphatic vessels

June 2013

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Dehydration

• Filtration pressure decreased

• Tissue fluid formation reduced

• Tissue nutrition impaired

• Metabolites accumulate

• If prolonged, tissue damage

June 2013

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Causes of Dehydration

• Not drinking water- no drinking water or ? no time to drink

• Haemorrhage- loss of blood

• Burns- loss of plasma

• Excessive sweating- loss of hypotonic fluid

• Diarrhoea, vomiting- loss of sodium and potassium with water

• Excessive urine flow-

– Lack of ADH, Diabetes MellitusJune 2013