Regulation of Fluid and Electrolyte Balance Prof K

Regulation of Fluid and Electrolyte Balance Prof. K. Sivapalan

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 Regulation of fluid and electrolytes 2

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 Regulation of fluid and electrolytes 3

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 Regulation of fluid and electrolytes 4

Effects of Drinking Water 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 June 2013 Regulation of fluid and electrolytes 5

Thirst Mechanism • Increase in plasma osmolarity [even small 2 m. Osm of Na. Cl] provokes thirst and drinking. • Salt appetite is increased consumption of salt in salt depletion. June 2013 Regulation of fluid and electrolytes 6

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 Regulation of fluid and electrolytes 7

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 Regulation of fluid and electrolytes 8

Aldosterone • Angeotensin II stimulates secretion of aldosterone from Adrenal Cortex. • Aldosterone increases sodium absorption in renal tubules, preventing excretion. June 2013 Regulation of fluid and electrolytes 9

Over Hydration- Oedema • Oedema is a condition seen as swelling • Occurs when interstitial fluid is more than usualincreased 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 Regulation of fluid and electrolytes 10

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 Regulation of fluid and electrolytes 11

Increased Tissue Fluid • Increased capillary filtration coefficient [capillary permiability]. • Increased capillary hydrostatic pressure. • Decreased plasma colloid osmotic pressure. June 2013 Regulation of fluid and electrolytes 12

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 Regulation of fluid and electrolytes 13

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 malnutrition June 2013 Regulation of fluid and electrolytes 14

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 Regulation of fluid and electrolytes 15

Blockage of lymph return • Cancer • Infections (e. g. , filaria nematodes) • Surgery • Congenital absence or abnormality of lymphatic vessels June 2013 Regulation of fluid and electrolytes 16

Dehydration • Filtration pressure decreased • Tissue fluid formation reduced • Tissue nutrition impaired • Metabolites accumulate • If prolonged, tissue damage June 2013 Regulation of fluid and electrolytes 17

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 Mellitus June 2013 Regulation of fluid and electrolytes 18

Fluids used in replacement • ORS- Oral rehydration fluid • 5% dextrose • Normal saline • Other solutions June 2013 Regulation of fluid and electrolytes 19