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Water and electrolyte balance

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Distribution of water and electrolytes in different compartments and their regulation

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Water and electrolyte balance

  1. 1. Water and Electrolyte Balance R. C. Gupta M.D. (Biochemistry) Jaipur (Rajasthan), India
  2. 2. Water is the most abundant component of our body Need for water is more urgent than that for any other nutrient Humans beings can live one month without food but only six days without water EMB-RCG
  3. 3. EMB-RCG In adults, water accounts for: 70% of the total body weight in males 60% of the total body weight in females
  4. 4. EMB-RCG Water content depends on age: Infants: 75% Adults: 60-70% Elderly: 45%
  5. 5. EMB-RCG Water content differs in different tissues: Muscles: 70% Adipose tissue: 30% Bones: 10% Water content is more in muscular persons than in obese persons
  6. 6. Water: Bathes all cells Gives shape and form to cells Serves as a lubricant Is the solvent for all ions and molecules Transports materials to and from cells Is the medium for all biochemical reactions
  7. 7. Latent heat of evaporation Specific heat Dielectric constant Solvent power Some properties of water which make it an ideal medium for body fluids are its: Water has been chosen as the universal solvent for all living organisms
  8. 8. Solvent power Water is an efficient and suitable solvent for most of the solutes present in our body Some compounds which do not dissolve readily in water can form colloidal solutions
  9. 9. Water has a high dielectric constant A large number of oppositely charged particles can co-exist in water due to this Dielectric constant
  10. 10. Specific heat Water has a very high specific heat which means that a large amount of heat is required to raise the temperature of water Due to this, body temperature doesn’t rise appreciably when thermal energy is released during oxidation of nutrients
  11. 11. Latent heat of evaporation Water has a high latent heat of evaporation relative to other liquids A large amount of thermal energy is required for evaporation of water When water evaporates from skin and lungs, a large amount of heat is lost This prevents a rise in body temperature
  12. 12. Distribution of water Compartment Water Total water in an average man 50 litres Water in intra-cellular compartment 35 litres Water in extra-cellular compartment 15 litres
  13. 13. Un-exchangeable fluid The water present outside the cells is known as extra-cellular fluid (ECF) The ECF is further distributed into some sub-compartments: Trans-cellular fluid Interstitial fluid Plasma
  14. 14. Sub-compartment Volume 3 litresPlasma (vascular compartment) Interstitial fluid (in between cells) 7 litres Trans-cellular fluid (in cavities) 1 litre 4 litres Un-exchangeable fluid (in bones, cartilages, dense connective tissue etc)
  15. 15. Osmolality Concentration of solutes/particles in fluid, expressed in milliosmol (mosm) per kg Determines distribution of water in different compartments Water moves from lower to higher osmolality
  16. 16. The major osmotically active solutes in body fluids are: Electrolytes have more osmotic power as they dissociate into at least two particles Non-electrolytes e.g. glucose, lipids etc Electrolytes e.g. inorganic salts and proteins
  17. 17. Intracellular Interstitial Plasma fluid fluid CATIONS (mEq/L) Sodium 10 137 142 Potassium 160 5 5 Magnesium 24 3 3 Calcium 6 5 5 Total 200 150 155 ANIONS (mEq/L) Chloride 5 113 100 Bicarbonate 5 27 27 Sulphate 15 1 1 Inorganic phosphate 25 2 2 Organic phosphates 70 – – Organic anions 15 5 5 Proteins 65 2 20 Total 200 150 155
  18. 18. Effective osmolality of a compartment is determined by the solutes restricted to that compartment Effective osmolality of the compartment is also known as its tonicity
  19. 19. Selective distribution of ions in different compartments is maintained by specific ion channels and ion pumps A lot of energy is spent for maintaining the differential distribution of ions in different compartments
  20. 20. Cations Sodium is the major cation in extracellular fluid Potassium is the major cation in intracellular fluid This differential is maintained by Na+, K+- exchanging ATPase EMB-RCG
  21. 21. Anions The major anions in extracellular fluid are chloride and bicarbonate The major anions in intracellular fluid are phosphates and proteins
  22. 22. Proteins Proteins are present in a: Fairly high concentration in intracellular fluid Smaller but significant concentration in plasma Negligible concentration in interstitial fluid
  23. 23. Effective osmolality is determined by: Sodium and its associated anions in the extracellular fluid Potassium and its associated anions in the intracellular fluid
  24. 24. The ions and molecules have specific distribution in the intracellular fluid These are vital for the functioning of the cells, and are zealously maintained
  25. 25. Changes in osmolality are usually due to shift of salts (mainly sodium) When salts shift, water follows salts
  26. 26. Shrinkage of cells due to shifting of water out of the cells can seriously affect the functioning of cells Swelling of cells due to shifting of water into the cells can also seriously affect the functioning of cells
  27. 27. Hyper-osmolality of extracellular fluid draws water out of cells into the extra- cellular compartment Hypo-osmolality of extracellular fluid drives water from extracellular compart- ment into the cells
  28. 28. Osmolality of plasma is 275-290 mosmol/kg A 0.9% solution of NaCl in water has the same osmolality (or tonicity) as plasma A 5% solution of glucose in water also has the same osmolality (or tonicity) as plasma These two are said to be isosmotic or isotonic with plasma
  29. 29. Oncotic pressure Osmotic pressure exerted by proteins is called oncotic pressure It is also known as colloid osmotic pressure The normal oncotic pressure of plasma is about 25 mm of Hg
  30. 30. A decrease in the concentration of proteins in plasma decreases oncotic pressure of plasma Water is forced out of capillaries at the arterial end due to greater hydrostatic pressure It cannot re-enter at the venous end if the oncotic pressure is less than the hydrostatic pressure This will result in oedema
  31. 31. Water intake and output Water balance of the body depends upon the relative intake and output of water Water is taken in as drinking water and in the form of food and beverages Some water is formed in the body during oxidative reactions (metabolic water)
  32. 32. Metabolic water Oxidation of 1 gm of carbohydrate produces 0.60 gm of water Oxidation of 1 gm of fat produces 1.07 gm of water Oxidation of 1 gm of protein produces 0.41 gm of water
  33. 33. In a temperate climate, intake of water is: Source Volume Drinking water about 1.5 L /day Water in food and beverages about 1.0 L /day Metabolic water about 0.3 L /day Total intake about 2.8 L /day
  34. 34. Route Volume Urine about 1.5 L /day Faeces about 0.1 L /day Water vapour in expired air about 0.4 L /day Water loss in the form of sweat about 0.8 L /day Total output about 2.8 L /day Water is lost from the body in the form of:
  35. 35. In hot climates, sweat loss is much more This is compensated by increased intake of drinking water If it is not compensated, urine output will decrease However, urine output cannot decrease below a certain level
  36. 36. Normal excretion of solutes by the kidneys is about 600 milliosmol/day Minimum water required to dissolve 600 milliosmol solutes is 500 ml If urine output is below 500 ml/day, excretion of metabolic waste decreases A urine output below 500 ml/day is called oliguria
  37. 37. Regulation of water balance Water balance is maintained by: The thirst centre in hypothalamus Antiduretic hormone of posterior pituitary These two receive signals about osmolality of plasma from osmoreceptors located in the hypothalamus
  38. 38. Osmo-receptors can perceive a change of even 1-2% in the osmolality of plasma If there is an increase in the osmolality of plasma: Thirst centre is stimulated which increases water intake Posterior pituitary secretes anti- diuretic hormone which decreases urine output
  39. 39. ADH secretion begins when the osmolality of plasma reaches about 285 mosmol/kg The thirst centre is stimulated when the osmolality of plasma reaches about 295 mosmol/kg
  40. 40. When blood circulates through the kidneys, 125 ml of glomerular filtrate is formed per minute About 180 litres of glomerular filtrate is formed in 24 hours Glomerular filtration rate
  41. 41. When the filtrate passes through the tubules, a large amount of solutes and water are absorbed The re-absorption can be divided into: Obligatory re- absorption Facultative re- absorption Tubular re-absorption
  42. 42. A large amount of solutes is absorbed when the filtrate passes through proximal convoluted tubules and loop of Henle A corresponding amount of water is re- absorbed due to osmotic effect of solutes This is known as obligatory re-absorption Obligatory re-absorption
  43. 43. Obligatory re-absorption equals: About 85% of the glomerular filtrate Or about 153 litres per day
  44. 44. Cells of distal convoluted tubules and collecting ducts are not permeable to water in the absence of ADH Binding of ADH to its receptors (V2 receptors) on the surface of these cells activates adenylate cyclase Facultative re-absorption
  45. 45. Active adenylate cyclase increases the intracellular concentration of cAMP cAMP activates protein kinase A Active protein kinase A phosphorylates some cytosolic proteins
  46. 46. The phosphorylated proteins translocate aquaporins from cytosol into cell membrane Aquaporins are water channels Water moves into the cell through these water channels
  47. 47. Movement of water into distal convoluted tubules and collecting ducts is proportional to plasma ADH concentration The ADH-regulated re-absorption is known as facultative re-absorption of water Normally, this is about 25.5 litres/day
  48. 48. About 1.5 litres of water is not absorbed by tubules This is excreted in the form of urine every day Facultative re-absorption can be adjusted to maintain the water balance of the body
  49. 49. Electrolyte balance Sodium, potassium and chloride are the major electrolytes Their plasma levels are: Sodium: 135 -145 mEq/L Potassium: 3.5 - 5.0 mEq/L Chloride: 96 -106 mEq/L
  50. 50. Sodium The most important cation in regulation of fluid and electrolyte balance The most abundant cation in the ECF Contibutes significant osmotic pressure
  51. 51. Potassium Critical to maintenance of membrane potential Compensates for shifts of hydrogen ions in or out of cells
  52. 52. Chloride The most abundant anion in the ECF Contributes significant osmotic pressure
  53. 53. Regulation of sodium Aldosterone promotes tubular re- absorption of sodium Oesrogens have a similar but weaker effect Atrial natriuretic peptide inhibits release of aldosterone
  54. 54. Plasma K+ level regulates potassium balance High plasma K+ level promotes tubular secretion of potassium Low plasma K+ level inhibits tubular secretion of potassium Aldosterone increases potassium secretion Regulation of potassium
  55. 55. Regulation of chloride Chloride is the major anion associated with sodium It moves with sodium Aldosterone increases the tubular reabsorption of chloride
  56. 56. Dehydration can result from diminished intake of water or excessive loss of water Excessive water loss is a far more common cause of dehydration Dehydration
  57. 57. Excessive water loss can be due to: • Excessive sweating • Vomiting • Diarrhoea • Haemorrhage • Burns
  58. 58. Excessive water loss can also occur in uncontrolled diabetes mellitus To dissolve the glucose being excreted in urine, urinary water output increases
  59. 59. Excess water loss in urine may also occur in renal diseases This happens when the kidneys fail to reabsorb water e.g. in chronic glomerulo- nephritis
  60. 60. Extremely severe water loss can occur in diabetes insipidus Diabetes insipidus can be: Central diabetes insipidus Nephrogenic diabetes insipidus
  61. 61. Central diabetes insipidus is due to decreased secretion of ADH Nephrogenic diabetes insipidus is due to decreased responsiveness of target cells to ADH
  62. 62. Dehydration is corrected by administra- tion of fluids The fluids may be given orally or intra- venously The composition of the fluid given should be similar to that of the fluid lost Correction of dehydration
  63. 63. Excessive retention of water can occur in acute renal failure Kidneys fail to excrete water in acute renal failure Sometimes, it can result from over- administration of intravenous fluids Water intoxication
  64. 64. Hypersecretion of ADH is a rare cause of water retention Apart from treatment of the primary cause, diuretics may be used to increase the output of urine
  65. 65. Most diuretics act by inhibiting the tubular reabsorption of some solutes Water is lost in urine to dissolve the extra solutes Diuretics
  66. 66. Some commonly used diuretics are: • Acetazolamide • Spironolactone • Thiazides • Furosemide • Ethacrynic acid • Mannitol
  67. 67. Acetazolamide is a competitive inhibitor of carbonic anhydrase It decreases the formation of carbonic acid in proximal convoluted tubules Normally, carbonic acid dissociates into H+ and HCO3 – Acetazolamide
  68. 68. H+ is secreted into tubular fluid in exchange for Na+ By disrupting this exchange, acetazola- mide increases urinary Na+ excretion Extra water is excreted to dissolve Na+ Excessive use of acetazolamide can cause acidosis due to H+ retention
  69. 69. Spironolactone is a structural analogue of aldosterone Due to structural resemblance, it binds to aldosterone receptors This prevents the action of aldosterone on distal convoluted tubules Spironolactone
  70. 70. When the action of spironolactone is blocked, excretion of sodium and chloride increases Water excretion is increased due to the osmotic effect of sodium and chloride
  71. 71. Thiazides inhibit sodium re-absorption in the distal convoluted tubules They also increase potassium loss Thiazides
  72. 72. Furosemide decreases reabsorption of sodium and chloride in the loop of Henle Hence, it is known as a loop diuretic It is a potassium-sparing diuretic as it does not cause potassium loss Furosemide
  73. 73. Action of ethacrynic acid is very similar to that of furosemide This is also a potassium-sparing loop diuretic Ethacrynic acid
  74. 74. Mannitol is an osmotic diuretic It is filtered by the glomeruli but is not re-absorbed by the tubules Extra water is lost in urine due to the osmotic effect of mannitol Mannitol
  75. 75. Dehydration described earlier is never due to a pure water loss The fluids lost from the body contain electrolytes also The loss usually occurs from the extra- cellular compartment as the intracellular fluid is tightly protected ECF contraction and expansion
  76. 76. Dehydration results in a decrease in ECF volume (ECF contraction) Depending upon the osmolality of the fluid lost, ECF contraction can be: Isotonic Hypotonic Hypertonic
  77. 77. Retention of water causes an increase in the volume of ECF (ECF expansion) ECF expansion can be: Isotonic Hypotonic Hypertonic
  78. 78. Isotonic contraction or expansion of ECF does not affect the ICF If ECF becomes hypotonic or hypertonic, secondary changes occur in the ICF
  79. 79. Isotonic fluid is lost from the body Can occur in diarrhoea due to loss of isotonic secretions Can occur in intestinal obstruction due to collection of secretions in the gut Isotonic ECF contraction
  80. 80. Hypertonic fluid is lost from the body Can occur in Addison’s disease due to excessive loss of sodium and chloride in urine Hypotonic ECF contraction
  81. 81. Hypotonic fluid is lost from the body Can occur in fevers and heat exposure due to excessive sweating or insensible perspiration Hypertonic ECF contraction
  82. 82. Isotonic fluid accumulates in interstitial tissue Can occur due to oedema caused by hypertension, congestive heart failure, nephrotic syndrome, cirrhosis of liver etc Isotonic ECF expansion
  83. 83. More water is retained than solutes Can occur in acute glomerulonephritis due to decreased glomerular filtration Hypotonic ECF expansion
  84. 84. Retention of solutes is more than that of water Can occur in primary aldosteronism and Cushing’s disease due to retention of sodium and chloride Hypertonic ECF expansion
  85. 85. ECF contraction clinically manifests as a decrease in blood volume (hypovolaemia) Sudden and excessive loss of fluids from the body can cause life-threatening hypo- volaemia Hypovolaemia
  86. 86. But hypovolaemia is not always due to loss of fluids It can occur when the total body water is normal, or even increased It may be due to shifting of water from the vascular compartment into interstitial tissue
  87. 87. A decrease in blood volume decreases the blood pressure Restoration of blood volume and blood pressure requires the actions of: Renin-angiotensin system Aldosterone ADH
  88. 88. Compensatory mechanisms may be unable to correct hypovolaemia: If it is too severe If the pathological condition causing hypovolaemia persists
  89. 89. Pathological conditions causing hypovolaemia may do so by causing: Fluid loss Redistribution of water
  90. 90. Renal Na and H2O loss can occur in: • Chronic renal diseases • Diabetes mellitus • Addison’s disease • Diabetes insipidus etc Extra-renal Na and H2O loss can occur in: • Fevers • Vomiting • Diarrhoea • Intestinal obstruction • Haemorrhage • Burns etc
  91. 91. A shift of water from the vascular compartment into interstitial tissue (oedema) can cause hypovolaemia Redistribution of water Oedema can occur due to a decrease in oncotic pressure of plasma or due to an increase in capillary permeability
  92. 92. Common causes of oedema are: Congestive heart failure Nephrotic syndrome Cirrhosis of liver
  93. 93. Hypovolaemia can be: Isotonic Hypotonic Hypertonic
  94. 94. Isotonic hypovolaemia can occur due to: Diarrhoea Intestinal obstruction
  95. 95. Hypotonic hypovolaemia can occur due to: Chronic renal disease Excessive use of diuretics Addison’s disease Congestive heart failure Nephrotic syndrome Cirrhosis of liver
  96. 96. Hypertonic hypovolaemia can occur due to: Fevers Heat exposure Severe burns
  97. 97. Treatment of hypovolaemia should comprise: Treatment of the primary cause Correction of fluid balance
  98. 98. In hypovolaemia due to shifting of water from vascular compartment, correction requires salt restriction and diuretics In hypovolaemia due to sodium and water loss, correction requires oral or intravenous administration of fluids
  99. 99. Oral rehydration is preferable if hypo- volaemia is mild Severe cases require intravenous fluids
  100. 100. In isotonic hypovolaemia, isotonic (0.9%) saline should be given In hypotonic hypovolaemia, hypertonic (3%) saline is preferable In hypertonic hypovolaemia, hypotonic (0.45%) saline or 5% GDW (glucose in distilled water) is preferable Intravenous fluids
  101. 101. While giving intravenous fluids, a watch should be kept on serum potassium Care should be taken not to over- hydrate the patient Fluid imbalance may be accompanied by disturbances in acid-base balance Acid-base imbalance should also be corrected along with the fluid imbalance

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