Fluids and Electrolytes

1. Fluids andElectrolytes MALIK ALQUB MD. PhD.

2. The Cell Has a Limited Repetoire H20 moves passively Across cell membrane According to the osmotic gradient K+140 meq/L 280 milliosmoles/L Na+140 meq/L 280 milliosmoles/L

3. High Osmolality Outside The Cell=Shrinkage H2O K+140 meq/L 280 milliosmoles/L Na+150 meq/L 300 milliosmoles/L

4. Low Osmolality Outside The Cell=Swelling H20 K+140 meq/L 280 milliosmoles/L Na+120 meq/L 240 milliosmoles/L RUPTURE

5. Body Fluid Compartments • 2/3 (65%) of TBW is intracellular (ICF) • 1/3 extracellular water • 25 % interstitial fluid (ISF) • 5- 8 % in plasma (IVF intravascular fluid) • 1- 2 % in transcellular fluids – CSF, intraocular fluids, serous membranes, and in GI, respiratory and urinary tracts;

6. Body Fluids • Water is most abundant body compound“Average” body water volume in reference tables based on healthy, nonobese 70-kg male • Volume averages 42 L in a 70-kg male • Plasma (3.5 L) • Interstitial fluid (10.5 L) • Intracellular fluid (28 L) • Water is about 80% of body weight in newborn; about 60% in adult males; and about 50% in adult females

8. Movement of fluids • Fluid compartments are separated by membranes that are freely permeable to water. • Movement of fluids due to: • hydrostatic pressure • osmotic pressure • Capillary filtration (hydrostatic) pressure • Capillary colloid osmotic pressure • Interstitial hydrostatic pressure • Tissue colloid osmotic pressure

9. Osmotic Pressure • When a solution containing non-penetrating solutes is separated from pure water by a membrane, the pressure that must be applied to the solution to prevent the net flow of water into the solution (prevent osmosis) is termed the osmotic pressure of the solution. • The greater the osmolarity, the greater its osmotic pressure. • The lower the water concentration, the higher the osmotic pressure. • Osmotic pressure of a solution is directly proportional to the concentration of osmotically active particles in that solution

10. OSMOSIS • Movement of the solvent or water across a membrane • Involves solution or water • Equalizes the concentration of ions on each side of membrane • Movement of solvent molecules across amembrane to an area where there is a higherconcentration of solute that cannot pass through the membrane

12. OSMOLALITY • Measure of solution’s ability to create osmotic pressure & thus affect movement of water • Number of osmotically active particles per kilogram of water • Plasma osmolality is 280-300* mOsm/ kg • ECF osmolality is determined by sodium • MEASURE used in clinical practice to evaluate serum & urine

13. Osmolality In Clinical Practice • Serum 280-300mOsm/kg; Urine 50-1400mOsm/kg • Serum osmolality can be estimated by doubling serum sodium • More prescisely • 2X Na + urea + glucose • Values are in mmol/L

14. Osmolarity Regulation • ICF Osm. = ECF Osm. • Interstitial Osm = Serum Osm. • Hypothalamus is the serum osmostat. It stimulates thirst and ADH secretion. • Primary Defense for Osmolarity = Thirst • Primary Defense for Osmolarity = Renal excretion of water via ADH effect

15. Osmolarity Regulation • Maximum concentrating ability of kidney is approximately 800-1600mOsm/kg H20 • Max. ADH effect decreases urine output to approximately 500 cc/day • No ADH release increases urine output to 15-20 Liters per day. Uosm = 40 – 80 mOsm/kg H20

16. Fluid Balance

17. Fluid Balance • Water circulates freely in ECF compartment • ECF and ICF are normally in osmotic equilibrium and no large-scale circulation occurs between compartments • If abnormal amounts of water move from plasma into interstitial fluid  called? • Increases in plasma osmolality trigger thirst and release of antidiuretic hormone (ADH)

18. Primary Regulatory Hormones • Affect fluid and electrolyte balance: • antidiuretic hormone (ADH) • aldosterone • natriuretic peptides

19. Antidiuretic Hormone (ADH) • Stimulates water conservation at kidneys: • reducing urinary water loss • concentrating urine • Stimulates thirst center: • promoting fluid intake

20. ADH Production • Osmoreceptors in hypothalamus monitor osmotic concentration of ECF (plasma, CSF) • Change in osmotic concentration in plasma and CSF alters osmoreceptor activity • Osmoreceptor neurons secrete ADH in proportion to osmotic concentraiton via the posterior pituitary

21. Aldosterone • Is secreted by adrenal cortex in response to: • rising K+ (sensed at the adrenal cortex) or falling Na+ levels in blood • activation of renin–angiotensin system (usually due to changes in blood volume) • Determines rate of Na+ absorption and K+ loss along DCT and collecting system • “Water Follows Salt” • High plasma aldosterone concentration causes kidneys to conserve salt • Conservation of Na+ by aldosterone also stimulates water retention

22. Aldosterone Figure 26.8

23. Natriuretic Peptides • ANP and BNP are released by cardiac muscle cells in response to abnormal stretching of heart walls due to elevated blood pressure or volume • Reduce thirst • Block release of ADH and aldosterone • Cause diuresis • Lower blood pressure and plasma volume

24. Fluid Shifts • Rapid water movements between ECF and ICF in response to an osmotic gradient • If ECF osmotic concentration increases: • ECF becomes hypertonic to ICF • water moves from inside cells to ECF • If ECF osmotic concentration decreases: • ECF becomes hypotonic to ICF • water moves from ECF into cells

25. Water Losses • Dehydration develops when water losses exceed water gains • If water is lost, but electrolytes retained: • ECF osmotic concentration rises • water moves from ICF to ECF in a fluid shift • Both ECF and ICF will be slightly more concentrated than before but they will be osmotically balanced • net change in ECF is small • homeostatic responses will occur to replace lost water

26. Water Losses • If water is lost, but electrolytes retained, ECF (and ICF) have higher concentrations, lower volumes • hypothalamus senses elevated ECF osmolarity this and releases ADH to restore fluid balance • New water in the ECF will shift into ICF and restore volumes and concentrations

27. Severe Water Loss • Causes: • excessive perspiration • inadequate water consumption • repeated vomiting • diarrhea

28. Water Gains • If water is gained, but electrolytes are not: • ECF volume increases • ECF becomes hypotonic to ICF • fluid shifts from ECF to ICF • Basically the opposite of water loss: • Reach osmotic balance but at lower concentrations, higher volumes • may result in overrhydration: • distorts cells • changes solute concentrations around enzymes • disrupts normal cell functions

29. Water Gains • If water is gained, but electrolytes are not: • ECF is at lower concentration, higher volume • This triggers decrease in ADH release, fluid is lost and ICF will lose some water back to ECF, restoring both volume and concentration balance

30. Causes of Overhydration • Ingestion of large volume of fresh water • Injection into bloodstream of hypotonic solution • Endocrine disorders like excessive ADH production • Inability to eliminate excess water in urine: • chronic renal failure • heart failure • cirrhosis

31. Disorders of Water Balance: Figure 26.7a

32. peripheral and presacral edema pulmonary edema jugular venous distension hypertension Decreased hematocrit decr. serum protein poor skin turgor dry mucous membranes flat neck veins hypotension increased hematocrit Increased serum prot. HypervolemiaHypovolemia

33. pitting edema

34. jugular venous distension

35. Electrolyte Balance

36. Electrolyte Balance • Requires equal rates of gain and loss for each electrolyte in the body • Electrolyte concentration directly affects water balance • Concentrations of individual electrolytes affect cell functions

37. Solutes – dissolved particles • Electrolytes – charged particles • Cations – positively charged ions • Na+, K+ , Ca++, H+ • Anions – negatively charged ions • Cl-, HCO3- , PO43- • Non-electrolytes . • Proteins, urea, glucose, O2, CO2

38. Rules of Electrolyte Balance • Most common problems with electrolyte balance are caused by imbalance between gains and losses of sodium ions • Problems with potassium balance are less common, but more dangerous than sodium imbalance • Changes in plasma sodium levels affect: • Plasma volume, blood pressure • ICF and interstitial fluid volumes

39. Na+, K+ • Sodium holds a central position in fluid and electrolyte balance • Sodium is the dominant cation in ECF • Sodium salts provide 90-95% of ECF osmolarity (concentration): • sodium chloride (NaCl) • sodium bicarbonate • Sodium concentration in the ECF normally remains stable • Potassium Is the dominant cation in ICF

40. SODIUM (NA) • Main extracellular fluid (ECF) cation • Helps govern normal ECF osmolality • Helps maintain acid-base balance • Activates nerve & muscle cells • Influences water distribution (with chloride)

41. Na+ Regulation So changes in sodium concentration are corrected by ADH (not aldosterone) Figure 27–4

42. Abnormal Na+Concentrations in ECF • Hyponatremia: • usu. body water content rises (overhydration) • Hypernatremia: • usu. body water content declines (dehydration) • Severe problems with electrolyte concentrations almost always occur secondary to fluid balance problems

43. HYPERNATREMIA • Serum Na + level > 148 mEq/L • serum osmolality > 295 mOsm/kg

44. Etiologies of Hypernatremia Primary Sodium Excess Primary Water Loss Poor Intake of Water Impaired access to water (i.e. infants, elderly patients with dementia or whom are bedbound) Impaired thirst sensation Hypothalamic lesions Increased Urinary Loss of Water ADH deficiency (Central Diabetes Isipidus DI) ADH resistance (Nephrogenic DI) Increased GI Loss of Water Increased Transcutaneous Loss of Water Transmembrane Shift of Water (most often due to rapid production of intracellular lactate) Excess Intake of Sodium Decreased Urinary Excretion of Sodium Hyperaldosteronism

45. HYPONATREMIA • Serum Na+ < 135 mEq/L (patient may be asymptomatic until level drops below 125)

46. Etiologies of Hyponatremia Primary Sodium Loss Primary Water Excess Excessive Intake of Water (1°polydipsia) Psychosis Decreased Urinary Excretion of Water Decreased GFR Increased ADH Heart failure Cirrhosis SIADH Transmembrane Shift of Water Hyperglycemia Poor Intake of Sodium Increased Urinary Loss of Sodium Diuretics Proximal RTA Aldosterone deficiency/resistance Increased GI Loss of Sodium (Fluid loss must be followed by repletion with free water). Vomitting Diarrhea Increased Transcutaneous Loss of Sodium (Fluid loss must be followed by repletion with free water).

47. Potassium Balance • 98% of potassium in the human body is in ICF • Cells expend energy to recover potassium ions diffused from cytoplasm into ECF • Factors • Rate of gain across digestive epithelium • Rate of loss into urine, regulated along distal portions of nephron and collecting system as Na+ from tubular fluid is exchanged for K+ in peritubular fluid

48. Mechanisms of regulation • Renal regulation • Transcellular shift between the intracellular and extracellular compartments

49. Factors in Tubular Secretion of K+ • Changes in concentration of ECF: • higher ECF concentration increases rate of secretion (just because there’s more of it) • Aldosterone levels affect K+ loss in urine • ion pumps reabsorb Na+ from filtrate in exchange for K+ from peritubular fluid • High K+ plasma concentrations stimulate aldosterone release, lower K+ but Na+ stays • Changes in pH: • low ECF pH lowers peritubular fluid pH • H+ rather than K+ is exchanged for Na+ in tubular fluid so ECF K+ increases

50. Transcellular shifts • Sodium-potassium ATPase • Both insulin and epinephrine increase the activity of sodium-potassium pump. (An increase in potassium level stimulates insulin release. --- a feedback mechanism) • Potassium-hydrogen exchange to maintain electrical neutrality • In acidosis • In alkalosis