1. Ch 24 Water, Electrolyte, & Acid-Base Balance Jen Tynes SELU

2. Balance cellular function requires a fluid medium with a carefully controlled composition cellular function requires a fluid medium with a carefully controlled composition three types of homeostatic balance three types of homeostatic balance –water balance average daily water intake and loss are equal average daily water intake and loss are equal –electrolyte balance the amount of electrolytes absorbed by the small intestine balance with the amount lost from the body, usually in urine the amount of electrolytes absorbed by the small intestine balance with the amount lost from the body, usually in urine –acid-base balance the body rids itself of acid (hydrogen ion – H + ) at a rate that balances metabolic production the body rids itself of acid (hydrogen ion – H + ) at a rate that balances metabolic production balances maintained by the collective action of the urinary, respiratory, digestive, integumentary, endocrine, nervous, cardiovascular, and lymphatic systems balances maintained by the collective action of the urinary, respiratory, digestive, integumentary, endocrine, nervous, cardiovascular, and lymphatic systems 24-2

3. Water, Electrolyte & Acid-Base Balance

4. Body Water newborn baby’s body weight is about 75% water newborn baby’s body weight is about 75% water young men average 55% - 60% young men average 55% - 60% women average slightly less women average slightly less obese and elderly people as little as 45% by weight obese and elderly people as little as 45% by weight total body water (TBW) of a 70kg (150 lb) young make is about 40 liters total body water (TBW) of a 70kg (150 lb) young make is about 40 liters 24-4

5. Water Balance Total body water for 150 lb. male = 40L Total body water for 150 lb. male = 40L Fluid compartments Fluid compartments –65% ICF –35% ECF 25% tissue fluid 25% tissue fluid 8% blood plasma, lymph 8% blood plasma, lymph 2% transcellular fluid (CSF, synovial fluid) 2% transcellular fluid (CSF, synovial fluid)

6. 24-6 Water Movement Between Fluid Compartments fluid continually exchanged between compartments fluid continually exchanged between compartments water moves by osmosis water moves by osmosis because water moves so easily through plasma membranes, osmotic gradients never last for very long because water moves so easily through plasma membranes, osmotic gradients never last for very long if imbalance arises, osmosis restores balance within seconds so the intracellular and extracellular osmolarity are equal if imbalance arises, osmosis restores balance within seconds so the intracellular and extracellular osmolarity are equal –if osmolarity of the tissue fluid rises, water moves out of the cell –if it falls, water moves in osmosis from one fluid compartment to another is determined by the relative concentrations of solutes in each compartment osmosis from one fluid compartment to another is determined by the relative concentrations of solutes in each compartment –electrolytes – the most abundant solute particles, by far –sodium salts in ECF –potassium salts in ICF electrolytes play the principal role in governing the body’s water distribution and total water content electrolytes play the principal role in governing the body’s water distribution and total water content

7. Water Movement in Fluid Compartments Electrolytes play principle role in water distribution and total water content Electrolytes play principle role in water distribution and total water content

8. Water Gain Preformed water Preformed water –ingested in food and drink Metabolic water Metabolic water –by-product of aerobic metabolism and dehydration synthesis

9. Water Loss Routes of loss Routes of loss –urine, feces, expired breath, sweat, cutaneous transpiration Loss varies greatly with environment and activity Loss varies greatly with environment and activity –respiratory loss  : with cold, dry air or heavy work –perspiration loss  : with hot, humid air or heavy work Insensible water loss Insensible water loss –breath and cutaneous transpiration Obligatory water loss Obligatory water loss –breath, cutaneous transpiration, sweat, feces, minimum urine output (400 ml/day)

10. Fluid Balance

11. Regulation of Fluid Intake Dehydration Dehydration –  blood volume and pressure –  blood osmolarity Thirst mechanisms Thirst mechanisms –stimulation of thirst center (in hypothalamus) angiotensin II: produced in response to  BP angiotensin II: produced in response to  BP ADH: produced in response to  blood osmolarity ADH: produced in response to  blood osmolarity hypothalamic osmoreceptors: signal in response to  ECF osmolarity hypothalamic osmoreceptors: signal in response to  ECF osmolarity –inhibition of salivation thirst center sends sympathetic signals to salivary glands thirst center sends sympathetic signals to salivary glands

12. Satiation Mechanisms Short term (30 to 45 min), fast acting Short term (30 to 45 min), fast acting –cooling and moistening of mouth –distension of stomach and intestine Long term inhibition of thirst Long term inhibition of thirst –rehydration of blood (  blood osmolarity) stops osmoreceptor response,  capillary filtration,  saliva stops osmoreceptor response,  capillary filtration,  saliva

13. Dehydration, Thirst, and Rehydration

14. Regulation of Output Controlling Na + reabsorption (changes volume) Controlling Na + reabsorption (changes volume) –as Na + is reabsorbed or excreted, water follows Action of ADH (changes concentration of urine) Action of ADH (changes concentration of urine) –ADH secretion (as well as thirst center) stimulated by hypothalamic osmoreceptors in response to dehydration –aquaporins synthesized in response to ADH membrane proteins in renal collecting ducts to channel water back into renal medulla, Na + is still excreted membrane proteins in renal collecting ducts to channel water back into renal medulla, Na + is still excreted –effects: slows  in water volume and  osmolarity

15. Secretion and Effects of ADH

16. Disorders of Water Balance Fluid deficiency Fluid deficiency –volume depletion (hypovolemia) total body water , osmolarity normal total body water , osmolarity normal hemorrhage, severe burns, chronic vomiting or diarrhea hemorrhage, severe burns, chronic vomiting or diarrhea –dehydration total body water , osmolarity rises total body water , osmolarity rises lack of drinking water, diabetes, profuse sweating, diuretics lack of drinking water, diabetes, profuse sweating, diuretics infants more vulnerable infants more vulnerable –high metabolic rate demands high urine excretion, kidneys cannot concentrate urine effectively, greater ratio of body surface to mass affects all fluid compartments affects all fluid compartments –most serious effects circulatory shock, neurological dysfunction, infant mortality circulatory shock, neurological dysfunction, infant mortality

17. Water Loss & Fluid Balance 1) profuse sweating 2) produced by capillary filtration 3) blood volume and pressure drop, osmolarity rises 4) blood absorbs tissue fluid to replace loss 5) fluid pulled from ICF

18. 24-18 Fluid Balance in Cold Weather the body conserves heat by constricting blood vessels of the skin forcing blood to deeper circulation the body conserves heat by constricting blood vessels of the skin forcing blood to deeper circulation –raises blood pressure which inhibits secretion of ADH –increases secretion of atrial natriuretic peptide –urine output is increased and blood volume reduced cold air is drier and increases respiratory water loss also reducing blood volume cold air is drier and increases respiratory water loss also reducing blood volume cold weather respiratory and urinary loses cause a state of reduced blood volume (hypovolemia) cold weather respiratory and urinary loses cause a state of reduced blood volume (hypovolemia) –exercise will dilate vessels in skeletal muscles –insufficient blood for rest of the body can bring on weakness, fatigue, or fainting (hypovolemic shock)

19. Fluid Excess Volume excess Volume excess –both Na + and water retained, ECF isotonic –aldosterone hypersecretion Hypotonic hydration Hypotonic hydration –more water than Na + retained or ingested, ECF hypotonic - can cause cellular swelling Most serious effects Most serious effects –pulmonary and cerebral edema

20. Blood Volume & Fluid Intake Kidneys compensate very well for excessive fluid intake, but not for inadequate intake

21. Fluid Sequestration Excess fluid in a particular location Excess fluid in a particular location Most common form: edema Most common form: edema –accumulation in the interstitial spaces Hematomas Hematomas –hemorrhage into tissues; blood is lost to circulation Pleural effusions Pleural effusions –several liters of fluid may accumulate in some lung infections

22. Electrolytes Function Function –chemically reactive in metabolism –determine cell membrane potentials –affect osmolarity of body fluids –affect body’s water content and distribution Major cations Major cations –Na +, K +, Ca 2+, H + Major anions Major anions –Cl -, HCO 3 -, PO 4 3-

23. 24-23 Electrolyte Concentrations Figure 24.7 145 4 103 5 4 300 (a) Blood plasma 12 150 4 < 1 75 300 (mEq/L) (b) Intracellular fluid Osmolarity (mOsm/L) Na + K+K+ Cl – Ca 2+ PiPi Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

24. Sodium - Functions Membrane potentials Membrane potentials Accounts for 90 - 95% of osmolarity of ECF Accounts for 90 - 95% of osmolarity of ECF Na + - K + pump Na + - K + pump –exchanges intracellular Na + for extracellular K + –creates gradient for cotransport of other solutes (glucose) –generates heat NaHCO 3 has major role in buffering pH NaHCO 3 has major role in buffering pH

25. Sodium - Homeostasis Primary concern - excretion of dietary excess Primary concern - excretion of dietary excess –0.5 g/day needed, typical diet has 3 to 7 g/day Aldosterone - “salt retaining hormone” Aldosterone - “salt retaining hormone” –  # of renal Na + /K + pumps,  Na+ and  K + reabsorbed –hypernatremia/hypokalemia inhibits release ADH -  blood Na + levels stimulate ADH release ADH -  blood Na + levels stimulate ADH release –kidneys reabsorb more water (without retaining more Na + ) ANP (atrial natriuretic peptide) – from stretched atria ANP (atrial natriuretic peptide) – from stretched atria –kidneys excrete more Na + and H 2 O, thus  BP/volume Others - estrogen retains water during pregnancy Others - estrogen retains water during pregnancy –progesterone has diuretic effect

26. Sodium - Imbalances Hypernatremia Hypernatremia –plasma sodium > 145 mEq/L from IV saline from IV saline –water retension, hypertension and edema Hyponatremia Hyponatremia –plasma sodium < 130 mEq/L –result of excess body water, quickly corrected by excretion of excess water

27. Potassium - Functions Most abundant cation of ICF Most abundant cation of ICF Determines intracellular osmolarity Determines intracellular osmolarity Membrane potentials (with sodium) Membrane potentials (with sodium) Na + -K + pump Na + -K + pump

28. Potassium - Homeostasis 90% of K + in glomerular filtrate is reabsorbed by the PCT 90% of K + in glomerular filtrate is reabsorbed by the PCT DCT and cortical portion of collecting duct secrete K + in response to blood levels DCT and cortical portion of collecting duct secrete K + in response to blood levels Aldosterone stimulates renal secretion of K + Aldosterone stimulates renal secretion of K +

29. Secretion and Effects of Aldosterone

30. Potassium - Imbalances Most dangerous imbalances of electrolytes Most dangerous imbalances of electrolytes Hyperkalemia-effects depend on rate of imbalance Hyperkalemia-effects depend on rate of imbalance –if concentration rises quickly, (crush injury) the sudden increase in extracellular K + makes nerve and muscle cells abnormally excitable –slow onset, inactivates voltage-gated Na + channels, nerve and muscle cells become less excitable Hypokalemia Hypokalemia –from sweating, chronic vomiting or diarrhea –nerve and muscle cells less excitable muscle weakness, loss of muscle tone,  reflexes, arrthymias muscle weakness, loss of muscle tone,  reflexes, arrthymias

31. Potassium & Membrane Potentials

32. Chloride - Functions ECF osmolarity ECF osmolarity –most abundant anions in ECF Stomach acid Stomach acid –required in formation of HCl Chloride shift Chloride shift –CO 2 loading and unloading in RBC’s pH pH –major role in regulating pH

33. Chloride - Homeostasis Strong attraction to Na +, K + and Ca 2+, which it passively follows Strong attraction to Na +, K + and Ca 2+, which it passively follows Primary homeostasis achieved as an effect of Na + homeostasis Primary homeostasis achieved as an effect of Na + homeostasis

34. Chloride - Imbalances Hyperchloremia Hyperchloremia –result of dietary excess or IV saline Hypochloremia Hypochloremia –result of hyponatremia Primary effects Primary effects –pH imbalance

35. Calcium - Functions Skeletal mineralization Skeletal mineralization Muscle contraction Muscle contraction Second messenger Second messenger Exocytosis Exocytosis Blood clotting Blood clotting

36. Calcium - Homeostasis PTH PTH Calcitriol (vitamin D) Calcitriol (vitamin D) Calcitonin (in children) Calcitonin (in children) –these hormones affect bone deposition and resorption, intestinal absorption and urinary excretion Cells maintain very low intracellular Ca 2+ levels Cells maintain very low intracellular Ca 2+ levels –to prevent calcium phosphate crystal precipitation phosphate levels are high in the ICF phosphate levels are high in the ICF

37. Calcium - Imbalances Hypercalcemia Hypercalcemia –alkalosis, hyperparathyroidism, hypothyroidism –  membrane Na + permeability, inhibits depolarization –concentrations > 12 mEq/L causes muscular weakness, depressed reflexes, cardiac arrhythmias Hypocalcemia Hypocalcemia –vitamin D , diarrhea, pregnancy, acidosis, lactation, hypoparathyroidism, hyperthyroidism –  membrane Na + permeability, causing nervous and muscular systems to be abnormally excitable –very low levels result in tetanus, laryngospasm, death

38. Phosphates - Functions Concentrated in ICF as Concentrated in ICF as –phosphate (PO 4 3- ), monohydrogen phosphate (HPO 4 2- ), and dihydrogen phosphate (H 2 PO 4 - ) Components of Components of –nucleic acids, phospholipids, ATP, GTP, cAMP, creatine phosphate Activates metabolic pathways by phosphorylating enzymes Activates metabolic pathways by phosphorylating enzymes Buffers pH Buffers pH

39. Phosphates - Homeostasis Renal control Renal control –if plasma concentration drops, renal tubules reabsorb all filtered phosphate Parathyroid hormone Parathyroid hormone –  excretion of phosphate Imbalances not as critical Imbalances not as critical –body can tolerate broad variations in concentration of phosphate

40. Acid-Base Balance Important part of homeostasis Important part of homeostasis –metabolism depends on enzymes, and enzymes are sensitive to pH Normal pH range of ECF is 7.35 to 7.45 Normal pH range of ECF is 7.35 to 7.45 Challenges to acid-base balance Challenges to acid-base balance –metabolism produces lactic acids, phosphoric acids, fatty acids, ketones and carbonic acids

41. Acids and Bases Acids Acids –strong acids ionize freely, markedly lower pH –weak acids ionize only slightly Bases Bases –strong bases ionize freely, markedly raise pH –weak bases ionize only slightly

42. Buffers Resist changes in pH Resist changes in pH –convert strong acids or bases to weak ones Physiological buffer Physiological buffer –system that controls output of acids, bases or CO 2 urinary system buffers greatest quantity, takes several hours urinary system buffers greatest quantity, takes several hours respiratory system buffers within minutes, limited quantity respiratory system buffers within minutes, limited quantity Chemical buffer systems Chemical buffer systems –restore normal pH in fractions of a second –bicarbonate, phosphate and protein systems bind H + and transport H + to an exit (kidney/lung)

43. Bicarbonate Buffer System Solution of carbonic acid and bicarbonate ions Solution of carbonic acid and bicarbonate ions –CO 2 + H 2 O  H 2 CO 3  HCO 3 - + H + Reversible reaction important in ECF Reversible reaction important in ECF –CO 2 + H 2 O  H 2 CO 3  HCO 3 - + H + lowers pH by releasing H + lowers pH by releasing H + –CO 2 + H 2 O  H 2 CO 3  HCO 3 - + H + raises pH by binding H + raises pH by binding H + Functions with respiratory and urinary systems Functions with respiratory and urinary systems –to lower pH, kidneys excrete HCO 3 - –to raise pH, kidneys excrete H + and lungs excrete CO 2

44. Phosphate Buffer System H 2 PO 4 -  HPO 4 2- + H + H 2 PO 4 -  HPO 4 2- + H + –as in the bicarbonate system, reactions that proceed to the right release H + and  pH, and those to the left  pH Important in the ICF and renal tubules Important in the ICF and renal tubules –where phosphates are more concentrated and function closer to their optimum pH of 6.8 constant production of metabolic acids creates pH values from 4.5 to 7.4 in the ICF, avg. 7.0 constant production of metabolic acids creates pH values from 4.5 to 7.4 in the ICF, avg. 7.0

45. Protein Buffer System More concentrated than bicarbonate or phosphate systems especially in the ICF More concentrated than bicarbonate or phosphate systems especially in the ICF Acidic side groups can release H + Acidic side groups can release H + Amino side groups can bind H + Amino side groups can bind H +

46. Respiratory Control of pH Neutralizes 2 to 3 times as much acid as chemical buffers Neutralizes 2 to 3 times as much acid as chemical buffers Collaborates with bicarbonate system Collaborates with bicarbonate system –CO 2 + H 2 O  H 2 CO 3  HCO 3 - + H + lowers pH by releasing H + lowers pH by releasing H + –CO 2 (expired) + H 2 O  H 2 CO 3  HCO 3 - + H + raises pH by binding H + raises pH by binding H +  CO 2 and  pH stimulate pulmonary ventilation, while an  pH inhibits pulmonary ventilation  CO 2 and  pH stimulate pulmonary ventilation, while an  pH inhibits pulmonary ventilation

47. Renal Control of pH Most powerful buffer system (but slow response) Most powerful buffer system (but slow response) Renal tubules secrete H + into tubular fluid, then excreted in urine Renal tubules secrete H + into tubular fluid, then excreted in urine

48. H + Secretion and Excretion in Kidney

49. Limiting pH Tubular secretion of H + (step 7) Tubular secretion of H + (step 7) –continues only with a concentration gradient of H + between tubule cells and tubular fluid –if H + concentration  in tubular fluid, lowering pH to 4.5, secretion of H + stops This is prevented by buffers in tubular fluid This is prevented by buffers in tubular fluid –bicarbonate system –phosphate system Na 2 HPO 4 + H +  NaH 2 PO 4 + Na + Na 2 HPO 4 + H +  NaH 2 PO 4 + Na + –ammonia (NH 3 ), from amino acid catabolism NH 3 + H + and Cl -  NH 4 Cl (ammonium chloride) NH 3 + H + and Cl -  NH 4 Cl (ammonium chloride)

50. Buffering Mechanisms in Urine

51. Acid-Base Balance

52. 24-52 Buffering Mechanisms in Urine Figure 24.11 Antiport Key Tubular fluid K+K+ Na + +NaH 2 PO 4 Na + Urine HCO 3 – + – – NH 4 Cl + Glomerular filtrate NH 3 H 2 CO 3 Blood of peritubular capillary Renal tubule cells (proximal convoluted tubule) H+H+ Amino acid catabolism H+H+ H 2 CO 3 Diffusion through a membrane channel Diffusion through the membrane lipid H + + NH 3 +Cl – H + + Na 2 HPO 4 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

53. Acid-Base & Potassium Imbalances Acidosis Acidosis –H + diffuses into cells and drives out K +, elevating K + concentration in ECF H + buffered by protein in ICF, causes membrane hyperpolarization, nerve and muscle cells are hard to stimulate; CNS depression may lead to death H + buffered by protein in ICF, causes membrane hyperpolarization, nerve and muscle cells are hard to stimulate; CNS depression may lead to death

54. Acid-Base & Potassium Imbalances Alkalosis Alkalosis –H + diffuses out of cells and K + diffuses in, membranes depolarized, nerves overstimulate muscles causing spasms, tetany, convulsions, respiratory paralysis

55. Disorders of Acid-Base Balances Respiratory acidosis (emphysema) Respiratory acidosis (emphysema) –rate of alveolar ventilation falls behind CO 2 production Respiratory alkalosis (hyperventilation) Respiratory alkalosis (hyperventilation) –CO 2 eliminated faster than it is produced Metabolic acidosis Metabolic acidosis –  production of organic acids (lactic acid, ketones seen in alcoholism, diabetes) –ingestion of acidic drugs (aspirin) –loss of base (chronic diarrhea, laxative overuse) Metabolic alkalosis (rare) Metabolic alkalosis (rare) –overuse of bicarbonates (antacids) –loss of acid (chronic vomiting)

56. Compensation for pH Imbalances Respiratory system adjusts ventilation (fast, limited compensation) Respiratory system adjusts ventilation (fast, limited compensation) –hypercapnia (  CO 2 ) stimulates pulmonary ventilation –hypocapnia reduces it Renal compensation (slow, powerful compensation) Renal compensation (slow, powerful compensation) –effective for imbalances of a few days or longer –acidosis causes  in H + secretion –alkalosis causes bicarbonate secretion

57. 24-57 Fluid Replacement Therapy One of the most significant problems in the treatment of seriously ill patients is the restoration and maintenance of proper fluid volume, composition, and distribution among fluid compartments One of the most significant problems in the treatment of seriously ill patients is the restoration and maintenance of proper fluid volume, composition, and distribution among fluid compartments Fluids may be administered to: Fluids may be administered to: –replenish total body water –restore blood volume and pressure –shift water from one fluid compartment to another –restore and maintain electrolyte and acid-base balance Drinking water is the simplest method Drinking water is the simplest method –does not replace electrolytes –broths, juices, and sports drinks replace water, carbohydrates, and electrolytes

58. 24-58 Fluid Replacement Therapy Patients (pt) who cannot take fluids by mouth Patients (pt) who cannot take fluids by mouth –enema – fluid absorbed through the colon –parenteral routes – fluid administration other than digestive tract intravenous (I.V.) route is the most common intravenous (I.V.) route is the most common subcutaneous (sub-Q) route subcutaneous (sub-Q) route intramuscular (I.M.) route intramuscular (I.M.) route other parenteral routes other parenteral routes Excessive blood loss Excessive blood loss –normal saline (isotonic, 0.9% NaCl) –raises blood volume while maintaining normal osmolarity takes 3 to 5 times the normal saline to rebuild normal blood volume because much of the saline escapes blood and enters interstitial fluid compartment takes 3 to 5 times the normal saline to rebuild normal blood volume because much of the saline escapes blood and enters interstitial fluid compartment can induce hypernatremia or hyperchloremia can induce hypernatremia or hyperchloremia Correct pH imbalances Correct pH imbalances –acidosis treated with Ringer’s lactate –alkalosis treated with potassium chloride

59. 24-59 Fluid Replacement Therapy Plasma volume expanders – hypertonic solutions or colloids that are retained in the bloodstream and draw interstitial water into it by osmosis Plasma volume expanders – hypertonic solutions or colloids that are retained in the bloodstream and draw interstitial water into it by osmosis –used to combat hypotonic hydration by drawing water out of swollen cells –can draw several liters of water out of the intracellular compartment within a few minutes pts who cannot eat pts who cannot eat –isotonic 5% dextrose (glucose) solution –has protein sparing effect – fasting patients lose as much as 70 to 85 grams of protein per day I.V. glucose reduces this by half I.V. glucose reduces this by half pts with renal insufficiency pts with renal insufficiency –given slowly through I.V. drip