Fluid, Electrolyte, Acid-Base Balance

1. Chapter 27 Fluid, Electrolyte, & Acid-Base Balance

2. Body Water Content Infants have low body fat, low bone mass, and are 73% or more water Total water content declines throughout life Healthy males are about 60% water; healthy females are around 50% This difference reflects females’: Higher body fat Smaller amount of skeletal muscle In old age, only about 45% of body weight is water

3. Fluid Compartments Water occupies two main fluid compartments Intracellular fluid (ICF) – about 2/3 by volume, contained in cells Extracellular fluid (ECF) – consists of two major subdivisions Plasma – the fluid portion of the blood Interstitial fluid (IF) – fluid in spaces between cells lymph, cerebrospinal fluid, eye humors, synovial fluid, serous fluid, and gastrointestinal secretions

4. Composition of body fluids Water is the “universal solvent” Solutes are either electrolytes or nonelectrolytes Electrolytes – ions w/electrical charge: inorganic salts, inorganic/organic acids & bases, & some proteins Nonelectrolytes – have bonds that prevent dissociation in solution (no electrical charge): glucose, lipids, creatinine, urea…

5. Osmosis Water moves from areas of lesser osmolality to areas of greater osmolality …electrolytes have a greater influence on movement of H2O b/c they dissociate into more particles than nonelectrolytes ie. NaCl?Na+ + Cl- glucose?glucose

6. Extracellular and Intracellular Fluids Each fluid compartment of the body has a distinctive pattern of electrolytes Extracellular fluids are similar (except for the high protein content of plasma) Sodium is the chief cation Chloride is the major anion Intracellular fluids have low sodium and chloride Potassium is the chief cation Phosphate is the chief anion

7. Water balance- see figure 27-1 Water input – 60% ingested liquids 30% ingested solids 10% metabolic water (water of oxidation) – produced via cellular metabolism Water output – 28% vaporized thru lungs/skin (insensible water loss) 8% perspiration 4% feces 60% thru kidneys as urine

8. Thirst mechanism Located in hypothalamus & is poorly understood Triggered by a decrease in plasma volume by >10% or increase in plasma osmolality by 1-2%

9. 3 Primary Regulatory Hormones Affect fluid and electrolyte balance: antidiuretic hormone aldosterone natriuretic peptides

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

11. Aldosterone Is secreted by adrenal cortex in response to: rising K+ or falling Na+ levels in blood activation of renin–angiotensin system Determines rate of Na+ absorption and K+ loss along DCT and collecting system

12. “Water Follows Salt” High aldosterone plasma concentration: causes kidneys to conserve salt Conservation of Na+ by aldosterone: also stimulates water retention Obligatory water loss – insensible (lung, skin, feces, etc.) & minimum sensible (~500 ml in urine) loss

13. Natriuretic Peptide ANP is released by cardiac muscle cells In response to abnormal stretching of heart walls caused by: elevated blood pressure an increase in blood volume Reduce thirst Block release of ADH and aldosterone Cause diuresis Lower blood pressure and plasma volume

14. Excess water intake Normal function: ~30’ after ingestion, kidneys start to eliminate excess water (time needed to (-) ADH release) Diuresis reaches peak in ~ 1 hour Urine output declines to its lowest levels after ~3 hours

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

16. Dehydration Also called water depletion Develops when water loss is greater than gain Severe water loss causes excessive perspiration inadequate water consumption repeated vomiting diarrhea

17. Electrolyte Balance Electrolytes are salts, acids, and bases, but electrolyte balance usually refers only to salt balance Salts are important for: Neuromuscular excitability Secretory activity Membrane permeability Controlling fluid movements Salts enter the body by ingestion and are lost via perspiration, feces, and urine

18. Electrolyte Balance When the body loses water: plasma volume decreases & electrolyte concentrations rise When the body loses electrolytes: water is lost by osmosis

19. Sodium Has the primary role in controlling ECF volume & water distribution in the body NaHCO3 & NaCl account for 90-95% of all solutes in the ECF The single most abundant cation in the ECF and accounts for virtually all of the osmotic P B/C all body fluids are in osmotic equilibrium, a change in Na [ ] affects plasma volume & BP as well as ICF & IF volumes as well

20. Na balance Aldosterone – makes DCT & CTs in kidneys more permeable to Na (65% Na reabsorbed in PCT & 25% reclaimed in Loop of Henle) H2O may or may not follow depending on levels of ADH (aldosterone usually allows for easier excretion of H2O) If needed, almost all of the Na may be reabsorbed in DCT leading to urine with high H2O content & little Na excretion CV system baroreceptors: High blood volume?carotid/aortic sinuses? alert brain stem ?decreased SNS output to kidneys?increased GRF?increased Na & H2O output?decreased BV & BP Low blood volume?constriction of afferent arterioles?reduced filtrate formation?decreased urinary output?increased BV & BP

21. Abnormal Na+ Concentrations in ECF Hyponatremia: body water content rises (overhydration) ECF Na+ concentration < 130 mEq/L Hypernatremia: body water content declines (dehydration) ECF Na+ concentration > 150 mEq/L

22. Na balance, cont. ADH – increases H2O reabsorption Atrial natriuretic peptide/factor (ANP/F) Released by certain cells of heart atria when stretched…reduces BV & BP by (-) nearly all events that promote vasoconstriction & Na/H2O retention Estrogens – chemically similar to aldosterone Progesterone – blocks effect of aldosterone so it has a diuretic effect Glucocorticoids – tends to have an aldosterone like effect & promotes edema

23. Potassium The chief intracellular cation Essential for protein synthesis & normal neuromuscular functioning Levels affect resting membrane potential (especially in the heart) Increased K+ levels in ECF decreases membrane potential?depolarization? reduced excitability Part of the body’s buffer system (ECF K+ levels rise w/acidosis as K+ leaves the cell & H+ enters the cell)

24. Regulation of potassium Levels maintained mostly by renal mechanisms Tubules reabsorb ~55% of filtered K+ Thick ascending limb reabsorbs~30 % Less than 15% excreted in urine …K+ balance falls on cortical collecting ducts by changing amount of K+ secreted in to filtrate Generally, K+ levels are high in the ECF & the thrust of kidney fnx is to excrete it…failure to ingest dietary K+ results in severe potassium deficiency

25. Potassium regulation Aldosterone – enhances K+secretion while causing Na reabsorption There is a one-for-one exchange of Na & K in the cortical collecting ducts to maintain electrolyte balance Adrenal cortical cells are extremely sensitive to K+ content of the ECF that baths them…K+ controls its own [ ] in the ECF via feedback regulation of aldosterone release

26. 2 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

27. Calcium balance 99% of body’s Ca is in bone in the form of calcium phosphate salts (most abundant mineral in the body) Ionic Ca in ECF important for normal blood clotting, cell membrane permeability, neuromuscular excitability & secretory behavior Hypocalcemia?increases excitability & causes mm tetany Hypercalcemia?(-) neurons & mm cells and may cause life-threatening arrhythmias Ca balance is regulated by 2 Hormones: PTH & calcitonin 98% of filtered Ca is reabsorbed under normal circumstances

28. Calcium balance, cont. PTH & Calcitriol– (+) by decreased plasma Ca levels Bones – activates osteoclasts Small intest. – enhances intestinal absorption of Ca by indirectly (+) kidneys to activate vit D Kidneys – increases Ca reabsorption by renal tubules while decreasing phosphate ion reabsorption Calcitonin – (+) by rising plasma Ca levels Antagonistic to PTH & calcitriol by causing deposition of Ca in bone but its effect is negligible

29. Anion regulation Chloride is major anion in ECF & maintains osmotic pressure of blood w/ Na 99% filtered Cl- is reabsorbed w/ blood pH w/in normal limits W/acidosis, fewer Cl ions accompany Na b/c bicarbonate ion reabsorption is stepped up to restore blood pH to normal Other anions seem to have a transport maximum & excesses are spilled over into the urine

30. Acid-base balance Arterial blood = 7.4 Venous blood & IF = 7.35 More acidic metabolites & CO2…more acidic ICF = 7.0 [ ] of H+ in blood regulated by: 1. Chemical buffers (rapid/fraction of a second), 2. Respiratory center of brain stem (1-3 min.), & 3. Renal mechanisms (most potent/require hours to a day or more)

31. Terms Relating to Acid–Base Balance

32. Strong or Weak Strong acids and strong bases: dissociate completely in solution Weak acids or weak bases: do not dissociate completely in solution some molecules remain intact

33. Sources of Hydrogen Ions Most hydrogen ions originate from cellular metabolism Breakdown of phosphorus-containing proteins releases phosphoric acid into the ECF Anaerobic respiration of glucose produces lactic acid Fat metabolism yields organic acids and ketone bodies Transporting carbon dioxide as bicarbonate releases hydrogen ions

34. 3 Types of Acids in the Body Volatile acids - Can leave solution and enter the atmosphere Carbonic acid is an important volatile acid in body fluid Fixed acids - Are acids that do not leave solution. Once produced they remain in body fluids until eliminated by kidneys Sulfuric Acid and Phosphoric Acid are most important fixed acids in the body generated during catabolism of AA’s, phospholipids, & nucleic acids Organic acids : Produced by aerobic metabolism are metabolized rapidly & do not accumulate Produced by anaerobic metabolism (e.g., lactic acid) build up rapidly

35. A Buffer System Consists of a combination of: a weak acid and the anion released by its dissociation The anion functions as a weak base

36. 3 major chemical buffers 1. Bicarbonate The only important ECF buffer 2. Phosphate Effective in urine & ICF (unimportant for buffering blood plasma – bicarb is more imp.) 3. Protein Proteins w/in cells & in plasma are the body’s most powerful & plentiful source of buffers Major ICF buffer

37. Carbonic Acid Is a weak acid In ECF at normal pH equilibrium state exists Is diagrammed H2CO3 ? H+ + HCO3—

38. Protein Buffer Systems Depend on ability of amino acids Respond to pH changes by accepting or releasing H+

39. The Hemoglobin Buffer System Is the only intracellular buffer system with an immediate effect on ECF pH Helps prevent major changes in pH when plasma PCO2 is rising or falling CO2 diffuses across RBC membrane: no transport mechanism required As carbonic acid dissociates: bicarbonate ions diffuse into plasma in exchange for chloride ions (chloride shift) Hydrogen ions are buffered by hemoglobin molecules

40. Problems with Buffer Systems Provide only temporary solution to acid–base imbalance Do not eliminate H+ ions Supply of buffer molecules is limited

41. Maintenance of Acid–Base Balance For homeostasis to be preserved, captured H+ must: be permanently tied up in water molecules: through CO2 removal at lungs removed from body fluids: through secretion at kidney

42. Respiratory regulation of H+ levels Acts slower than chemical buffers but has 1-2 x the buffering power than all the chemical buffers combined CO2 + H2O ??H2CO3??H+ + HCO3- Alkalosis (rise in pH) causes shift to right (more H+ [ ] ) …acidosis (drop in pH) causes shift to left (more CO2 removed from blood by increased ventilation) Respiratory alkalosis or acidosis can occur from anything that impairs pulmonary fnx

43. Renal regulation of H+ levels Chemical buffers can tie up acids or bases temporarily but cannot rid the body of them Lungs can dispose of carbonic acid by eliminating CO2 Only the kidneys can rid the body of other acids generated by cellular metabolism: phosphoric acid, uric acid, lactic acid, & ketone bodies Although the kidneys act slowly, they are the ultimate organs of acid-base regulation

44. Renal acid-base regulation 1. Conserve (reabsorb) or generation of new bicarbonate ions To reabsorb bicarb, H+ must be secreted For each H+ secreted into tubule lumen, one Na is reabsorbed from filtrate (maintaining electrolyte balance) To excrete bicarb, H+ must be retained 2. Excreting bicarbonate ions When body is in alkalosis the collecting ducts will secrete HCO3 while reclaiming H+ to acidify the blood Overall effect in nephrons & collecting ducts as a whole is to reabsorb more HCO3 than is excreted (even in alkalosis)

45. Acidosis and Alkalosis Affect all body systems: particularly nervous and cardiovascular systems Both are dangerous: but acidosis is more common because normal cellular activities generate acids

46. Acidosis – pH<7.35 Respiratory acidosis Most common cause of acid-base imbalance CO2 accumulates in blood…shallow breathing, hampered gas exchange: pneumonia, emphysema, cystic fibrosis Metabolic acidosis All causes other than respiratory Too much alcohol (converts to acetaldehyde?acetic acid), excessive loss of HCO3 (diarrhea), lactic acidosis (exercise), ketoacidosis (starvation)

47. Alkalosis – pH>7.45 Respiratory alkalosis From hyperventilation…rarely result of disease process Metabolic alkalosis Less common than metabolic acidosis Caused by excessive vomiting or GI suctioning, intake of excessive bases (antacids), constipation (more HCO3- is reabsorbed by the colon)

48. Diagnostic Chart for Acid-Base Disorders

49. Blood Chemistry and Acid–Base Disorders

50. Limits of acidosis/alkalosis pH below 7.0?depression of CNS?coma or death pH above 7.8?overexcitation of nervous system?mm tetany, extreme nervousness, & convulsions?death often from respiratory arrest Acid-base imbalance due to inadequacy of a physiological buffer system is compensated for by the other system The respiratory system will attempt to correct metabolic acid-base imbalances The kidneys will work to correct imbalances caused by respiratory disease

51. Respiratory Compensation In metabolic acidosis: The rate and depth of breathing are elevated Blood pH is below 7.35 and bicarbonate level is low As carbon dioxide is eliminated by the respiratory system, PCO2 falls below normal In respiratory acidosis, the respiratory rate is often depressed and is the immediate cause of the acidosis In metabolic alkalosis: Compensation exhibits slow, shallow breathing, allowing carbon dioxide to accumulate in the blood Correction is revealed by: High pH (over 7.45) and elevated bicarbonate ion levels Rising PCO2

52. Renal Compensation To correct respiratory acid-base imbalance, renal mechanisms are stepped up Acidosis has high PCO2 and high bicarbonate levels The high PCO2 is the cause of acidosis The high bicarbonate levels indicate the kidneys are retaining bicarbonate to offset the acidosis Alkalosis has Low PCO2 and high pH The kidneys eliminate bicarbonate from the body by failing to reclaim it or by actively secreting it

53. Occur in the young, reflecting: Low residual lung volume High rate of fluid intake and output High metabolic rate yielding more metabolic wastes High rate of insensible water loss Inefficiency of kidneys in infants Problems with Fluid, Electrolyte, and Acid-Base Balance