Saladin Ch. 24

1. Saladin Ch. 24 Fluid, Electrolyte, & Acid-Base Balance

2. Fluid Compartments • Fluid occupies two main compartments.[55-60% body weight] • Intracellular fluid (ICF) – about two thirds by volume, contained in cells = 65% of body fluids • Extracellular fluid (ECF) – consists of two major subdivisions = 35% • Plasma – the fluid portion of the blood. • Interstitial fluid (IF) – fluid in spaces between cells.

3. Fluid Compartments • Other fluids. [lymph, CSF, GI fluids, synovial fluid, ocular humors, pleural, pericardial and peritoneal fluids, glomerular filtrate]

4. Figure 26.1

5. Fluid Movement Among Compartments • Water moves by osmosis which is determined by electrolyte movement & balance

6. Water Balance & ECF Osmolality • For proper hydration, water intake must equal water output. • Total ingested water [2500 mL/day] • Metabolic [from aerobic resp. and dehydration synthesis] [200mL/day] • Food & drink – [2300 mL]

7. Water Balance & ECF Osmolality • Average outputs – increase with activity, environment, etc. • Total output [2500mL] • Kidneys – urine [1500mL/day] • Skin & sweat – 500mL/day • Lungs – esp. [300mL/day] • GI tract – feces [200mL/day]

8. Figure 26.4

9. REGULATION OF WATER UPTAKE • Regulated by adjusting water ingestion – thirst • Local responses – decreased saliva  dry mouth • Increased blood osmotic pressure  hypothalamus  thirst [may get ADH] • Decreased blood volume  release of renin  angiotensin II  thirst

10. REGULATION OF WATER OUTPUT • Regulated by urinary water and NaCl loss – kidneys can’t replace loss, just reduce further loss. • ADH increases water reabsorption in renal collecting ducts  aquaporins inserted into principal cell membranes  water reabsorption.

12. Disorders of Water Balance: Dehydration • Water loss exceeds water intake & the body is in negative fluid balance. • Causes include: hemorrhage, severe burns, prolonged vomiting or diarrhea, profuse sweating, water deprivation, & diuretic abuse.

13. Disorders of Water Balance: Dehydration • Dehydration – get increased osmolarity; Diabetes mellitus & insipidus, overuse of diuretics, diarrhea; [can lead to shock] Use isotonic salts solutions for replacement of extreme fluid loss – prevents hypotonic swelling.

14. 2 ECF osmotic pressure rises Cells lose H2O to ECF by osmosis; cells shrink Excessive loss of H2O from ECF 3 1 (a) Mechanism of dehydration Figure 26.7a

15. Disorders of Water Balance: Fluid Excess • Water intoxication – take in faster than can remove – hypotonic ECF • Hyponatremia]  cell swelling and cerebral edema

16. 3 H2O moves into cells by osmosis; cells swell ECF osmotic pressure falls 2 1 Excessive H2O enters the ECF (b) Mechanism of hypotonic hydration Figure 26.7b

17. Disorders of Water Balance: Edema • Atypical accumulation of fluid in interstitial spaces, leading to tissue swelling. • Caused by anything that increases flow of fluids out of the bloodstream or hinders their return. • Increases blood pressure. • May involve vessel blockage, CHF.

19. Electrolyte Balance • Electrolyte - substance that dissociates into cations & anions in water & conducts electricity in solution.

20. Functions of Electrolytes • Essential minerals - enzyme cofactors. • Control of osmosis. • Maintenance of acid/base balance. • Conduction of electrical current.

21. Extracellular & 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). • Na+ is the chief cation. • Cl- is the major anion.

22. Extracellular & Intracellular Fluids • Intracellular fluids have low Na+and Cl- . • K+ is chief cation. • Proteins & phosphate are chief anions.

23. Figure 26.2

24. Sodium & Electrolyte Balance • Na+ holds central position in fluid & electrolyte balance. [Plasma levels = 142 mEq/L] • Sodium salts account for 90 of all solutes in the ECF. • Na+ is the only cation exerting significant osmotic pressure - most important solute in determining water distribution.

25. Sodium & Electrolyte Balance • Adult need 0.5 g/day – in America always have EXCESS – problem is getting rid of it • Concentrations are maintained by maintaining water levels – “water follows salt” • 65% of Na+ in renal filtrate is automatically reabsorbed.

26. Regulation of Sodium Balance: • Levels regulated by: • Aldosterone - [increases reabsorption of Na+ and excretion of K+], ascending loop, DCT & collecting ducts • ADH [increases water reabsorption in response to increases in Na] • ANP [increases Na+ excretion by kidneys by inhibing ADH & aldosterone]

27. Regulation of Sodium Balance: • Estrogens – enhances Na reabsortion and water retention • Progesterone may decrease Na+ reabsorption by blocking aldosterone. • Glucocorticoids – can cause edema

28. Na+ Imbalances • Hypernatremia  water retention, hypertension & edema • Hyponatremai hypotonic hydration if not corrected

29. K+ and Electrolyte Balance • Most abundant intracellular cation. • Functions in impulse conduction, OP regulation, protein synthesis & pH. • Levels regulated in the kidneys by aldosterone.

30. Influence of Aldosterone • Aldosterone stimulates potassium ion secretion by “principal cells.” • In collecting ducts, for each Na+ reabsorbed, a K+ is secreted. • Increased K+ in ECF around adrenal cortex causes release of aldosterone & potassium secretion.

31. K+ Imbalances • Hyperkalmia – get different responses if fast or slow onset. Fast - The heart is really sensitive to too much [can cause membrane depolarization - hyperexcitability]; Slow – cells become LESS excitable • Hypokalmia - too little [causes hyperpolarization and non-responsiveness].

32. Regulation of Chloride • Cl- is the major anion accompanying Na+ in the ECF. • Functions in OP regulation, HCl formation. • Levels regulated by Na+ movements.

33. Regulation of Calcium & Phosphate • Calcium-most abundant mineral. • Abundant extracellular cation. • Functions in bone, teeth, blood clotting, impulse conduction, muscle contraction.

34. Regulation of Calcium & Phosphate • Levels controlled by: • PTH • Calcitrol • Calcitonin

35. Regulation of Calcium & Phosphate • Hypercalcemia – from alkalosis or hypothyroidism – inhibits depolarization  weakness, arrhythmia • Hypocalcemia – Vit. D deficiency, lactation, pregnancy, etc. Too little can increase excitability & may lead to tetanus;

36. Regulation of Calcium & Phosphate PHOSPHATE • Functions: Nucleic acids, ATP, phospholipids, bone, etc. • Control: Kidneys reabsorb if levels drop, Parathyroid hormone stimulates excretion

37. Acid-Base Balance • Normal pH of blood is 7.35 – 7.45. • 3 mechanisms to maintain proper pH: • Buffer systems • Changes in respiration • Excretion by kidney

38. Acid-Base Balance • Acid = proton [H+] donor;. • Most H+ originates as metabolic products. • Strong dissociates completely • Weak acid only dissociates a little • Base = proton acceptor • Strong – binds a lot, • Weak binds less

39. Chemical Buffer Systems • Buffers resist changes in pH when strong acid or strong base are added. • Weak acid + salt of that acid. • Works by taking up or releasing H+ ions to maintain pH at a given level. • Three major chemical buffer systems: • Bicarbonate/carbonic acid buffer system

40. Chemical Buffer Systems • Phosphate buffer system • Protein buffer system • Any changes in pH are resisted by the entire chemical buffering system.

41. Bicarbonate Buffer System • A mixture of carbonic acid (H2CO3) & its salt, usually sodium bicarbonate (NaHCO3) (potassium or magnesium bicarbonates work as well). • This system is the only important ECF buffer. H++ HCO3¯ H2CO3 CO2 + H2O

42. Phosphate Buffer System • This system is an effective buffer in urine & intracellular fluid. OH ¯ + H2PO4- H2O +CO2 H++ HPO42- H2PO4-

43. Protein Buffer System • Plasma & intracellular proteins are the body’s most plentiful & powerful buffers - does ¾ of all. • Protein molecules are amphoteric and can function as both weak acids & a weak bases. • The acid group (COOH) can give up an H+ to neutralize a base. • The amine group can accept an H+, raising pH.

44. Respiration • The respiratory system regulation of acid-base balance is a physiological buffering system. • Exhalation of CO2 lowers H+ by lowering carbonic acid • With alkalosis – get shallow slow breathing to help ACCUMULATE CO2

45. Kidneys • Kidneys can expel H’s by secretion – neutralize it in tubules with bicarb, phosphate, etc. • Only way to remove non-carbonic acid H+ [phosphoric uric, lactic acid, ketone bodies] • Also only way to regulate alkaline substances – can break down bicarb to CO2 and reabsorb that.

47. Blood pH • Acidosis: pH below 7.35 • Alkalosis: pH above 7.45 • Compensation-physiological changes that occur to bring pH back to normal.

48. Blood pH • Normal Blood Values • pH 7.35 – 7.45 • PCO2 35 – 45 mm Hg • HCO3- 22 – 26 mEq/L

49. Blood pH • Blood Acidosis • pH below 7.35 • depression of CNS  coma • Blood Alkalosis • pH above 7.45 • Over excitability of CNS and PNS  nervousness  spasms  convulsions

50. Respiratory Acidosis and Alkalosis • Respiratory acidosis is the most common cause of acid-base imbalance. • Occurs when a person breathes shallowly, or gas exchange is hampered by diseases such as pneumonia, cystic fibrosis, or emphysema.