Chapter 27: Fluid, Electrolyte and Acid-base Balance

1. Chapter 27: Fluid, Electrolyte and Acid-base Balance BIO 211 Lecture Instructor: Dr. Gollwitzer

2. Today in class we will discuss: • The importance of water and its significance to fluid balance in the body • Definitions and the importance of: • Fluid Balance • Electrolyte balance • Acid-base balance • Extracellular fluid (ECF) and intracellular fluid (ICF) and compare their composition • Fluid and electrolyte balance • Hormones that regulate them • Importance of key electrolytes

3. Introduction • Water critical to survival • 50-60% total body weight • 99% of extracellular fluid (ECF) • Essential component of cytosol (intracellular fluid, ICF) • All cellular operations rely on water • Diffusion medium for gases, nutrients, waste products

4. Body Fluid Compartments • Body must maintain normal volume and composition of: • ICF • ECF = all other body fluids • Major - IF, plasma • Minor - lymph, CSF, serous and synovial fluids • ICF > total body water than ECF • Acts as water reserve

5. Body Fluid Compartments Figure 27–1a-2

6. Body Fluid Balance • Must maintain body fluid: • Volume (fluid balance) • Ionic concentration (electrolyte balance) • pH (acid-base balance) • Gains (input) must equal loss (output) • Balancing efforts involve/affect almost all body systems

7. Fluid (Water) Balance • = amount of H20 gained each day equal to amount of H2O lost • Regulates content and exchange of body water between ECF and ICF • Gains • GI (from food, liquid)* • Catabolism • Losses • Urine* • Evaporation (from skin, lungs) • Feces * Primary route

8. Fluid Gains and Losses Figure 27–3

9. Electrolyte (Ion) Balance • Balances gains and losses of all electrolytes (ions that can conduct electrical current in solution) • Gains • GI (from food, liquid) • Losses • Urine • Sweat • Feces

10. Acid-base (pH) Balance • Balances production and loss of H+ • Gains • GI (from food and liquid) • Metabolism • Losses • Kidneys (secrete H+) • Lungs (eliminate CO2)

11. Fluid Components • ECF components (plasma and IF) very similar • Major differences between ECF and ICF • ICF very different because of cell membrane • Selectively permeable • Specific channels for ions • Active transport into/out of cell • Water exchange between ECF and ICF occurs across cell membranes by: • Diffusion • Osmosis • Carrier-mediated transport

12. Cations in Body Fluids Figure 27–2 (1 of 2)

13. Anions in Body Fluids Figure 27–2 (2 of 2)

14. Cations and Anions in Body Fluids • In ECF • Na+ • Cl- • HCO3- • In ICF • K+ (98% of body content) • Mg2+ • HPO42- • Negatively charge proteins

15. Principles of Fluid and Electrolyte Regulation • All homeostatic mechanisms that monitor and adjust body fluid composition respond to changes in ECF, not ICF • Because: • A change in ECF spreads throughout body and affects many or all cells • A change in ICF in one cell does not affect distant cells

16. Principles of Fluid and Electrolyte Regulation • No receptors directly monitor fluid or electrolyte balance • Electrolyte balance = electrolytes gained equals the electrolytes lost • Monitor secondary indicators • Baroreceptors – for plasma volume/pressure • Osmoreceptors – for osmotic (solute) concentration • Solutes = ions, nutrients, hormones, all other materials dissolved in body fluids

17. Principles of Fluid and Electrolyte Regulation • Cells cannot move water by active transport • Passive in response to osmotic gradients • Fluid balance and electrolyte balance are interrelated • Body’s content of water and electrolytes: • Increases if gains exceed losses • Decreases if losses exceed gains

18. Primary Hormones for Fluid and Electrolyte Balance • ADH • Aldosterone • Natriuretic peptides (e.g., ANP)

19. ADH • Produced by osmoreceptor neurons in supraoptic nuclei in hypothalamus (and released by posterior pituitary) • Osmoreceptors monitor osmotic concentrations in ECF • Osmotic concentration increases/decreases when: • Na+ increases/decreases or • H2O decreases/increases • Increased osmotic concentration  increased ADH

20. ADH •  Water conservation • Increases water absorption  decreased osmotic concentration (by diluting Na+) • Stimulates thirst center in hypothalamus  increased fluid intake

21. Figure 27–4

22. Aldosterone • Mineralocorticoid secreted by adrenal cortex • Produced in response to: • Decreased Na+ or increased K+ • In blood arriving at: • Adrenal cortex • Kidney (renin-angiotensin system

23. Renin-Angiotensin System • Renin released in response to: • Decreased Na+ or increased K+ in renal circulation • Decreased plasma volume or BP at JGA • Decreased osmotic concentration at DCT • Renin   angiotensin II activation in lung capillaries • Angiotensin II  • Adrenal cortex  increased aldosterone • Posterior pituitary  ADH • Increased BP (hence it’s name)

24. Aldosterone • In DCT and collecting system of kidneys  • Increased Na+ absorption (and associated Cl- and H2O absorption) • Increased K+ loss •  Increased sensitivity of salt receptors on tongue  crave salty foods

25. Natriuretic Peptides • Released by cardiac muscle cells stretched by: • Increased BP or blood volume • Oppose angiotensin II and cause diuresis •  Decreased ADH  increased H2O loss at kidneys •  Decreased aldosterone  increased Na+ and H2O loss at kidneys • Decreases thirst  decreased H2O intake • Net result = decreased stretching of cells

26. Figure 27–5, 7th edition

27. Fluid and Electrolyte Balance • When body loses water: • Plasma volume decreases • Electrolyte concentrations increase • When body loses electrolytes: • Electrolyte concentrations decrease • Water also lost

28. Disorders of Fluid and Electrolyte Balance • Dehydration = water depletion • Due to: • Inadequate water intake • Fluid loss, e.g., vomiting, diarrhea • Inadequate ADH (hypothalamic/pituitary malfunction) • Leads to: • Too high Na+ = hypernatremia • Thirst, wrinkled skin • Decreased blood volume and BP • Fatal circulatory shock

29. Disorders of Fluid and Electrolyte Balance • Overhydration = water excess • Due to: • Excess water intake (>6-8 L/24 hours) • Seen in hazing rituals (water torture) • Marathon runners/paddlers • Ravers on ecstasy who overcompensate for thirst • Chronic renal failure • Excess ADH • Leads to • Too low Na+ = hyponatremia • Increased blood volume and BP • CNS symptoms (water intoxication); can proceed to convulsions, coma, death

30. Disorders of Fluid and Electrolyte Balance • Hypokalemia • Too low K+ • Caused by diuretics, diet, chronic alkalosis (plasma pH >7.45) • Results in muscle weakness and paralysis • Hyperkalemia • Too high K+ • Caused by diuretics (that block Na+ reabsorption) • Renal failure, chronic acidosis (plasma pH<7.35) • Results in severe cardiac arrhythmias

31. Summary: Disorders of Electrolyte Balance • Most common problems with electrolyte balance • Caused by imbalance between gains/losses of Na+ • Uptake across digestive epithelium • Excretion in urine and perspiration • Problems with potassium balance • Less common, but more dangerous

32. Today in class we will discuss: • Acid-base balance and • Three major buffer systems that balance pH of ECF and ICF • Compensatory mechanisms involved in maintaining acid-base balance • Respiratory compensation • Renal compensation • Causes, effects, and the body’s response to acid-base disturbances that occur when pH varies • Respiratory acid-base disorders • Respiratory acidosis • Respiratory alkalosis • Metabolic acid-base disorders • Metabolic acidosis • Metabolic alkalosis

33. Acid-Base Balance • Control of pH • Acid-base balance = = production of H+ is precisely offset by H+ loss • Body generates acids (H+) during metabolic processes • Decrease pH • Normal pH of ECF = 7.35 – 7.45 • <7.35 = acidosis (more common than alkalosis) • >7.45 = alkalosis • <6.8 or >7.7 = lethal

34. Acid-Base Balance • Deviations outside normal range extremely dangerous • Disrupt cell membranes • Alter protein structure (remember hemoglobin?) • Change activities of enzymes • Affects all body systems • Especially CNS and CVS

35. Acid-Base Balance • CNS and CVS especially sensitive to pH fluctuations • Acidosis more lethal than alkalosis • CNS deteriorates  coma  death • Cardiac contractions grow weak and irregular  heart failure • Peripheral vasodilation  decreased BP and circulatory collapse

36. Acid-Base Balance • Carbonic acid (H2CO3) • Most important factor affecting pH of ECF • CO2 + H2O  H2CO3  H+ + HCO3-

37. Relationship between PCO2 and pH • PCO2 inversely related to pH Figure 27–9

38. Acid-Base Balance • H+ • Gained • At digestive tract • Through cellular metabolic activities • Eliminated • At kidneys by secretion of H+ into urine • At lungs by forming H2O and CO2 from H+ and HCO3- • Sites of elimination far from sites of production • As H+ travels through body, must be neutralized to avoid tissue damage • Accomplished through buffer systems

39. Buffers • Compounds dissolved in body fluids • Stabilize pH • Can provide or remove H+

40. Buffer Systems in Body Fluids • Phosphate buffer system (H2PO4-) • In ICF and urine • Protein buffer systems • In ICF and ECF • Includes: • Hb buffer system (RBCs only) • Amino acid buffers (in proteins) • Plasma protein buffers (albumins, globulins…) • Carbonic acid-bicarbonate buffer system • Most important in ECF

41. Figure 27–7

42. Carbonic Acid–Bicarbonate Buffer System Figure 27–9

43. Maintenance of Acid-Base Balance • For homeostasis to be preserved: • H+ gains and losses must be balanced • Excess H+ must be: • Tied up by buffers • Temporary; H+ not eliminated, just not harmful • Permanently tied up in H2O molecules • Associated with CO2 removal at lungs • Removed from body fluids • Through secretion at kidneys • Accomplished by: • Respiratory mechanisms • Renal mechanisms

44. Conditions Affecting Acid-Base Balance • Disorders affecting buffers, respiratory or renal function • Emphysema, renal failure • Cardiovascular conditions • Heart failure or hypotension • Can affect pH, change glomerular filtration rates, respiratory efficiency • Conditions affecting CNS • Neural damage/disease that affects respiratory and cardiovascular reflexes that regulate pH

45. Disturbances of Acid-Base Balance • Serious abnormalities have an: • Acute (initial) phase • pH moves rapidly out of normal range • Compensated phase • If condition persists • Physiological adjustments move pH back into normal range • Cannot be completed unless underlying problem corrected • Types of compensation • Respiratory • Renal

46. Respiratory Compensation • Changes respiratory rate • Increasing/decreasing respiratory rate changes pH by lowering/raising PCO2 • Helps stabilize pH of ECF • Occurs whenever pH moves outside normal limits • Has a direct effect on carbonic acid-bicarbonate buffer system

47. Respiratory Compensation • Increased PCO2 • Increased H2CO3  increased H+  decreased pH (acidosis) • Increased respiratory rate  more CO2 lost at lungs  CO2 decreases to normal levels • Decreased PCO2 • Decreased H2CO3  decreased H+  increased pH (alkalosis) • Decreased respiratory rate  less CO2 lost at lungs  CO2 increases to normal levels

48. Carbonic Acid–Bicarbonate Buffer System Figure 27–9

49. Renal Compensation • Changes renal rates of H+ and HCO3- • Secretion • Reabsorption • In response to changes in plasma pH • Increased H+ or decreased HCO3- • Decreased pH (acidosis)  more H+ secreted and/or less HCO3- reabsorbed • Decreased H+ or increased HCO3- • Increased pH (alkalosis)  less H+ secreted and/or more HCO3- reabsorbed

50. The Carbonic Acid–Bicarbonate Buffer System and Regulation of Plasma pH Figure 27–11a