Fluid, Electrolyte, and Acid-Base Balance

1. Fluid, Electrolyte, and Acid-Base Balance Chapter 26

2. Body Water Content • Varies with weight, age, and sex: • Early embryo (97%) • Newborn (77%) • Adult male (60%) • Adult female (54%) • Elderly (45%) • Adipose tissue versus skeletal muscle largely determines adult differences.

3. Body Fluids and Compartments

4. Composition of Body Fluids • Electrolytes • Cations: sodium, potassium, hydrogen, magnesium, and calcium • Anions: chloride, bicarbonate, phosphate, and sulfate • Non-electrolytes • Glucose • Urea • Protein • Lipids • Creatinine

5. Comparison of Compartments

6. Fluid Movement Between Compartments

7. Fluid Movements Exchange between blood and interstitial spaces

8. Fluid Movements Exchange between extracellular fluids and cells

9. WaterBalance

10. Regulation of Water Intake

11. Regulation of Water Output • Obligatory Water Losses • Skin and lungs • Urine and feces • Fluid intake • Diet • Levels of Anti-diuretic hormone (ADH)

12. Regulation of Water Output

13. Disturbances in Water Balance • Dehydration • Hypotonic hydration • Edema

14. Electrolyte Balance Regulation of Sodium Balance Regulation of Potassium Balance Regulation of Calcium and Phosphate Balance Regulation of Magnesium Balance Regulation of Anions

15. Regulation of SodiumBy Aldosterone

16. Regulation of Sodium By ANP

17. Regulation of SodiumBy Baroreceptors

18. Regulation of Sodium • Aldosterone • ANP • Baroreceptors • Other hormones: • Estrogen=enhances Na+ reabsorption • Progesterone=decreases Na+ reabsorption • Glucocorticoids=enhances Na+ reabsorption

19. 75-80% of sodium (NaCl) in renal filtrate is reabsorbed in proximal tubules of kidneys. • Aldosterone aids in actively reabsorbing remaining Na+Cl- in distal convoluted tubule/collecting tubule by increasing tubule permeability; therefore aldosterone promotes both sodium and water retention • Mechanism: • increase in K or decease in Na in blood plasma renin-angiotensin Mechanism • stimulates adrenal cortex to release aldosterone • aldosterone targeted towards the kidney tubules • increase in Na reabsorption increase in K secretion • restores homeostatic plasma levels of Na and K

20. Influences on aldosterone synthesis and release: • Elevated potassium levels in ECF directly stimulates adrenal cells to secrete aldosterone • Juxtaglomerular apparatus of renal tubes release renin in response to: • decreased stretch (due to decrease in blood pressure) • decreased filtrate osmolarity • sympathetic nervous system stimulation Cardiovascular system • As blood volume (and pressure) rises, the baroreceptors in the heart and in the large vessels of the neck and thorax (carotid arteries and aorta) communicate to the hypothalamus • Sympathetic nervous system impulses to kidneys decrease, allowing afferent arterioles to dilate; as the glomerular filtration rate rises, sodium and water output increases (causing pressure diuresis) • Reduced blood volume and pressure results

21. Influence of ADH • Amount of water reabsorbed in the distal segments of the kidney tubules is proportional to ADH release (increase in ADH secretion = increase in water resorption) • Osmoreceptors of the hypothalamus sense the ECF solute concentrations and trigger or inhibit ADH release from the pituitary • Mechanism: • decrease in sodium concentration in plasma (decreased osmolarity) • stimulates osmoreceptors in hypothalamus • stimulates posterior pituitary to release ADH • ADH targeted toward distal and collecting tubules of kidney • the effect is increased water resorption • plasma volume increases, osmolarity decreases • scant urine produced

22. Influence of atrial natriuretic factor (ANF) Reduces blood pressure and blood volume by inhibiting nearly all events that promote vasoconstriction and sodium and water retention

23. Regulation of Potassium The regulatory site of potassium is in the renal tubules Influence of aldosterone Influence of plasma potassium concentrations

24. Regulation of Potassium Balance • Potassium is the chief intracellular cation • Relative intracellular-extracellular potassium concentrations directly affects a cell's resting membrane potential, therefore a slight change on either side of the membrane has profound effects (ie. on neurons and muscle fibers) • Potassium is part of the body's buffer system, which resists changes in pH of body fluids; ECF potassium levels rise with acidosis (decrease pH) as potassium leave cells and fall with alkalosis (increase pH) as potassium moves into cells • Potassium balance is maintained primarily by renal mechanisms • Potassium reabsorption from the filtrate is constant - 10-15% is lost in urine regardless of need; because potassium content of ECF is low (compared to sodium concentration), potassium balance is accomplished by changing amount of potassium secreted into the filtrate; therefore regulated by collecting tubules

25. Regulation of Calcium • Influence of Parathyroid Hormone • Influence of Calcitonin

26. Regulation of Calcium Balance • 99% of calcium found in bones as an apatite • Calcium needed for blood clotting, nerve transmission, enzyme activation, etc... • Calcium ion concentration is regulated by interaction of two hormones: parathyroid hormone and calcitonin • Calcium ion homeostasis: effects of PTH and calcitonin • PTH - released by the parathyroid cells, promotes increase in calcium levels by targeting... • bones - PTH activates osteoclasts, which breakdown the matrix • small intestines - PTH enhances intestinal absorption of calcium ions indirectly by stimulating the kidneys to transform vitamin D to its active form which is a necessary cofactor for calcium absorption • Kidneys - PTH increases calcium reabsorption by renal tubes while simultaneously decreasing phosphate ion reabsorption • Calcitonin - targets bone to encourage deposition of calcium salts and inhibits bone reabsorption (therefore an antagonist of PTH

27. Regulation of Phosphate • Influence of Parathyroid Hormone • Decreases plasma phosphate concentrations while increasing calcium concentrations • Influence of Calcitonin • Increases plasma phosphate concentration while decreasing calcium concentrations

28. Regulation of Magnesium Balance and Anions • Magnesium • PTH increases plasma magnesium concentrations by causing a decrease in the amount of magnesium excreted by the kidneys • Anions • Chloride is indirectly increased by Aldosterone because it passively follows sodium

29. Acid-Base Balances

30. Strong Acids versus Weak Acids

31. Abnormalities in Acid-Base Balance • Respiratory acidosis • Increased CO2=increased H+=decreased pH • Hypoventilation • To compensate: increase excretion of H+ or by increased reabsorption of HCO3- • Respiratory alkalosis • Decreased CO2=decreased H+=increased pH • Hyperventilation • To compensate: decreased H+ excretion or by decreased reabsorption of HCO3-

32. Abnormalities in Acid-Base Balance • Metabolic acidosis • Decreased HCO3-=increased H+=decreased pH • Diarrhea, ketosis, renal dysfunction • Hyperventilation • Metabolic alkalosis • Increased HCO3-=decreased H+=increased pH • Vomiting, diuretics, alkaline drug use • Hypoventilation

33. Chemical Buffers Three Types: Bicarbonate Buffers Phosphate Buffers Protein Buffers

34. Bicarbonate Buffers • Major extracellular buffering system • HCO3- functions as a weak base while H2CO3 functions as a weak acid. • Example: HCl + NaHCO3- H2CO3 + NaCl

35. Phosphate Buffers • Important in urine and intracellular buffering systems • However NaH2PO4 acts as the weak acid and Na2HPO4 serves as the weak base. • Example: HCl + Na2HPO4 NaH2PO4 + NaCl

36. Protein Buffers Most abundant buffering system in the body including intracellular and extracellular compartments. Carboxyl groups (COOH) and amine groups (NH3) act as either an acid or a base respectively.

37. Physiological Buffers Two Types: Respiratory Buffering System Renal Buffering System

38. Physiological Buffer Systems • Respiratory System • Rising plasma H+ causes deeper, rapid breathing which decreases CO2 blood thereby decreasing H+ ions.

39. Physiological Buffer Systems • RenalSystem • To counteract acidosis, H+ is secreted into the renal tubules and excreted in urine or NH4+ is excreted rather than reabsorbed. • To counteract alkalosis, bicarbonate ions are secreted into the filtrate and H+ is reabsorbed.

40. Recall Tubular Reabsorption and Secretion of Ions