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Fluid, Electrolyte, and Acid-Base Homeostasis

Fluid, Electrolyte, and Acid-Base Homeostasis

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Fluid, Electrolyte, and Acid-Base Homeostasis

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  1. Fluid, Electrolyte, and Acid-Base Homeostasis • A. Fluid compartments and fluid balance • B. Water 1. Regulation of fluid intake (gain) 2. Regulation of fluid output (loss) • C. Electrolytes 1. Distribution • D. Movement of body fluids 1. Exchange between plasma and interstitial fluid 2. Exchange between interstitial and intracellular fluids • E. Acid-base balance 1. Buffer systems 2. Exhalation of carbon dioxide 3. Kidney excretion of H+

  2. Body Fluid Compartments • 1. intracellular fluid (67%) • 2. extracellular fluid (33% a. interstitial fluid (80%) b. plasma (20%) c. other fluids

  3. Selectivelypermeablecellmembranes separate body fluids into the fluid compartments. Which cells do this? venularend plasma arteriolar end endothelium reabsorption filtration interstitial fluid osmosis tissue cells Fluids are in constant motion between the three compartments.

  4. Water is the main component of all body fluids. • 1. What does the term fluid balance mean? • 2. What is the primary mechanism by which water moves from one compartment to the next? • 3. What controls osmosis and therefore fluid balance? • 4. Fluid balance, then, means water balance, but it implies electrolyte balance as well; the two are inseparable in this regard.

  5. Water makes up 45 - 75% of total body weight. The exact percentage is dependent upon: • Age- newborn = 75%, lean adult man = 60%, old age = 45% • Body Size- obese = as little as 45%, very lean as much as 75% • Gender- lean adult man = 60%, lean adult women = 50%

  6. Water gain and loss(2,500 ml/day) • 1. water gain a. preformed vs b. metabolic • 2. water loss sensible vs. insensible

  7. Dehydration, Thirst, and Rehydration

  8. Regulation of fluid intake (Thirst) decreased salivation decreased blood volume increased blood osmotic pressure dry mouth and throat decreased blood pressure stimulation of hypothalamic osmoreceptors stimulation of tactile receptors increased angiotensin II stimulation of hypothalamic thirst center conscious awareness of thirst increased water intake

  9. Regulation of Fluid Output (adjusted by 3 hormones) ANTIDIURETIC HORMONE ALDOSTERONE ATRIAL NATRIURETIC PEPTIDE dehydration increased blood volume dehydration increased angiotensin II stretch of right atrium increased blood osmolarity stimulation of hypothalamic osmoreceptors increased aldosterone secretion of ANP secretion of ADH from posterior pituitary gland increased Na+ reabsorption decreased Na+ reabsorption increased water reabsorption decreased water reabsorption increased thirst increased water reabsorption rehydration rehydration decreased blood volume

  10. Electrolytes versus Non-electrolytes • Electrolytes have a greater effect on osmosis than do nonelectrolytes. Consider the following: • C6H12O6 in water = C6H12O6 • NaCl in water = Na+ and Cl- • CaCl2 in water = Ca++ and Cl- and Cl- • Which of these three compounds will exert the greater effect on osmosis? • Just as important, once the electrolyte dissociates, its ions can attract other ions of the opposite charge, creating an electrochemical gradient.

  11. Electrolyte Distribution

  12. Electrolytes serve four general functions: • 1. essential minerals • 2. exert a greater effect on osmosis • 3. help maintain acid-base balance • 4. carry electrical current

  13. Exchange of body fluids between plasma and interstitial fluid occurs across capillary membranes in what three ways? • 1. diffusion • 2. vesicular transport • 3. bulk flow • Bulk flow is dependent on what four pressures that determine the net filtration pressure? • NFP = (BHP + IFOP) - (BCOP + IFHP) = +10 mm Hg (net filtration) = - 9 mm Hg (net reabsorption)

  14. Exchange between ICF and interstitial fluid • 1. based on osmosis • 2. different ionic compositions • 3. net flow = 0 • 4. Na+ and K+ most important • 5. example -- decreased [Na+]

  15. Relationship Between Sodium Balance and Water Balance RELATIONSHIP BETWEEN SODIUM BALANCE AND WATER BALANCE + excessive sweating, vomiting, diarrhea intake of plain water decreased interstitial sodium decreased interstitial osmotic pressure net flow of water into cells from interstitial space increased filtration from blood capillaries overhydration of cells convulsions  coma decreased blood volume decreased blood pressure circulatory shock DEATH

  16. Acid-base balance • Normal body fluid pH is 7.35 - 7.45. • This range is maintained by three major mechanisms: • 1. buffer systems • 2. exhalation of carbon dioxide (respiratory compensation) • 3. kidney excretion of H+ (renal compensation) • Metabolic acidosis vs. respiratory acidosis • Metabolic alkalosis vs. respiratory alkalosis

  17. In a buffer system, a strong acid is converted to a weaker one, using a weak base, or, a strong base is converted to a weaker one using a weak acid. • The carbonic acid-bicarbonate system: • H2CO3 HCO3- + H+ • HCO3- (bicarbonate ion acts as a weak base.) • H2CO3 (carbonic acid acts as a weak acid.) H+ + HCO3- H2CO3 (Used if there is an excess of H+.) H2CO3 H+ + HCO3- (Used if there is a shortage of H+)

  18. The phosphate buffer system is an important regulator of pH in the intracellular fluid. H2PO4- HPO42- + H+ H2PO4- (dihydrogen phosphate acts as a weak acid) HPO42- (monohydrogen phosphate acts as a weak base) OH- + H2PO4- H2O + HPO42- (used as a weak acid to buffer strong bases) H+ + HPO42- H2PO4- (used as a weak base to buffer a strong acid)

  19. The protein buffer system is the most abundant buffer in cells and plasma. Proteins act as both acidic and basic buffers because they have a free carboxyl group and a free amine group.

  20. Inside red blood cells, hemoglobin is an especially good buffer. In the tissues, the Bohr effect ensures that oxygen is delivered to the tissues as hydrogen ions are buffered.

  21. Exhalation of Carbon Dioxide • What would be the net effect of hyperventilation? reaction shifts to the left increased H+ used to reform carbonic acid used to reform CO2 pH increases increased CO2 lost from the body • What would be the net effect of hypoventilation? • reaction shifts to the right as CO2 accumulates in the body • H+ accumulate in the body • pH decreases CO2 + H2O H2CO3 H+ + HCO3- CO2 + H2O H2CO3 H+ + HCO3-

  22. Kidney excretion of hydrogen ions • pH is adjusted by changing the rate of H+ secretion by the renal tubules. • Kidneys slow to respond to imbalances • Imbalances need to last for a few days or longer • More permanent

  23. Respiratory Acidosis/Alkalosis versus Metabolic acidosis/alkalosis