Fluid, Electrolyte, and Acid-Base Balance

1. Fluid, Electrolyte, and Acid-Base Balance

2. Osmosis: Water molecules move from the less concentrated area to the more concentrated area in an attempt to equalize the concentration of solutions on two sides of a membrane.

3. Diffusion: The movement of molecules through a semipermeable membrane from an area of higher concentration to an area of lower concentration.

4. Solvent (H20) Movement • Cell membranes are semipermeable allowing water to pass through • Osmosis- major way fluids transported Water shifts from low solute concentration to high solute concentration to reach homeostasis (balance).

5. Osmolarity • Concentration of particles in solution • The greater the concentration (Osmolarity) of a solution, the greater the pulling force (Osmotic pressure) • Normal serum (blood) osmolarity = 280-295 mOSM/kg • A solution that has HIGH osmolarity is one that is > serum osmolarity = HYPERTONIC solution • A solution that has LOW osmolarity is one that is < serum osmolarity = HYPOTONIC solution • A solution that has equal osmolarity as serum = ISOTONIC solution

6. Hypertonic Fluids • Hypertonic fluids have a higher concentration of particles (high osmolality) than ICF • This higher osmotic pressure shifts fluid from the cells into the ECF • Therefore Cells placed in a hypertonic solution will shrink

7. Hypertonic Fluids • Used to temporarily treat hypovolemia • Used to expand vascular volume • Fosters normal BP and good urinary output(often used post operatively) • Monitor for hypervolemia !Not used for renal or cardiac disease. THINK – Why not? • D5% 0.45% NS • D5% NS • D5% LR Pulmonary Edema

8. Cell in a hypertonic solution 8

9. Hypotonic Fluids • Hypotonic fluids have less concentration of particles (low osmolality) than ICF • This low osmotic pressure shifts fluid from ECF into cells • Cells placed in a hypotonic solution will swell

10. Hypotonic Fluids • Used to “dilute” plasma particularly in hypernatremia • Treats cellular dehydration • Do not use for pts with increased ICP risk or third spacing risk • 0.45%NS • 0.33%NS

11. Cell in a hypotonic solution 11

12. Isotonic Fluid • Isotonic fluids have the same concentration of particles (osmolality) as ICF (275-295 mOsm/L) • Osmotic pressure is therefore the same inside & outside the cells • Cells neither shrink nor swell in an isotonic solution, they stay the same

13. Isotonic Fluid • Expands both intracellular and extracellular volume • Used commonly for: excessive vomiting,diarrhea • 0.9% Normal saline • D5W • Ringer’s Lactate

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15. Schematic of filtration pressure changes within a capillary bed. On the arterial side, arterial blood pressure exceeds colloid osmotic pressure, so that water and dissolved substances move out of the capillary into the interstitial space. On the venous side, venous blood pressure is less than colloid osmotic pressure, so that the water and dissolved substances move into the capillary.

16. Filtration pressure is the difference between colloid osmotic pressure and blood hydrostatic pressure. These pressures are important in understanding how fluid leaves arterioles, enters the interstitial compartment, and eventually returns to the venules. The filtration pressure is positive in the arterioles, helping to force or filter fluids into interstitial spaces; it is negative in the venules and thus helps fluid enter the venules.

17. Distribution of Body Fluids

20. Body Fluids • Water= most important nutrient for life. • Water= primary body fluid. Adult weight is 55-60% water. • Loss of 10% body fluid = 8% weight loss SERIOUS • Loss of 20% body fluid = 15% weight loss FATAL • Fluid gained each day should = fluid lost each day (2 -3L/day average)

21. Intracellular fluid (ICF) Fluid inside the cell Most (2/3) of the body’s H20 is in the ICF. Extracellular Fluid (ECF) Fluid outside the cell. 1/3 of body’s H20 More prone to loss 3 types: Interstitial- fluid around/between cells Intravascular- (plasma) fluid in blood vessels Transcellular –CSF, Synovial fluid etc Fluid Compartments

26. Acid - Base Balance • Blood - normal pH of 7.2 – 7.45 • < 7.2 = acidosis > 7.45 = alkalosis • 3 buffer systems to maintain normal blood pH • Buffers • Removal of CO2 by lungs • Removal of H+ ions by kidneys

27. Buffers • Protein Buffer Systems • Amino Acid buffers • Hemoglobin buffers • Plasma Protein buffers • Phosphate Buffer Systems • Carbonic Acid – Bicarbonate Buffer System

28. Buffer systems are used to keep the body in pH balance (homeostasis) • It consists of a weak acid (H+)and its dissociation products (an anion) • 3 major buffer systems in human • Protein buffer system (includes hemoglobin buffer system) • Regulates ICF & ECF (both plasma & interstitial fluid) • Most important in ICF & hemoglobin • Hemoglobin buffer system = carbonic anhydrase in RBC * it absorbs CO2 from ECF & get immediate effect • Amino acids have carboxyl group (gives up H+) and • Amino acids have amino group(can accept H+)

29. Carbonic acid-bicarbonate buffer system • Important in ECF • Lots of carbon dioxide from metabolic acids • It mixes with water & get carbonic acid which dissociates into H+ & HCO3- • Metabolic acids have H+ ; Our body has “bicarbonate reserve” • Bicarbonate reserve = ample supply of bicarb in ECF • These combine to form CO2 + H2O • CO2 excreted via lungs • Think of CO2 as an acid since it readily combines with water to become carbonic acid

31. Maintenance of Acid-Base Balance • Respiratory System: removal of CO2 by lungs – stabilizes the ECF, has direct effect on Carbonic Acid – Bicarbonate Buffer System • Urinary System: removal of H+ ions by kidneys

32. Regulation of blood pH by the respiratory system

33. Kidney excretion of H+ • Metabolic reactions produce nonvolatile acids • One way to eliminate this huge load is to excrete H+ in urine • In the proximal convoluted tubule, Na+/H+ antiporters secrete H+ as they reabsorb Na+ • Intercalated cells of collecting duct include proton pumps that secrete H+ into tubule fluid; reabsorb K+ and HCO3- • Urine can be up to 1000 times more acidic than blood • 2 other buffers can combine with H+ in collecting duct • HPO42- and NH3

34. Secretion of H+ by intercalated cells in the collecting duct