Fluid, Electrolyte Balance

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.

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

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). Water is a solvent Concentration of particles in solution (pulling action = osmolarity) Isotonic have almost same osmolarity as plasma therefore there is no pull

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 Osmosis, by the way, is the reason that drinking salt water will kill you. The HIGH osmolarity salt water in the GI system rapidly pulls water into the GI system and excretion – rapidly dehydrating cells SEE NEXT SLIDES FOR FURTHER DISCUSSION

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

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 Used for post op, decreases intracellular edema, fosters normal BP and good urinary output. D51/2NS, D5NS, D5RL Hyperal Pulmonary Edema

Cell in a hypertonic solution 8

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 ECF- extracellular fluids

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

Cell in a hypotonic solution 11

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 ICF intracellular fluid - fluid inside the cell D5W isotonic /Normal saline solution is isotonic because it has almost the same concentration of sodium as blood. Used to replace Ecvlume

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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Schematic of filtration pressure changes within a capillary bed 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.

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.

Distribution of Body Fluids Figure 52-1 Total body fluid represents 40 L in an adult male weighing 70 kg (154 lb).

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)

Intracellular fluid (ICF) Fluid Compartments 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 Intracellular fluid (ICF) Fluid inside the cell Most (2/3) of the body’s H20 is in the ICF. NOTE: Potter & Perry speaks to the “percentage of body weight” 40% of BODY WEIGHT = ICF fluid 20% of BODY WEIGHT = ECF fluid Transcellular fluid is a negligible amount

Solute Homeostasis Where sodium goes, water follows. Diffusion – movement of particles down a concentration gradient. Osmosis – diffusion of water across a selectively permeable membrane Active transport – movement of particles up a concentration gradient; requires energy

Electrolytes