F LUID AND E LECTROLYTES DR. WALID AL-MEKHLAFI 202 5 o To achieve homeostasis, the body maintains strict control of water and electrolyte distribution and of acid-base balance. o This control is a function of the complex interplay of cellular membrane forces, specific organ activities and systemic and local hormone actions. Water constitutes an average 50 to 70% of the total body weight ( about 42L in an adult of 70kg) : • Young males → 60% of total body weight • Older males → 52% • Young females → 50% of total body weight • Older females → 47% Variation of ±15% in both groups is normal Obese have 25 to 30% less body water than lean people Infants 75 to 80% • Gradual physiological loss of body water Sources of body fluid: • Preformed (intake) = 2300ml/day • Metabolic water (produced through the catabolic breakdown of nutrients occurring during cellular respiration) = 200 ml/day 1. Provide transportation of nutrients to cells e.g., O2 2. Carry waste products away from cells e.g., CO2 3. Provide the environment for electrolyte chemicals reactions to occur 4. Aids in regulation of body temperature 5. Maintain blood volume 6. Solvent for minerals, vitamins and glucose Composition of Body Fluids Nonelectrolytes include most organic molecules, do not dissociate in water, and carry no net electrical charge. Electrolytes dissociate in water to ions, and include inorganic salts, acids and bases, and some proteins. The major cation in extracellular fluids is sodium, and the major anion is chloride; in intracellular fluid the major cation is potassium, and the major anion is phosphate. Electrolytes are the most abundant solutes in body fluids, but proteins and some nonelectrolytes account for 60-–97% of dissolved solutes Normally, there is a balance achieved between our total daily intake and output of water. Total fluid intake is modified by the induction of the sensation of thirst. This is produced by a reaction of cells in Hypothalamus to the increased osmotic pressure of the blood passing through this region. Another stimulus of thirst would be the degree of dryness of the oral mucosa Influence of ADH The amount of water reabsorbed in the renal collecting ducts is proportional to ADH release. When ADH levels are low, most water in the collecting ducts is not reabsorbed, resulting in large quantities of dilute urine. When ADH levels are high, filtered water is reabsorbed, resulting in a lower volume of concentrated urine. ADH secretion is promoted or inhibited by the hypothalamus in response to changes in solute concentration of extracellular fluid, large changes in blood volume or pressure, or vascular baroreceptors. Problems of Fluid Balance Deficient fluid volume Hypovolemia Dehydration Excess fluid volume Hypervolemia Water intoxication Intravenous fluid therapy: Crystalloids Colloids Definitions • Solute: a particle, usually a salt • Solvent: liquid, usually water • Solution: solute and solvent mixed together What solutions’ tonicity can be: Hypotonic: low solute, high solvent Isotonic: equal solute and solvent ratio Hypertonic: high solute, low solvent Isotonic The cell has the same concentration on the inside and outside which in normal conditions the cell’s intracellular and extracellular are both isotonic. Isotonic solutions are used: to increase the extracellular fluid volume due to • blood loss • surgery • dehydration Isotonic fluids o 0.9% NaCl (Normal Saline) o 5% dextrose in water (D5W) (In the bag) also used as a hypotonic solution after it is administered because the body absorbs the dextrose BUT it is considered isotonic o 5% Dextrose in 0.225% saline (D5W1/4NS) o Lactated Ringer’s Hypotonic The cell has a low amount of solute extracellularly and it wants to shift inside the cell to get everything back to normal via osmosis. This will cause cell swelling which can cause the cell to burst or lyses. Hypotonic solutions are used when the cell is dehydrated and fluids need to be put back intracellularly. • diabetic ketoacidosis (DKA) • hyperosmolar hyperglycemia Hypotonic solutions o D5W (in the body) o 0.25% NaCl o 0.45% NaCl (half normal saline) o 2.5% Dextrose