Fluid and Electrolytes Concepts

Chapter 41: Fluid and Electrolytes Basics Concepts

Overview

  • Instructor: Bozena Kik, RN, MSN, NE-BC

  • Course: N206 San Diego State University

Importance of Water

  • Vital Role: Most crucial nutrient for life, humans can only survive a few days without it.

  • Weight Measurement: A liter of water weighs 2.2 lbs (1 kg).

  • Assessment Method: The most reliable method for assessing fluid status is daily weights.

Functions of Water in the Body

  • Nutrient Transport: Delivers nutrients and oxygen to cells.

  • Waste Removal: Eliminates waste products from cells.

  • Hormonal Transport: Transports hormones, enzymes, blood platelets, and red/white blood cells.

  • Cellular Metabolism: Facilitates cellular metabolism and chemical reactions.

  • Solvent Role: Acts as a solvent for both electrolytes and nonelectrolytes.

  • Temperature Regulation: Helps in maintaining normal body temperature.

  • Digestion Aid: Facilitates digestion and aids in elimination processes.

  • Lubrication: Serves as a lubricant for tissues.

Compartments of Fluid in the Body

Intracellular Fluid (ICF)

  • Definition: Fluid contained within cells.

  • Percentage: Comprises 65-70% of total body water, 35-40% of total body weight.

Extracellular Fluid (ECF)

  • Definition: Fluid located outside the cells.

  • Percentage: Makes up 30-35% of total body water, 15-20% of total body weight.

  • Components of ECF:

    • Intravascular/Plasma: 5% of body weight (liquid component of blood).

    • Interstitial: 10-15% of body weight (fluid surrounding tissue cells).

    • Transcellular: <1% of body weight (includes fluids such as cerebrospinal fluid, pleural fluid, etc.).

Total Body Fluid

  • Comprises approximately 50-60% of body weight in a healthy adult.

Variations in Fluid Content

  • General Population: Total body water is 50-60% of body weight.

  • Infants: Have significantly higher body fluid percentage and extracellular fluid compared to adults.

  • Gender Differences: Women and individuals with higher fat cells have lower body water content; fat cells contain less water.

  • Elderly: Frequently demonstrate decreased total body fluid, which can drop as low as 45% due to loss of muscle mass and increased adipose tissue. This demographic is at increased risk for fluid imbalances.

Fluid Balance

Sources of Water

  • Ingested Liquids: Largest source of water intake, regulated by the thirst mechanism, which is influenced by dehydration or decreased blood volume.

  • Ingested Food: Second major source; certain foods like melons and citrus contain high water content, while dried fruits and cereals offer minimal water.

  • Metabolic Water: Resulting from metabolic processes such as the oxidation of carbohydrates, fats, and proteins.

Fluid Intake and Losses

  • Fluid Intake (mL):

    • Ingested water: 1,300

    • Ingested food: 1,000

    • Metabolic oxidation: 300

    • Total Intake: 2,600 mL

  • Fluid Output (mL):

    • Kidneys: 1,500

    • Skin: 600

    • Lungs: 300

    • Gastrointestinal: 200

    • Total Output: 2,600 mL

Electrolytes and Non-electrolytes

  • Homeostasis: Total cations equal total anions.

  • Definitions:

    • Solvents: Liquids that dissolve substances (e.g., water).

    • Solutes: Substances dissolved in a solution, including electrolytes and nonelectrolytes.

Electrolytes

  • Definition: Substances that dissociate into charged particles known as ions.

    • Cations: Positive charges (e.g., Sodium (Na+), Potassium (K+), Calcium (Ca2+), Hydrogen (H+), Magnesium (Mg2+)).

    • Anions: Negative charges (e.g., Chloride (Cl-), Bicarbonate (HCO3-), Phosphate (PO4^3-)).

  • Charged particles are integral for vital chemical reactions and metabolic processes. Non-electrolytes include molecules like urea and glucose that do not carry charges.

Osmosis

  • Definition: Movement of water through a semipermeable membrane from lower solute concentration to higher solute concentration until equilibrium is achieved.

  • Mechanism: Water traverses cell and capillary membranes through osmosis, crucial in fluid movement and intravenous (IV) fluid therapy.

Osmolality and Osmotic Pressure

Osmolality

  • Definition: Concentration of solute per kg of water; main determinants are sodium and protein.

  • Estimation of Serum Osmolality: Estimated as two times the serum sodium level; significant particles like urea and glucose elevate serum osmolality.

Osmolarity

  • Definition: Concentration of solute per liter of solution.

  • Osmotic Pressure: The 'pulling power' of a solution for water; higher osmolality correlates to greater osmotic pressure.

Osmolality of IV Solutions

  • Isotonic IV Solutions: Same concentration as plasma, no osmotic pressure difference; used for ECF and electrolyte replacement (e.g., Normal saline - 0.9% NaCl, Lactated Ringer’s).

  • Hypertonic IV Solutions: Higher concentration than plasma, draws water from cells into blood vessels, increasing vascular volume. Careful monitoring needed to prevent overload (e.g., 3% saline, Dextrose > 5%).

  • Hypotonic IV Solutions: Lesser concentration, causes water to move into cells, increasing cell water and decreasing vascular volume. Monitoring is essential to avoid cellular edema (e.g., 5% dextrose in water, 0.45% saline).

Diffusion

  • Definition: Movement of particles from an area of higher concentration to an area of lower concentration until equilibrium is reached.

  • Differential Permeability: Electrolytes can easily diffuse, while larger molecules such as urea and glucose do not diffuse through semipermeable membranes efficiently.

Capillary Fluid Movement

  • Hydrostatic Pressure: The force of fluid against vessel walls, primarily generated by the heart.

  • Oncotic Pressure: The pulling force exerted by colloids (e.g., proteins like albumin) within a solution, vital for maintaining blood volume and serum oncotic pressure.

Active Transport

  • Definition: Energy-requiring process that moves substances against a concentration gradient (lesser solute concentration to higher solute concentration).

  • Example: Na+/K+ pump; maintains higher extracellular sodium (Na+) and intracellular potassium (K+) concentrations, crucial for cellular function.

Chemical Regulation of Fluid Balance

Antidiuretic Hormone (ADH)

  • Source: Synthesized in hypothalamus, secreted by the posterior pituitary gland.

  • Function: Regulates water by enhancing reabsorption in kidney distal tubules during low cardiac output conditions, resulting in increased blood volume.

Aldosterone

  • Source: Hormone from adrenal glands.

  • Function: Conserves sodium, causing kidneys to retain sodium and excrete potassium; water follows sodium due to osmosis, indirectly affecting water retention.

    • Mechanism: Renin-angiotensin-aldosterone system (RAAS) pathway prompts sodium (and therefore water) retention.

Glucocorticoids (Cortisol)

  • Function: Released in stress, promotes sodium and water retention.

Natriuretic Peptides (ANP & BNP)

  • Source: Hormones released from the heart (atria/ventricles) when there is tension from increased blood volume.

  • Function: Reduce blood volume and blood pressure by promoting vasodilation and suppressing renin-angiotensin mechanism, decreasing aldosterone and ADH release, and increasing renal filtration rate.

Thirst Mechanism

  • Location: Thirst receptors located in the hypothalamus respond to plasma concentration changes.

  • Impact: Initiates ADH and aldosterone release to promote water reabsorption. Notably less effective in older adults.

Primary Organs of Homeostasis

  • Kidneys: Filter approximately 180 L of plasma daily, excreting about 1.5 L of urine.

  • Cardiovascular System: Pumps nutrients and water throughout the body.

  • Lungs: Regulate oxygen and carbon dioxide levels.

  • Adrenal Glands: Conserve sodium, chloride, and water; excrete potassium.

  • Pituitary Gland: Stores/releases ADH.

  • Thyroid Gland: Enhances blood flow and renal circulation.

  • Nervous System: Influences mechanisms regulating fluid balance.

  • Parathyroid Glands: Maintain calcium levels in ECF.

  • GI Tract: Absorbs water/nutrients entering the body via digestion.

Fluid Imbalances

Fluid Volume Deficit (Dehydration)

  • Definition: Loss of body fluids raises solute concentrations in blood, increasing serum Na+ levels.

  • Risk Factors: Hyperventilation, prolonged fever, diabetic ketoacidosis, enteral feeding without sufficient water.

  • Critical Populations: Individuals such as the confused, comatose, bedridden, as well as infants and the elderly.

  • Indicators of Dehydration: Fever, dry skin, sunken eyes, poor skin turgor, tachycardia, hypotension, hypothermia, irritability, confusion, dizziness, weakness, extreme thirst, decreased urine output.

  • Interventions: Fluid replacement (oral or IV), monitoring vital signs and symptoms, maintaining intake/output, daily weights, skin and mouth care.

Hypovolemia (Fluid Volume Deficit)

  • Definition: Isotonic fluid loss from the extracellular space that leads to hypovolemic shock.

  • Causes: Excessive fluid losses, decreased intake, third space fluid shifts, excessive renal losses.

  • Indicators: Mental status deterioration, thirst, tachycardia, delayed capillary refill, orthostatic hypotension, urine output < 30 mL/hr, cool/pale extremities, weight loss.

  • Interventions: Fluid and/or albumin replacement, blood transfusions, use of dopamine to maintain blood pressure, MAST trousers in severe shock, monitoring for fluid overload.

Hypervolemia (Fluid Volume Excess)

  • Definition: Excess fluid in extracellular compartment due to retention or excessive intake, leading to heart failure and pulmonary edema.

  • Causes: Heart failure, cirrhosis, renal failure, excessive sodium intake.

  • Indicators: Tachypnea, dyspnea, crackles, rapid/bounding pulse, hypertension, S3 gallop, weight gain, JVD, increased CVP and pulmonary artery pressures.

  • Interventions: Fluid and sodium restriction, diuretics, monitoring vital signs, strict input/output, assessment of breath sounds, supplemental oxygen as needed.

Edema

  • Definition: Fluid accumulation in tissues driven by hydrostatic pressure.

  • Types: Can present as anasarca (severe generalized edema) or pitting edema; pulmonary edema also falls under this category.

  • Interventions: Similar to hypervolemia management, focus on fluid and sodium control.

Water Intoxication

  • Definition: Hypotonic extracellular fluid enters cells, resulting in cellular swelling.

  • Causes: Conditions like SIADH, rapid infusion of hypotonic solutions, excessive tap water administration, or psychological conditions leading to polydipsia.

  • Indicators: Signs of increased intracranial pressure, early signs include changes in level of consciousness, N/V, muscle weakness. Late signs can escalate to bradycardia, widened pulse pressure, seizures, or coma.

  • Interventions: Preventative measures, neuro status assessment, monitoring of inputs/outputs, vital signs, seizure precautions, and restriction of fluids.

Laboratory Values for Fluid Volume Issues

  • Important laboratory values include (but not limited to):

    • Hematocrit (HCT)

    • Serum Osmolarity

    • Urine Specific Gravity

    • Serum Sodium