Ch 41: Fluid, Electrolyte, and Acid–Base Balance

Functions of Water in the Body

Water is essential for numerous physiological processes and is the most abundant component of the human body. It serves several critical functions:

• Transport: Carries nutrients to cells and removes waste products.

• Circulation: Transports hormones, enzymes, blood platelets, and blood cells.

• Metabolism: Facilitates cellular metabolism and chemical reactions.

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

• Temperature Regulation: Helps maintain normal body temperature.

• Digestion and Elimination: Aids in digestion and promotes elimination of waste.

• Lubrication: Acts as a lubricant for tissues.

Body Fluid Compartments

Body fluids are distributed in two main compartments:

• Intracellular Fluid (ICF): Fluid within cells, accounting for about 70% of total body fluid.

• Extracellular Fluid (ECF): Fluid outside cells, about 30% of total body fluid. Includes:

  • Intravascular Fluid: Plasma within blood vessels.

  • Interstitial Fluid: Fluid between cells.

Variations in Fluid Content:

• Infants have more body fluid and ECF than adults, making them more susceptible to fluid deficits.

• Women and obese individuals have less body water due to higher fat content.

Fluid Balance and Imbalances

Fluid balance is maintained when fluid intake equals fluid loss. Key terms:

• Solvent: The liquid (usually water) that dissolves substances.

• Solute: Substances dissolved in a solution (electrolytes and nonelectrolytes).

Fluid Losses:

• Kidneys: Excrete urine.

• Intestinal Tract: Excretes feces.

• Skin: Loses water through perspiration.

• Insensible Loss: Water lost through evaporation and respiration.

Types of Fluid Imbalances:

• Hypovolemia: Deficiency of water and electrolytes in ECF with near-normal proportions.

• Dehydration: Decreased volume of water with electrolyte changes.

• Third-space Fluid Shift: Fluid moves into potential spaces (e.g., peritoneal cavity), not available for normal use.

Electrolytes and Their Functions

Electrolytes are ions that carry an electrical charge and are vital for physiological functions.

• Cations: Positively charged ions (e.g., sodium, potassium, calcium, magnesium).

• Anions: Negatively charged ions (e.g., chloride, bicarbonate, phosphate).

• Homeostasis: The total number of cations equals the total number of anions.

Osmolarity and Solution Types

Osmolarity refers to the concentration of solute particles in a solution:

• Isotonic: Same concentration as plasma; no net movement of water.

• Hypertonic: Higher concentration than plasma; water moves out of cells, causing them to shrink.

• Hypotonic: Lower concentration than plasma; water moves into cells, causing them to swell.

Transport Mechanisms of Body Fluids

• Osmosis: Movement of water from lower to higher solute concentration until equilibrium is reached.

• Diffusion: Movement of solutes from higher to lower concentration.

• Active Transport: Movement of substances against a concentration gradient, requiring energy.

• Capillary Filtration: Movement of fluid through a membrane from higher to lower pressure.

Acid–Base Balance

The body maintains a narrow pH range (7.35–7.45) for optimal function.

• Acid: Substance that releases hydrogen ions (H+).

• Base: Substance that accepts or traps hydrogen ions.

• Acidosis: Excess hydrogen ions (low pH).

• Alkalosis: Deficit of hydrogen ions (high pH).

Major Homeostatic Regulators:

• Buffer Systems: Carbonic acid–sodium bicarbonate, phosphate, and protein buffers.

• Respiratory Mechanisms: Regulate carbon dioxide (CO2) exhalation.

• Renal Mechanisms: Regulate bicarbonate (HCO3-) excretion or retention.

Key Buffer Equation:

Additional info: The carbonic acid–bicarbonate buffer system is the most important in ECF, buffering up to 90% of hydrogen ions.

Types of Acid–Base Imbalances

• Respiratory Acidosis: Excess carbonic acid due to hypoventilation.

• Respiratory Alkalosis: Deficit of carbonic acid due to hyperventilation.

• Metabolic Acidosis: Deficit of bicarbonate or excess acid.

• Metabolic Alkalosis: Excess bicarbonate or loss of acid.

Fluid Volume Excess and Deficit

• Hypervolemia: Excessive retention of water and sodium in ECF.

• Overhydration: Excess water in extracellular spaces.

• Edema: Accumulation of ECF in tissue spaces.

• Interstitial-to-Plasma Shift: Fluid moves from interstitial spaces to blood.

Electrolyte Imbalances

Primary Organs of Homeostasis

• Kidneys: Filter plasma and excrete urine, regulating fluid and electrolyte balance.

• Cardiovascular System: Circulates nutrients and water.

• Lungs: Regulate oxygen and carbon dioxide levels.

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

• Pituitary Gland: Stores and releases antidiuretic hormone (ADH).

• Thyroid Gland: Increases blood flow and renal circulation.

• Nervous System: Regulates mechanisms influencing fluid balance.

• Parathyroid Glands: Regulate calcium levels in ECF.

• Gastrointestinal Tract: Absorbs water and nutrients.

Assessment and Diagnosis

• Nursing History: Fluid intake and elimination patterns, hydration status, disease and medication history.

• Physical Assessment: Skin/tongue turgor, oral moisture, tearing, skin appearance, edema, vital signs.

• Laboratory Studies: CBC, serum electrolytes, BUN, creatinine, urine pH/specific gravity, arterial blood gases.

Risk Factors for Imbalances

• Acute and chronic illnesses

• Abnormal fluid losses (vomiting, diarrhea, burns, trauma, surgery)

• Therapies affecting fluid/electrolyte balance

Expected Outcomes and Interventions

• Maintain fluid intake/output balance (approx. 2,500 mL over 3 days).

• Maintain urine specific gravity (1.010–1.025).

• Practice self-care for balance.

• Report symptom relief and recognize recurrence.

Interventions:

• Prevent imbalances through dietary planning and fluid modification.

• Administer medications (mineral–electrolyte preparations, diuretics).

• Provide intravenous (IV) fluid therapy as needed.

Intravenous Therapy and Blood Administration

• Central Venous Access Devices: Provide access for IV fluids, medications, blood products, and total parenteral nutrition (TPN); allow hemodynamic monitoring and blood sampling.

• Types: PICCs, nontunneled/tunneled central venous catheters, implanted ports.

• Vein Site Selection: Based on accessibility, condition, type of fluid, and duration.

• Blood Administration: Requires typing and cross-matching (A, B, AB, O; Rh factor), donor selection, transfusion initiation, and monitoring for reactions.

Key Terms and Definitions

• Electrolyte: Substance that dissociates into ions in solution and conducts electricity.

• Nonelectrolyte: Compound that does not dissociate into ions in solution.

• Osmosis: Movement of water across a semipermeable membrane from low to high solute concentration.

• Diffusion: Movement of molecules from high to low concentration.

• Active Transport: Energy-dependent movement of substances across membranes.

• Buffer: Substance that minimizes pH changes by absorbing or releasing hydrogen ions.

Example: Hyponatremia

Hyponatremia is a sodium deficit in ECF, often caused by excessive loss of sodium or gain of water. Symptoms may include confusion, muscle weakness, and seizures. Treatment involves correcting the underlying cause and restoring sodium balance.

Example: Carbonic Acid–Bicarbonate Buffer System

This buffer system maintains blood pH by balancing carbonic acid (H2CO3) and bicarbonate (HCO3-):

It is the primary buffer in extracellular fluid, responsible for buffering up to 90% of hydrogen ions.