Human Body Fluid Balance and Electrolytes

Body Fluid Compartments

• The human body is approximately 60% water.
• 67% of total body water is intracellular (ICF), while 33% is extracellular (ECF).
• ECF is further divided into intravascular (25%, blood vessels & lymph) and extravascular (75%) compartments.
• Extravascular compartment includes interstitial fluid (95%) and transcellular fluid (5%).

Fluid Balance

• Maintaining water balance requires equal rates of water intake and loss.
• Intake includes eating and drinking, while excretion occurs through urine, feces, sweat, and respiration.
• Dehydration arises from excessive water loss or inadequate intake.
• Regulation of fluid balance involves thirst mechanism, electrolyte gradients, antidiuretic hormones, and renal excretion/reabsorption.

Electrolytes

• Electrolytes are charged atoms or molecules (ions) that regulate various bodily functions.
• These functions include water distribution, osmotic pressure, nerve transmission, cell permeability, oxidation-reduction reactions, and blood pH maintenance.
• Cation electrolytes include sodium (Na+) and potassium (K+).
• Anion electrolytes include chloride (Cl-) and bicarbonate (HCO3-).

Electrolyte Imbalances

• Electrolyte imbalances can be life-threatening.
• Causes include vomiting, excessive urination, sweating, diarrhea, bleeding, and exudation from burns or injuries.

Electrolyte Compartmentalization

• Each body fluid compartment contains specific electrolyte concentrations.
• Plasma and interstitial fluid are rich in sodium, chloride, and bicarbonate.
• Intracellular fluid is high in potassium.

Osmosis

• Osmosis is the diffusion of solvent molecules across a semipermeable membrane from a higher to a lower concentration.
• Osmotic pressure results from the force exerted by solute particles across the membrane.
• Higher solute concentration leads to increased osmotic pressure.
• Water movement between intracellular and extracellular compartments depends on osmotic pressure.

Anion Gap

• The anion gap is a mathematical calculation used to assess electrolyte balance and acid-base status.
• It estimates the difference between measured cations (Na+ and K+) and anions (Cl- and HCO3-).
• A normal anion gap ranges from 4 to 12 mEq/L.

Sodium (Na+)

• Sodium accounts for 90% of plasma cations.
• It enters the body through various sources (e.g., salt).
• Sodium absorption temporarily increases extracellular fluid volume.
• Sodium levels are tightly regulated within a narrow range (136-145 mEq/L).
• The Na+/K+ pump helps exchange sodium for potassium across cell membranes, maintaining a gradient.

Sodium Regulation

• Aldosterone, a hormone secreted by the adrenal gland, promotes sodium resorption and exchange with potassium.
• Atrial natriuretic peptide (ANP) is a heart hormone that increases sodium excretion and lowers blood pressure.

Sodium Functions

• Sodium plays a vital role in nerve impulse transmission, maintaining extracellular fluid osmotic pressure, facilitating muscle contractions, and maintaining acid-base balance and blood viscosity.

Sodium Imbalances

• Hyponatremia (low sodium) is more common and may be caused by low sodium intake, diarrhea, vomiting, excessive sweating, or renal disease.
• Hypernatremia (high sodium) is less frequent and can occur due to severe dehydration, Cushing Syndrome, inappropriate saline therapy, or high sodium intake with limited water consumption.

Sodium Lab Procedures

• Sodium levels are typically measured using ion-selective electrodes (ISEs).
• Direct ISEs are utilized in undiluted samples, while indirect ISEs use diluted samples.
• Specimen types for sodium measurement include serum, lithium heparinized plasma, whole blood, sweat, urine, feces, and gastrointestinal fluids.
• Errors in measurement can arise from high lipid or protein content in indirect ISEs, electrode protein coating, and competing ions.