HS 147 - Chapter 8

Chapter 8: Disorders of Fluid, Electrolyte, and Acid-Base Balance

Overview

• Fluid, electrolyte, and acid-base balance are crucial for all cell functions and are primarily managed by the kidneys.

• An imbalance in hydrogen concentration can lead to acidosis (excess acid) or alkalosis (excess base).

• Body fluids are present in cells, tissue spaces, and the blood vessels.

• Environmental stressors and diseases can disrupt the regulation of fluid volume, composition, and distribution in the body.

Causes of Disorders

• Disorders can arise from:

  • Respiratory or renal disease

  • Obesity

  • Medications

• Key electrolyte ions involved include sodium, potassium, calcium, magnesium, chloride, hydrogen, phosphate, and bicarbonate.

• Imbalances may be acute or chronic and can vary in severity, possibly overwhelming the body's compensatory mechanisms.

Regulation of Electrolyte Balance

• Primarily regulated by:

  • Hypothalamus

  • Kidneys

  • Hormones (e.g., ADH, aldosterone, parathyroid hormone)

• Electrolytes consist of ionized constituents of cells, while non-electrolytes (e.g., glucose, urea) do not dissociate into ions.

• Ions are charged particles: Cations (positively charged) and Anions (negatively charged).

Body Fluid Compartments

• Total body water (TBW) comprises about 60% of body weight in males (55% in females), lower in obese and elderly individuals.

• Distribution:

  • Intracellular Fluid (ICF): Approx. 2/3 of TBW; high in potassium (K+).

  • Extracellular Fluid (ECF): Approx. 1/3 of TBW; includes interstitial fluid and plasma, high in sodium (Na+).

Fluid Movement Mechanisms

• Diffusion: Movement of particles along a concentration gradient from high to low concentration.

• Osmosis: Movement of water across a semi-permeable membrane towards a higher concentration of solute.

• Tonicity:

  • Isotonic: No change in cell volume.

  • Hypotonic: Cells swell.

  • Hypertonic: Cells shrink.

Water Movement and Edema

• Capillary Dynamics:

  • Four forces control fluid movement at the capillary level:

    • Capillary filtration pressure (pushes fluid out)

    • Capillary colloidal osmotic pressure (pulls fluid in)

    • Tissue hydrostatic pressure (opposes fluid outflow)

    • Tissue colloidal osmotic pressure (pulls fluid out)

• Edema: Accumulation of interstitial fluid; treated by diuretics and managing underlying causes.

  • Causes include heart failure, decreased osmotic pressure, increased permeability, lymphatic obstruction.

  • Types of edema:

    • Pitting Edema: Leaves an indentation when pressure is applied.

    • Non-pitting Edema: Firm to touch without indentations, associated with thyroid or lymphatic disorders.

    • Brawny Edema: Thickening and discoloration due to chronic venous insufficiency.

Sodium and Water Balance

• Sodium is the most abundant cation in ECF and is key for ECF volume and acid-base balance.

• Sodium Regulation:

  • Controlled by kidneys, influenced by hormonal systems (e.g., renin-angiotensin-aldosterone).

  • An increase in sodium can lead to fluid retention, affecting blood volume and pressure.

Water Intake and Loss

• Water gain: through oral intake and metabolism.

• Water loss: primarily through the kidneys, also through skin, lungs, and GI tract.

Hyponatremia and Hypernatremia

• Hyponatremia: Reduced sodium concentration; symptoms include confusion, fatigue, and seizures. Causes include diuretics, injuries, and SIADH.

• Hypernatremia: Elevated sodium levels; symptoms include excessive thirst and confusion, common in patients unable to express thirst.

Potassium Balance

• Potassium is critical in ICF for cell function. Regulation involves renal mechanisms.

• Hypokalemia: Low potassium levels; can lead to muscle weakness and cardiac disturbances.

• Hyperkalemia: High potassium levels; symptoms include muscle dysfunction and cardiac irregularities.

Calcium, Phosphorus, and Magnesium

• Calcium is essential for bone health and many body functions, primarily regulated by PTH and vitamin D.

  • Hypocalcemia: Symptoms include muscle cramps and seizures.

  • Hypercalcemia: Causes appetite loss, confusion, and increased thirst; often a result of cancer or hyperparathyroidism.

• Phosphorus plays a key role in energy and is influenced by dietary intake and renal function.

  • Hypophosphatemia: Can lead to weakness and increased infection risk.

  • Hyperphosphatemia: Often associated with chronic kidney disease.

Acid-Base Balance

• Managed by the kidneys and lungs; slight variations can have serious consequences.

• Measured on a pH scale. Acidosis and alkalosis can be metabolic (kidney-related) or respiratory (lung-related).

• Metabolic Acidosis: Caused by renal failure, dehydration, and diabetic ketoacidosis.

• Respiratory Acidosis: Caused by impaired CO2 removal.

• Compensatory mechanisms include chemical buffers and respiratory/renal adjustments.

Summary of Disorders

• Familiarize with disorders affecting fluid and electrolytes, their signs, causes, and implications for treatment.

• Important to recognize interdependence in sodium and water balance, and their key role in maintaining homeostasis.