05.2 Disorders of sodium balance

Disorders of Sodium Balance

• Sodium balance is crucial for regulating extracellular fluid (ECF) volume
• Sodium levels influence fluid shifts, causing either hypovolemia (volume contraction) or hypervolemia (volume expansion)
• Kidneys regulate sodium excretion to maintain euvolemia (normal fluid balance)
• Edema is excess fluid accumulation, often due to kidney dysfunction or changes in Starling forces

Learning Objectives

• Describe renal sodium handling and its impact on ECF volume
• Explain how plasma sodium variations affect ECF volume (hypo/hypervolemia)
• Differentiate between euvolemia, hypovolemia, and hypervolemia
• Describe kidney regulation of sodium excretion towards euvolemia
• Define edema and its contributing factors

Key Concepts

• Sodium Balance: Sodium (Na+) and its anions (Cl-, HCO3-) are primary ECF solutes, influencing ECF volume. High sodium increases ECF volume and blood pressure; low sodium reduces them.
• Volume Contraction (Hypovolemia): Decrease in ECF volume due to sodium loss, reducing blood volume and blood pressure.
• Volume Expansion (Hypervolemia): Increase in ECF volume due to sodium retention, increasing blood volume and blood pressure.
• Osmolarity: Concentration of osmotically active particles; plasma osmolarity influences fluid distribution.
• Glomerulotubular Balance: Mechanism ensuring proportionate sodium reabsorption to glomerular filtration rate (GFR), maintaining stable sodium levels with intake fluctuations.
• Edema: Excess fluid accumulation in interstitial spaces due to changes in Starling forces or kidney-regulated sodium/water retention.

Clinical Applications

• Isosmotic Volume Contraction (e.g., diarrhea): Loss of isotonic fluid decreases ECF volume (hypovolemia) without osmolarity change; prevents fluid shifts.
• Hyperosmotic Volume Contraction (e.g., water deprivation): Loss of more water than sodium increases osmolarity; fluid shifts from intracellular to extracellular spaces, reducing both volumes.
• Diagnostic approach involves evaluating sodium, osmolarity, and other indicators to determine volume status and isotonic, hypertonic, or hypotonic nature of disturbances.

Pathophysiology

• Hypovolemia: Low ECF volume activates the sympathetic nervous system, renin-angiotensin-aldosterone system (RAAS), and vasopressin release, promoting renal sodium and water retention to restore ECF volume.
• Hypervolemia: Increased ECF volume suppresses RAAS and stimulates atrial natriuretic peptide (ANP), promoting sodium and water excretion.
• Edema formation: Can result from increased capillary hydrostatic pressure or reduced oncotic pressure, leading to fluid shifts into interstitial spaces. Congestive heart failure (CHF) is an example, where venous congestion elevates capillary pressure.

Pharmacology

• Diuretics (e.g., furosemide, thiazides) enhance renal sodium and water excretion, useful in hypervolemia (CHF, edema).
• Aldosterone antagonists (e.g., spironolactone) block aldosterone action, reducing sodium reabsorption; useful in hypervolemia with RAAS factors (CHF).

Differential Diagnosis

• Hypovolemia: Conditions like diarrhea, vomiting, sweating, and Addison's disease
• Hypervolemia: Conditions like CHF, nephrotic syndrome, and cirrhosis
• Euvolemia: Normal blood pressure and ECF volume; minimal sodium retention or loss

Investigations

• Serum sodium levels, plasma osmolarity, urinary sodium, and blood urea nitrogen (BUN) and creatinine levels assess sodium balance and kidney function.

Explore the critical role of sodium balance in regulating extracellular fluid volume and its impact on conditions like hypovolemia and hypervolemia. This quiz covers renal sodium handling, the effects of sodium levels on fluid balance, and the definition and factors related to edema.