Water and sodium balance is the account of how the body holds and moves water and salt, and of what happens when the two go wrong together or apart. The disorders that result fall into four patterns, and the split between them is the organising idea:
- Hyponatremia — a low plasma sodium concentration, from water intake that the body does not excrete.
- Hypernatremia — a high plasma sodium concentration, from water loss that intake does not replace.
- Edema — water and sodium retained.
- Hypovolemia — water and sodium lost.
Hyponatremia and hypernatremia are disorders of water balance, while edema and hypovolemia are disorders of sodium balance.
A few terms recur in every note. The extracellular fluid (ECF) is the fluid outside the cells, and sodium is its main cation. Tonicity, or effective osmolality, counts only the solutes that cannot cross the cell membrane, because only those exert osmotic pressure. The effective intravascular volume is the part of the vascular volume that is actually perfusing tissue. SIADH is the syndrome of inappropriate antidiuretic hormone secretion, the most common cause of euvolemic hyponatremia. Osmotic demyelination is a rare demyelinating brain injury that correcting chronic hyponatremia too quickly can cause.
Read the notes in this order, beginning with body water, because the compartments and the effective intravascular volume are the background for everything that follows. The first three explain the physiology, the fourth sorts the disorders, and the last four take the reader from presentation through diagnosis to treatment:
- Body Water, Its Compartments, and Effective Intravascular Volume — how much water the body holds, how it is divided between the intracellular and extracellular compartments, and why the extracellular fluid volume does not always reflect tissue perfusion.
- Sodium Regulation and Steady State — what the plasma sodium concentration actually depends on, and the systems and steady state that keep the sodium content of the extracellular fluid stable.
- Urea and the Osmolality–Tonicity Paradox — the difference between osmolarity, osmolality and tonicity, and why urea raises plasma osmolality without raising plasma tonicity.
- Disorders of Water and Sodium Balance — the causes of hyponatremia, hypernatremia and hypovolemia, and how the osmolality of the fluid lost or retained decides the effect on the body fluids.
- Clinical Features of Hyponatremia and Hypernatremia — how the two dysnatremias present, and why severity depends less on the number than on how fast it changed.
- Finding the Cause of a Sodium Disorder — how to confirm the sodium value, classify by volume status, and read urine osmolality and urine sodium.
- Correcting Sodium Safely in Hyponatremia and Hypernatremia — the treatments, and the correction limits that avoid osmotic demyelination and cerebral edema.
- Choosing Saline in SIADH: Respect to Blood or Urine? — why in SIADH the tonicity of the saline has to be judged against the urine rather than the plasma.