What does this topic cover?
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. Disorders of this balance fall into four patterns. How are they organised?
What one question sorts the four patterns?
Sodium is the main cation of the fluid outside the cells. Plasma sodium is a ratio of sodium to the water that holds it, as the sodium-regulation note showed. So the number moves when water moves against the sodium.
That gives one organising question: did the water change against the salt, or did the salt and the water change together?
Water against salt gives hyponatremia and hypernatremia, the two disorders of water balance. Hyponatremia is caused by more water in the body than sodium, and hypernatremia by a water deficit against the sodium.
Salt and water together give edema and hypovolemia, the two disorders of sodium balance. Hypovolemia is a loss of sodium and water together.
The split between these four patterns is the idea the rest of the topic hangs on. Is the body out of step in water, or in salt and water together?
What do the notes of this cluster explain?
The notes follow that split. The first three explain the physiology, the fourth sorts the disorders, and the last four go from presentation through diagnosis to treatment.
- Body Water, Its Compartments, and Effective Intravascular Volume: how much water the body holds, how it is shared between the cells and the fluid outside them, and why the volume outside the cells does not always mean the tissues are well supplied.
- Sodium Regulation and Steady State: what sets plasma sodium besides sodium and water, and how the body keeps the sodium of the fluid outside the cells steady.
- Urea and the Osmolality–Tonicity Paradox: why osmolarity, osmolality and tonicity are different measures, and why urea raises one but not another.
- Disorders of Water and Sodium Balance: the causes of hyponatremia, hypernatremia, hypovolemia and edema, and what each does to the fluid left in the body.
- Clinical Features of Hyponatremia and Hypernatremia: how the two sodium disorders present, and why the speed of the change matters more than the number.
- Finding the Cause of a Sodium Disorder: how to confirm a sodium result, sort the patient by volume status, and read urine osmolality and urine sodium.
- Correcting Sodium Safely in Hyponatremia and Hypernatremia: how the disorder is treated, and the limits that protect the brain while sodium is brought back.
- Choosing Saline in SIADH: Respect to Blood or Urine?: in SIADH, saline is judged against the urine rather than the plasma.
Where should you start?
Start with body water, because its compartments and effective intravascular volume are the background for everything that follows. How much water does the body hold, and how is it shared between the cells and the fluid outside them?