Fluid shifts: hyponatremia, hypernatremia and hyperglycemia

The lecture draws each disorder as a pair of boxes (a Darrow–Yannet diagram): width is volume, height is osmolality, ICF on the left and ECF on the right. The dashed outline is the starting state and the solid outline is the new one. Read any of them with two questions, asked separately:

1. Does ECF tonicity change? If what was gained or lost has the same tonicity as plasma, nothing pulls water across cell membranes and the ICF is untouched. If the ECF becomes hypotonic, water moves into cells; if hypertonic, water moves out of cells.
2. Did ECF volume go up or down? That follows the change in total body sodium (and, for a moment, glucose or mannitol) plus any water shift from step 1.

An isotonic loss removes sodium and water in plasma proportions: secretory diarrhea (cholera) or bleeding. The remaining ECF has the same tonicity, so there is no gradient and no shift; the ECF shrinks and the ICF is unchanged. Serum Na⁺ stays normal (hypovolemic normonatremia), and the patient shows the volume-depletion signs of 4.1.

An isotonic gain, typically too much 0.9% saline, does the reverse: the ECF expands, the ICF is unchanged, serum Na⁺ is normal. The extra ECF accumulates in the interstitium as pitting edema and effusions, most readily in older people and those with renal dysfunction, who excrete sodium slowly.

In all three, the ECF is hypotonic, so water moves into cells and the ICF expands. They differ in total body sodium, and therefore in ECF volume.

Why does the hypovolemic patient's ICF expand while the ECF shrinks? He loses salty fluid and replaces it with water (thirst and ADH are both stimulated by volume loss). Sodium has been taken out of the ECF; the water left behind is now hypotonic, so some of it moves into cells. This is the only common hyponatremia in which the two compartments move in opposite directions.

Why does the SIADH patient look euvolemic? Pure water spreads through all body water, so two-thirds of it goes into cells. The small ECF expansion also suppresses renin and raises natriuretic peptides, so the kidney excretes sodium (urine Na⁺ > 20 mEq/L), which keeps the ECF from growing enough to produce edema.

Lecture image

In all three the ECF is hypertonic, so water moves out of cells and the ICF contracts. Thirst is the main defense: hypernatremia is rare in anyone awake with access to water, and common in infants, the very old and the unconscious.

Pure water loss shrinks the ECF too, but only by a third of the water lost (the rest comes out of cells), so volume signs are mild and the patient is called euvolemic. A hypertonic sodium gain is the mirror image of the hypovolemic hyponatremia: the compartments move in opposite directions, ECF up and ICF down.

Lecture image

A spot urine answers two questions about how the kidney is responding. Urine osmolality shows whether ADH is acting (concentrated urine) or switched off (dilute urine). Urine sodium shows whether the kidney is conserving sodium (< 20 mEq/L, as it does when it senses a poorly filled circulation) or excreting it (> 20 mEq/L, because of a diuretic, missing aldosterone, a sodium load or SIADH). Read them against the patient's volume.

Osmotic diuresis (glucosuria, mannitol) sits between the two: large volumes of urine that is neither dilute nor highly concentrated (roughly plasma-like osmolality, carrying a big solute load), with water lost in excess of sodium.

Without insulin, glucose cannot enter muscle and fat cells, so it acts as an effective osmole in the ECF. Water moves out of cells, the ICF contracts and, at first, the ECF expands and its sodium is diluted. This real dilution is translocational (hypertonic) hyponatremia: the sodium is low but measured osmolality is high. Soon the filtered glucose exceeds tubular reabsorption, and osmotic diuresis removes water (and sodium and potassium) in the urine, so both compartments end up contracted.

Choose an event. Before anything moves, predict for each compartment whether volume and osmolality rise, fall or stay the same, and which way water crosses the cell membrane. Then reveal and watch the boxes change.