Osmotic Agents: Pulling Water With Mannitol and Hypertonic Saline
Most drugs work by binding a receptor, an enzyme, a channel — a molecular lock and key. Osmotic agents don't bother with any of that. They work by a law of physics: put enough of a solute in one place, and water will move toward it, all on its own. There is no receptor, no signalling, no metabolism to speak of — just a gradient and the water that follows it. From shrinking a swollen brain in a neuro-emergency, to pulling fluid off the eye in acute glaucoma, to loosening the bowel, the same single trick is doing all the work. Understand osmosis and you understand every one of these agents at once.
A 24-year-old man arrives in the emergency department after a motorbike crash, deeply unconscious, one pupil blowing wide and sluggish. The CT shows a swelling, bleeding brain pressing against the rigid skull that cannot expand to accommodate it — intracranial pressure is climbing toward the point where the brainstem is squeezed and breathing stops. The neurosurgeon is minutes away, but the brain cannot wait. A nurse hangs a bag of mannitol and runs it in fast. Within twenty minutes the pupil begins to react again. Nothing was cut, no receptor was blocked — a sugar that the body cannot use simply sat in the bloodstream and pulled water out of the waterlogged brain, back across the vessel wall and into the circulation, buying the time the surgeon needs. It is one of the oldest tricks in medicine, and it is pure osmosis.
The one principle behind all of them
An osmotic agent is nothing but a solute that stays where you put it. Water moves across a semipermeable membrane from where solute is dilute to where it is concentrated, chasing balance. What decides the pull is not the total number of particles but the number of effective osmoles — solutes that are trapped on one side and cannot cross to even things out. A trapped osmole is like an anchor for water. Sodium is the great effective osmole of the extracellular fluid; glucose becomes one when insulin is missing; urea, by contrast, crosses membranes freely and so pulls water only briefly — it is an "ineffective" osmole. Every osmotic drug is engineered around this: give a substance that lodges in one compartment, does not get reabsorbed or metabolised, and it will drag water toward itself for as long as it stays there. That is the entire mechanism, and everything below is a variation on it.
Think of a compartment of water separated by a fine mesh, and you toss a handful of dry sponges onto one side — but sponges too big to slip through the mesh. They swell as water crawls toward them, and the level on that side rises while the other side falls. The sponge never pumps anything; it just sits there being thirsty, and geometry does the rest. Mannitol in the bloodstream is that trapped sponge: too big and too un-metabolised to leave, it makes the blood "thirsty" and water migrates out of the swollen brain to join it. Pull the sponge out to a new compartment — the bowel — and the same thirst loosens stool. One object, one behaviour, many uses.
Mannitol: the filtered sugar that won't come back
Mannitol is a sugar alcohol given intravenously. Two properties make it the classic osmotic drug. First, it distributes only in the extracellular space and does not cross healthy cell membranes, so it stays outside cells and holds water there. Second, it is freely filtered at the glomerulus but then barely reabsorbed by the tubule — so it lingers in the tubular fluid and drags water along the whole length of the nephron, producing an osmotic diuresis. It is, in effect, a diuretic that works from inside the tubular lumen rather than by blocking a transporter, which is why it appears alongside the loop and thiazide agents in the Diuretics chapter as its own mechanistic class. Its usefulness and its dangers both flow from the same fact: it pulls water wherever it happens to be sitting.
What mannitol is actually used for
Three uses, one mechanism, three different compartments to empty. The headline use is raised intracranial pressure and cerebral oedema. Because the blood–brain barrier is intact and keeps mannitol out of the brain, an infusion raises the osmolality of the blood relative to the swollen brain tissue, and water is drawn out of the brain, across that intact barrier, into the vasculature — shrinking the brain and lowering the pressure. It is a mainstay of neuro-emergencies and links directly to the Central nervous system section on managing raised ICP. Second, osmotic diuresis: historically mannitol was used to keep urine flowing in threatened oliguria and to flush certain toxins out through the kidney, though this role has narrowed. Third, it lowers intraocular pressure in acute angle-closure glaucoma by pulling water out of the vitreous — a use that connects to the Ophthalmology chapter, where the same osmotic pull (mannitol or oral glycerol) buys time before the eye can be treated definitively.
Mannitol has two phases, and forgetting the first one is dangerous. The instant it enters the blood it acts as a plasma expander — it pulls water into the circulation before a drop of urine is made, so the intravascular volume briefly rises. In a healthy brain-injured patient that is fine. In a patient with heart failure or pulmonary oedema, that sudden extra volume can tip them into acute pulmonary oedema before the diuresis ever catches up. So the mental model is not "mannitol dries you out"; it is "mannitol first floods the vessels, then empties them." You must respect both halves.
The cautions: from volume overload to the osmolar gap
After the early expansion comes the diuresis, and with it the opposite risk: if the water lost in urine is not replaced, the patient swings into hypovolaemia, and the brisk loss of water relative to electrolytes disturbs sodium and potassium — dehydration with a rising serum sodium is a real hazard. Two further traps are specific to the brain. If the blood–brain barrier is disrupted — as it often is in severe injury — mannitol can leak into the damaged brain tissue itself, reverse its own gradient, and pull water into the brain, a rebound worsening of oedema; and with repeated dosing it accumulates in the tissue, blunting later doses. Monitoring is therefore mandatory: track serum osmolality and the osmolar gap (the gap between measured and calculated osmolality, which mannitol widens as it builds up), keep an eye on renal function, and stop when the gap grows too large. Give too much for too long and mannitol can itself injure the kidney.
- An osmotic agent is a solute trapped in one compartment; it drags water toward itself along the gradient.
- Mannitol is a filtered-but-not-reabsorbed sugar alcohol — an osmotic diuretic acting from inside the tubule.
- Its flagship use is raised intracranial pressure: it pulls water out of the brain across an intact blood–brain barrier.
- Also used for osmotic diuresis and to lower intraocular pressure in acute glaucoma.
- Mannitol first expands intravascular volume — dangerous in heart failure/pulmonary oedema — then diureses.
- Monitor serum osmolality and the osmolar gap; watch for hypovolaemia, rebound cerebral oedema with a broken barrier, and renal injury.
Hypertonic (3%) saline: sodium as the osmole
Where mannitol uses a foreign sugar, hypertonic saline uses the body's own great osmole — sodium. 3% saline is far saltier than blood, so infusing it raises plasma sodium and osmolality, and water is drawn out of cells — including brain cells — into the extracellular fluid and circulation. That gives it two distinct roles. The first is raised intracranial pressure, as an alternative or adjunct to mannitol: it too shrinks the brain, but with a crucial difference in the volume it leaves behind. Mannitol ultimately depletes the intravascular volume through diuresis; hypertonic saline expands it. So in the head-injured patient who is also hypovolaemic or hypotensive — where you cannot afford to empty the tank — hypertonic saline is often the better choice, lowering the intracranial pressure while supporting the blood pressure at the same time. This decision sits squarely in the Central nervous system chapter on neuro-critical care.
The second role is severe, symptomatic hyponatraemia — a sodium so low the patient is seized or comatose from brain swelling. Here 3% saline is given as small, controlled boluses aimed only at pulling the sodium up just enough to relieve the cerebral oedema, not at normalising it. And this is where the danger of the osmotic principle bites back: if you raise a chronically low sodium too fast, water rushes out of brain cells so abruptly that they shrink and their myelin is stripped — osmotic demyelination syndrome (central pontine myelinolysis), a catastrophic, often irreversible injury. The correction is therefore rate-limited — no more than roughly 8–10 mmol/L in 24 hours — a number the Hyponatraemia chapter treats at length. The same physics that saves the brain in the first hour can destroy it over the next day if you pull the water out too quickly.
Both lower intracranial pressure by pulling water out of the brain across an intact barrier — the difference is what they do to the rest of the body. Mannitol: potent, familiar, but it diureses, so it can deplete intravascular volume and drop the blood pressure; monitor the osmolar gap and renal function; risk of rebound if the barrier is broken. Hypertonic saline: shrinks the brain while expanding the circulation, so it suits the hypovolaemic or hypotensive trauma patient; it raises serum sodium (which must be watched) and needs reliable, often central, access. Rule of thumb: reach for mannitol in the volume-replete patient with good kidneys, and for hypertonic saline when the patient is dry, bleeding, or hypotensive and you cannot afford a diuresis.
The same trick, in the gut and elsewhere
Once you see the principle, you see it everywhere. Glycerol and urea are older osmotic agents used, like mannitol, to pull water out of the brain or eye, though they have largely been displaced. The osmotic laxatives are the same physics moved into the bowel: lactulose (a non-absorbable sugar), macrogol (polyethylene glycol), and the magnesium salts all hold water in the gut lumen by osmosis, softening stool and speeding transit — the identical "trapped osmole draws water" mechanism you meet in full in the Gastrointestinal section on constipation. And the body generates its own osmotic diuresis in disease: in uncontrolled diabetes, glucose spilling into the urine acts exactly like mannitol, dragging water with it and causing the classic polyuria and dehydration of hyperglycaemia; a high urea in renal failure does something similar. The recurring theme could not be simpler — create a gradient with a solute that cannot escape, and water follows it, whether that water is leaving a swollen brain, a glaucomatous eye, or a constipated colon.
- Hypertonic (3%) saline uses sodium as the trapped osmole to pull water out of cells, including the brain.
- For raised ICP it shrinks the brain while expanding intravascular volume — mannitol depletes it via diuresis.
- Prefer hypertonic saline in the hypovolaemic/hypotensive head-injured patient; mannitol in the volume-replete one.
- Hypertonic saline also treats severe symptomatic hyponatraemia — small controlled boluses, not full normalisation.
- Correct sodium slowly (≈8–10 mmol/L per 24h) — too fast causes osmotic demyelination (central pontine myelinolysis).
- Osmotic laxatives (lactulose, macrogol, magnesium salts) and glucosuric/uraemic diuresis are the same principle in other compartments.
- Giving mannitol to a patient in heart failure or pulmonary oedema without heeding the early plasma-expansion phase — the sudden intravascular load can precipitate acute pulmonary oedema before any diuresis.
- Correcting a chronically low sodium too quickly with hypertonic saline — pulling water out of brain cells too fast strips their myelin and causes osmotic demyelination; cap the rise near 8–10 mmol/L per 24h.
- Continuing repeated mannitol doses without tracking the osmolar gap and renal function — accumulation blunts later doses, can reverse into rebound cerebral oedema when the barrier is broken, and can injure the kidney.
A 30-year-old with a severe traumatic brain injury has a rising intracranial pressure. He is also hypotensive and hypovolaemic from other injuries. Which osmotic therapy is the more appropriate first choice, and why?
- Osmotic agents work by pure physics: a solute trapped in one compartment drags water toward it — no receptor, no metabolism, just a gradient.
- Mannitol, a filtered-but-not-reabsorbed sugar alcohol, pulls water out of the brain (raised ICP), the tubule (osmotic diuresis) and the eye (acute glaucoma) — but first expands, then depletes, intravascular volume.
- Hypertonic (3%) saline uses sodium as the osmole: for raised ICP it shrinks the brain while expanding volume (better than mannitol in the hypovolaemic patient), and it treats severe symptomatic hyponatraemia — but sodium must be raised slowly (≈8–10 mmol/L/24h) to avoid osmotic demyelination.
- The same principle spans glycerol/urea, the osmotic laxatives (lactulose, macrogol, magnesium salts), and the glucosuric/uraemic diuresis of disease: create a gradient, and water follows.
- Rang & Dale's Pharmacology — Diuretics: osmotic diuretics (mannitol).
- Katzung Basic & Clinical Pharmacology — Agents that alter water excretion: osmotic diuretics.
- British National Formulary (BNF) — Mannitol; sodium chloride hypertonic solutions.
- Brain Trauma Foundation Guidelines for the Management of Severe Traumatic Brain Injury — hyperosmolar therapy (mannitol and hypertonic saline).
- UK Kidney Association / Renal Association Clinical Practice Guideline — Investigation and Management of Hyponatraemia in Adults.
- Ganong's Review of Medical Physiology — Body fluid compartments, osmolality and the osmolar gap.
