osmotic
Meaning of osmotic: Water follows salt and sugar rather than the other way around, and most of the body's fluid shifts follow from that one rule.
Definition of osmotic
Osmotic describes anything to do with osmosis, the movement of water across a membrane that lets water through but holds back dissolved particles. Water moves toward the side where those particles are more concentrated, diluting it, until the two sides come closer to balance. The movement is passive, so no cell has to spend energy driving it.
What does osmosis do in the body?
Cell membranes let water pass freely but hold back most dissolved particles, so water moves constantly to even out concentrations that cannot be equalized any other way. Body fluid is normally kept at a matched concentration inside and outside cells, and sodium is the main particle setting that level on the outside. When the outside becomes dilute, water moves into cells and they swell. When it becomes concentrated, water leaves and they shrink. Brain cells are the most vulnerable, since the skull leaves no room for swelling, which is why disturbances of blood sodium show up as confusion and drowsiness rather than as symptoms in the limbs. The same pull operates across capillary walls, where proteins too large to escape hold water inside the bloodstream; when those proteins fall, fluid collects in the tissues instead.
Where does the word turn up in medicine?
| Use | What is happening |
|---|---|
| Osmotic laxative | A poorly absorbed sugar or salt pulls water into the bowel |
| Osmotic diuresis | Glucose spilling into urine when blood sugar is very high drags water out with it |
| Osmotic agents for brain swelling | A concentrated solution in the blood draws water out of swollen brain tissue |
| Osmotic demyelination | Nerve damage following correction of a very low blood sodium too quickly |
How does an osmotic laxative differ from a stimulant one?
The two work on entirely different principles. An osmotic laxative is barely absorbed, so it stays in the gut and holds water alongside it, softening the stool and increasing its bulk in the large intestine. The bowel then moves that larger, softer volume along in the ordinary way. A stimulant laxative instead acts on the nerves and muscle of the bowel wall to increase contraction directly. Because an osmotic agent pulls water out of the body and into the bowel, heavy use shifts fluid and salts away from the circulation, which is the source of its main drawbacks. The same principle explains the preparations taken before an examination of the bowel, which use large volumes of a poorly absorbed solution to flush the intestine clear.
Used in a sentence
An osmotic laxative was recommended to help soften the patient's stool.
Good to know
Osmotic laxatives work by drawing water into the intestine through osmotic pressure, a different mechanism from stimulant laxatives, which act by increasing the contraction of the intestinal wall.
Word origin
Where “osmotic” comes from
From Greek osmos, meaning a pushing or thrust, derived from othein, meaning to push.