Osmotic diuresis occurs when substances of high molecular weight, such as glucose, enter the kidney tubules. The substances cause an increase in the hydrostatic pressure or osmotic pressure within the tubule, which reduces the reabsorption of water and increases urine output. The same effect can be seen in therapeutics such as Mannitol. High molecular weight substances in the circulation can also increase the amount of circulating fluid by increasing the osmolarity of the blood. This has the effect of pulling water from the interstitial space, making more water available in the blood and causing the kidney to compensate by removing it as urine. In hypotension, often colloids are used intravenously to increase circulating volume in themselves, but as they exert a certain amount of osmotic pressure, water is therefore also moved, further increasing circulating volume. As blood pressure increases, the kidney removes the excess fluid as urine.

• Article at MedlinePlus

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