Mannitol is a monosaccharide that is not metabolized in the body. After being filtered by the glomerulus, it is rarely reabsorbed in the renal tubules and plays an osmotic diuretic effect.

(1) Tissue dehydration. Increase plasma osmotic pressure, causing water in tissues (including eyes, brain, cerebrospinal fluid, etc.) to enter blood vessels, thereby reducing tissue edema, reducing intraocular pressure, intracranial pressure, cerebrospinal fluid volume and pressure. 1g of mannitol can produce an osmotic concentration of 5.5mOsm. An injection of 100g of mannitol can transfer 2000ml of intracellular water to the outside of the cell, and 50g of urinary sodium can be excreted.

The osmotic concentrations of mannitol solutions of different concentrations are as follows

Mannitol concentration (%) Osmotic concentration (mOsm/L) Mannitol concentration (%) Osmotic concentration (mOsm/L) < /p>

5 275 20 1100

10 550 25 1375

15 825

(2) Diuretic effect. The diuretic mechanism of mannitol is divided into two aspects:

① Mannitol increases blood volume and promotes prostaglandin I2 secretion, thereby dilating renal blood vessels and increasing renal blood flow, including renal medullary blood flow. The glomerular afferent arterioles dilate, the glomerular capillary pressure increases, and the cortical glomerular filtration rate increases.

② This drug is rarely (<10%) reabsorbed by the renal tubules after being filtered by the glomerulus, so it can increase the osmotic concentration of the fluid in the renal tubules and reduce the amount of water, Na+, Cl-, Reabsorption of K+, Ca2+, Mg2+ and other solutes. In the past, it was believed that this drug mainly acts on the proximal tubule, but puncture animal experiments found that after the application of large doses of mannitol, the water and Na+ passing through the proximal tubule only increased by 10% to 20% and 4% to 5% respectively; The water and Na+ in the distal tubule increased by 40% and 25% respectively, suggesting that the reduction of water reabsorption and Na+ in the loop of Henle plays an important role in the diuretic effect of mannitol. This may be due to increased renal medullary blood flow and increased loss of urea and Na+ in the medullary, thereby destroying the medullary osmotic pressure gradient difference.

Because the renal tubular fluid flow increases after infusion of mannitol, when certain drugs and poisons are poisoned, the concentration of these substances in the renal tubules decreases, the toxicity to the kidneys is reduced, and the excretion through the kidneys is accelerated.