Mannitol is a sugar that does not pass through the walls of capillaries in the b

Question

Mannitol is a sugar that does not pass through the walls of capillaries in the brain. It also does not cross the wall of kidney tubules, the structures that transport blood filtrate to become urine. Explain why mannitol can be considered as osmotically active. How might its clinical administration help to prevent swelling of the brain in head trauma? Also, explain the effect it might have on the water content of urine Mannitol is a sugar that does not pass through the walls of capillaries in the brain. It also does not cross the wall of kidney tubules, the structures that transport blood filtrate to become urine. Explain why mannitol can be considered as osmotically active. How might its clinical administration help to prevent swelling of the brain in head trauma? Also, explain the effect it might have on the water content of urine

Explanation / Answer

Mannitol is a non electrolyte of low molecular weight it is pharmacologically non active it is osmotic ally active an osmotically active substance draws water towards in osmosis solvent move from an area of lower solute concentration to an area of higher solute concentration here mannitol is solute here fluid moves from place with no solute that is mannitol to place with high concentration of mannitol so if mannitol is present in blood stream it draws water towards it as it does not cross the blood brain barrier it increases thr osmolality of blood plasma.it is not metabolized amd it is freely excreated from the body with least resorption

Clinical administration in brain and head trauma

Swelling means excess of water, mannitol is given which circulates within the blood plasma between blood and brain there are present capillaries which are semi-permeablr membrane when mannitol is given the excess water passes through the capillaries into blood stream due yo presence of mannitol which increases the osmolality 1-1.5g/kg mannitol is infused over 1 hour as 20 percentage solution to transiently raise plasma osmolarity. This is how mannitol is used to treat edemas .

Effect on water content of urine

Mannitol cannot be reabsorbed in the tubules within thr tubules increase s osmolarity and hence more water is absorbed into the tubules it also retains osmolarity in proximal tubule dilutes the luminal fluid ehich opposes sodium absorption.increases the blood flow to medulla decrease passive salt absorption. Increase s the volume of urine output.

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