Osmotic diuretics cause water to be retained within the proximal tubule and descending limb of loop of Henle (freely permeable to water). These compounds are filtered in the glomerulus, but cannot be reabsorbed. Their presence leads to an increase in the osmolarity of the filtrate. To maintain osmotic balance, water is retained in the urine. Example: mannitol, glucose (especially in uncontrolled diabetes).