Diuretics Quiz

See Osmotic Diuretics. Option E is correct: an osmotic diuretic such as mannitol is a substance that is freely filtered at the glomerulus (the kidney's tiny filter) but is hardly reabsorbed, so it stays inside the tubule (the long working tube of each nephron, the kidney's functional unit) and keeps the osmolality of the tubular fluid high. Osmolality is the concentration of dissolved particles in a fluid, and water tends to move towards the side where particles are more concentrated. Normally water follows sodium and other solutes out of the tubule and back into the blood, especially in the proximal tubule (the first, very water-permeable stretch). Mannitol particles that cannot leave hold water in the tubular fluid, so less water is reclaimed and more water, along with sodium, is passed as urine. This is called an osmotic diuresis. Mannitol is given by intravenous infusion (a drip), and the BNF lists cerebral oedema (brain swelling) and raised pressure inside the eye as its uses. The effect comes from simple osmosis, not from blocking a transporter or a hormone receptor. The steps are:

1. Mannitol is freely filtered at the glomerulus
2. It is not reabsorbed, so it stays in the tubular fluid and raises its osmolality
3. Water is held back in the tubule, so urine volume rises

• Mechanism: An osmotic diuretic such as mannitol is freely filtered but hardly reabsorbed.
• Effect: It keeps tubular fluid osmolality high, so water is held in the tubule.
• Result: An osmotic diuresis with more urine, without blocking a transporter or receptor.