Diuretics are drugs that increase urine production. Many diuretics exert their effect by decreasing sodium reabsorption in the kidney tubules. In other words the concentration of sodium ions in the urine is increased. Usual knowledge of osmolality to explain why diuresis occurs.

Reabsorption of water in the kidney tubules occurs along osmotic gradients. It is helpful to imagine that the water molecules want to be where the dissolved particles are. If we absorb sodium ions in the kidney tubules then water will follow. If we don't absorb sodium ions and they remain in the year in, then water will also remain and urine volume will increase. Increased urine production (polyuria) is characteristic of diabetes mellitus. In diabetes mellitus the amount of glucose passing through the renal tubules exceeds the capacity of the tubules to resorb it. Hence the glucose acts as an extra particle in the urine and water molecules stay with it. This is known as an osmotic diuresis. Extreme thirst is another symptom of diabetes mellitus. It occurs because the excessive loss of water in the urine has triggered homeostatic mechanisms that aim to return the amount of body water to normal. One way that this can be done is by increasing thirst.