Compare and contrast water diuresis and osmotic diuresis, giving one example of each.

Outline:

·        How they are produced

·        Urine flow and osmolality

Essay:

            Diuresis is the loss of water in the urine. Water diuresis is the result of a reduced osmotic reabsorption of water, leading to increased solute-free water clearance and a hypoosmotic urine. Water diuresis can be produced by drinking large amounts of hypotonic fluid.

Osmotic diuresis is the result of reduced solute reabsorption, leading to the presence of large quantities of unreabsorbed solutes in the renal tubules, causing an increase in urine volume. Osmotic diuresis is produced by the administration of compounds such as mannitol and related polysaccharides that are filtered out but not reabsorbed. It is also produced by naturally occurring substances when they are present in amounts exceeding the capacity of the tubules to reabsorb them. In diabetes, for example, the glucose that remains in the tubules causes polyuria.

It is important to recognize the difference between osmotic diuresis and water diuresis so as to determine the underlying cause of it. In water diuresis, the amount of water reabsorbed in the proximal tubules id normal, and the maximal urine flow that can be produced is about 16 mL/min. In osmotic diuresis, increased urine flow is due to decreased water reabsorption in the proximal tubules and loops and very large urine flows can be produced.