Impairments In Urinary Concentrating And Diluting Ability

Deficiencies in the ability of the kidney to regulate plasma osmolality by appropriately forming either dilute or concentrated urine may have several causes. There may be a defect in production or regulation of vasopressin release, an inability of the collecting duct to respond to vasopressin, or a failure to form a medullary osmolality gradient.

Diabetes Insipidus

Diabetes insipidus refers to high rates of production of dilute urine either because the posterior pituitary fails to release vasopressin or because the kidney does not respond to the hormone. This should be contrasted with diabetes mellitus in which increased urine flow is a consequence of the osmotic diuresis produced by the filtration and incomplete reabsorption of large amounts of glucose (see Chapter 26, the section on ''Passive Reabsorptive Processes''). (The modifier melli-tus refers to the fact that urine was sweet to the taste, whereas insipidus refers to a urine without taste using archaic diagnostic procedures!) The failure to produce or release vasopressin is referred to as central diabetes insipidus. This is a rare congenital disorder, but it may also be caused by surgical procedures (such as hypo-physectomy to treat a pituitary tumor), infections, cerebrovascular accidents, or trauma. Removal of the pituitary does not always cause a total lack of vaso-pressin production because some of the nerves responsible for releasing the hormone end in the pituitary stalk and are not lost.

Inability to release vasopressin in response to plasma hyperosmolality leaves only the thirst mechanism to regulate plasma osmolality. Because the urine cannot be concentrated, dilute urine is continually produced, with volumes that can reach 18 L/day. To combat the

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