Okay, so two people were admitted to the Endocrinology ward. One of them is 35 year old Imre, who came in with intense polyuria and polydipsia. Imre was dehydrated and presented with dry mouth, headache, dry skin and dizziness. Several tests were done and results showed increased serum osmolality and further on, a desmopressin test was done. During the test, an ADH analogue was administered and urine osmolality increased. The other person is 45 year old Sienna who came in to do some routine tests because she started taking cyclophosphamide and wanted to make sure that there are no complications. Her lab results showed hyponatremia, decreased blood osmolality, and her urine osmolality was higher than serum osmolality.
Now, both individuals are unable to maintain normal osmolality. But to understand this we need to go over a bit of physiology first. In the brain, specifically in the hypothalamus, there are osmoreceptors which can sense the osmolality of the blood, or how concentrated it is. Osmolality is the concentration of dissolved particles in the blood plasma, or the liquid portion of blood. There are a number of dissolved particles in the blood plasma, but the major ones are glucose, sodium, and blood urea nitrogen, and a normal osmolality is between 285 and 295 milliOsmoles per kilogram.
Now, during periods of dehydration there is an increase in concentration of these particles in the blood and osmolality increases. The osmoreceptors in the hypothalamus sense the change in osmolality and this triggers the sensation of thirst. The water that we drink gets absorbed and dilutes the blood, bringing the osmolality back to normal.
In addition to osmoreceptors, the hypothalamus also contains the supraoptic and paraventricular nuclei that produce antidiuretic hormone, or ADH, which is then sent to the posterior pituitary for storage. ADH is also called vasopressin because it causes smooth muscle around the blood vessels to contract, which increases resistance and raises blood pressure.