Diabetes insipidus

With diabetes insipidus, “diabetes” means an increased passing of urine, and “insipidus” means tasteless; so diabetes insipidus is a condition characterized by the production of large quantities of dilute and tasteless urine.

The tasteless urine of diabetes insipidus distinguishes it from diabetes mellitus which describes sweet tasting urine- and, yes, urine was really tasted at one point in time to make that distinction!

Now, in the brain there’s a region called the hypothalamus.

Inside the hypothalamus 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 milli Osmoles per kilogram.

During periods of dehydration there is an increase in concentration of these particles in the blood and osmolality increases.

The osmoreceptors in the hypothalamus detect the increased osmolality and that triggers the sensation of thirst, which tells us to drink more water. The water then gets absorbed and dilutes the blood, bringing the osmolality back to normal.

In addition to osmoreceptors, the hypothalamus also contains a cluster of neurons that are found in a specific spot called the supraoptic nucleus.

These neurons produce a hormone called antidiuretic hormone, or ADH. ADH is also called vasopressin because it causes smooth muscle around the blood vessels to contract, which increases blood resistance and raises blood pressure.

When the osmoreceptors detect high osmolality, they signal the supraoptic nucleus to send ADH down the supraoptico-hypophyseal tract, which runs through the infundibulum or pituitary stalk, and into the posterior pituitary gland, where it is then released into the blood.

ADH travels to the kidneys, specifically to the distal convoluted tubule and collecting ducts of the nephrons and binds to a receptor called vasopressin receptor 2, or AVPR2.