Antidiuretic hormone, or ADH, is a peptide hormone that is anti- or against -diuresis which is excessive urine production.
Antidiuretic hormone is also called vasopressin because it causes vasoconstriction - constriction of blood vessels.
So antidiuretic hormone prevents making too much urine, which leads to water retention, and vasoconstriction, and together these two actions help increase the blood pressure.
Now, the brain has two interconnected structures: the hypothalamus and the pituitary gland. These two structures are connected by the pituitary stalk.
The hypothalamus is a part of the brain that contains several nuclei, or clusters of neurons. And two of these nuclei, the paraventricular and supraoptic nuclei, contain neurons that secrete ADH.
When ADH is produced, it travels down the axons of these neurons, and these axons have small dilations called Herring bodies, which is where ADH is stored.
When the body needs more ADH, the stored hormone is released and continues down the axon through the pituitary stalk.
From there it’s released into the posterior pituitary gland which is interstitial tissue near capillary beds, so that the ADH can easily enter the bloodstream.
Let’s say that it's a super sunny day out and you forget to bring water with you. Well first, as you walk around, you’re constantly losing water through sweat as well as water vapor from your mouth and nose as you breathe out - these are insensible water losses. Without drinking water, you can quickly get dehydrated. This causes your plasma osmolarity to increase, because the fluid levels in your blood drop, but the total number of solute particles remains roughly the same.
Now, two things now begin to happen simultaneously. First, a region in the brain called the anterior hypothalamus has a cluster of neurons called supraoptic nuclei, which have osmoreceptors that sense even tiny changes in osmolarity, as small as 1 mOsm/L. These neurons are always sampling the blood that passes by and they have a special channel called aquaporin 4 which allows water to freely enter or exit the cell.