Vasopressin or antidiuretic hormone (ADH) is a very critical hormone to understand because of the role it plays in water and blood pressure regulation. ADH is synthesized by the neurons in the supraoptic nucleus of the hypothalamus. The synthesized products are then stransported down the axons of these neurons into the posterior pituitary (neurohypophysis). When the neurons of the supraoptic nuclei, called osmoreceptors, become excited they generate action potentials that stimulate the secretion of ADH. The names of this hormone directly guide you to what its physiologic effects are: vascular smooth muscle contraction (vasopression) and retention of water by the kidneys (ADH).
In the kidneys the receptors for ADH are found on the nephrons; the microscopic functional units of the kidneys. The parts of the nephron where the receptors are specifically located are on the distal convoluted tubule (DCT) and collecting duct (CD). The hypothalamus stimulates the secretion of ADH when our osmolarity or extracellular sodium levels rise. This is a sign that we are losing water and becoming dehydrated. Once this hormone is secreted and interacts with the kidneys, this stimulates them to retain water which will in turn lower the person's overall urine output. This will also make the urine more concentrated and give it a darker color. Once the body's osmolarity returns back to normal then the secretion of ADH will halt.
ADH is also as part of an important hormonal response to a severe drop in blood volume and pressure called the renin-angiotensin aldosterone system. I won't cover this whole system now but for now you just need to understand that when this system is engaged, it stimulates the secretion of ADH which will in turn increase body water levels and the contraction of vascular smooth muscle.
Source: Mind of Aaron
Source: Self made
Source: Self made