Also known as vasopressin, this hormone is vital in the regulation of plasma volume. This same hormone was given two names because it was originally associated with what were thought to be separate mechanisms, but in actuality are pretty much indistinguishable. This hormone is indeed secreted from the posterior pituitary, but like all hormones secreted from that particular part of the gland is actually formed in the hypothalamus, from whence it travels down the axons of neurons, and is released into the capillary bed within the posterior pituitary. The release is stimulated by a change in plasma osmolality. If plasma osmolality increases above the desired level, ANP is released.

It travels through the bloodstream, eventually reaching the kidneys, where it causes insertion of aquaporin proteins in the apical membrane of the distal tubule. This allows water from the distal tubule to reenter the hypertonic renal medulla, via facilitated diffusion. This leads to a decrease in plasma osmolality, and decreases dehydration.

While baroreceptors can also have an effect on ANP secretion, the osmoreceptors are by far the dominant stimulus.

Finally, should water diuresis be desired, a low level of vasopressin wil do the trick, causing aquaporins to be removed from the apical membrane.

These are my interpretation of my lecture notes, but I may have used some references from Hole's Anatomy and Physiology (Shier, Butler, Lewis) and Human Physiology (Vander, Sherman, Luciano)