The body has specialized cells (called "osmoreceptors") in certain areas that measure the osmolality of blood. (An example is the organum vasculosum of the lamina terminalis, or OVLT, if you're curious). Losing water increases blood osmolality (usually around 290 mOsm/L), which is sensed by the osmoreceptors. The osmoreceptors in turn activate two mechanisms:

1. They activate the thirst center (in the hypothelamus)
2. They cause the secretion of vasopressin (antidiuretic hormone / ADH) from the pituitary gland, which increases the retention of free water (ie water without solutes) in the kidneys

These mechanisms increase the free water concentration of blood, causing blood osmolality to return to normal.

Please note that different mechanisms exist to address a decrease in blood volume, and if both blood volume decreases and osmolality increases, both categories of mechanisms will activate. There's considerable overlap between these mechanisms.