Final answer:
Dehydration causes increased body fluid osmolarity, which is detected by osmoreceptors in the hypothalamus. This triggers the release of ADH from the posterior pituitary gland, leading to water reabsorption in the kidneys and balancing osmolarity.
Step-by-step explanation:
An increase in the osmolarity of body fluids due to dehydration is sensed by osmoreceptors in the hypothalamus, leading to the release of antidiuretic hormone (ADH) from the posterior pituitary gland. The osmoreceptors detect changes in the concentration of solutes in the blood, which occurs during states of dehydration. When these receptors sense a higher osmolarity, they signal the hypothalamus, which in turn prompts the posterior pituitary gland to release ADH, also known as vasopressin. ADH then acts on the kidneys, increasing water reabsorption, thereby diluting the blood plasma and helping to restore osmotic balance.