Osmoreceptors in the _______________ signal the posterior pituitary to release ADH when blood pressure is low.

Osmoreceptors are specialized sensory receptors that detect changes in the osmolarity of the blood. They play a crucial role in the regulation of body fluid balance and blood pressure. The hypothalamus contains these osmoreceptors, which are sensitive to the concentration of solutes in the blood. When blood pressure is low, it often coincides with increased osmolarity, prompting the osmoreceptors to signal the posterior pituitary gland to release antidiuretic hormone (ADH), also known as vasopressin.

ADH acts on the kidneys to promote water reabsorption, which helps to increase blood volume and consequently raises blood pressure. This regulatory pathway is essential for maintaining homeostasis in the body's fluid levels and blood pressure.

The other regions mentioned, such as the cortex, hippocampus, and cerebellum, do not play primary roles in the osmoregulation mechanism involving ADH release. The cortex is more involved in higher cognitive functions, the hippocampus is related to memory formation, and the cerebellum primarily coordinates voluntary movements. Therefore, the hypothalamus is clearly the correct choice as the site of osmoreceptor function influencing ADH secretion.