Highly sensitive cells within the hypothalamus that react to changes in blood composition and cause the release of antidiuretic hormone (ADH) when appropriate are called.

osmoreceptors

Explanation:

Osmoreceptors are nerve cells (neurons) capable of detecting changes in the extracellular fluid (ECF) osmolality, i.e., in the amount of solute in free water within the ECF. Osmoreceptors are known to contribute to osmoregulation by controlling body-fluid balance. These sensory receptors are mainly localized in the hypothalamus of homeothermic animals where they detect variations in the osmotic pressure and stimulate the release of antidiuretic hormone (ADH) in blood in response to the increase in plasma osmolality.