Final answer:
Antidiuretic hormone (ADH), secreted by the pituitary gland, regulates intracellular fluids by stimulating reabsorption of fluids. Its secretion increases in conditions like dehydration.
Step-by-step explanation:
The hormone involved in the regulation of intracellular fluids, produced by the pituitary gland, stimulating reabsorption of fluids, with secretion increasing when blood is concentrated (e.g., in dehydration) is antidiuretic hormone (ADH). ADH is secreted by the hypothalamus and acts on the kidneys to make the collecting ducts more permeable to water, allowing water molecules to be reabsorbed from urine into the blood through osmosis into capillaries. This helps the body conserve water and maintain overall fluid balance.