Final answer:
Antidiuretic hormone (ADH) significantly influences the osmolarity of blood by promoting water reabsorption in the kidneys, which helps to concentrate urine and manage blood osmolarity.
Step-by-step explanation:
The hormone that most affects the osmolarity of blood is antidiuretic hormone (ADH), also known as vasopressin. ADH plays a crucial role in water regulation in the body by acting on the kidneys to influence the reabsorption of water. It makes the collecting ducts in the kidneys permeable to water, allowing for water molecules to be reabsorbed from the urine back into the blood, thereby concentrating the urine and reducing the osmolarity of blood.