Hormone

A hormone is one of the body's chemical messengers. It is made by the endocrine glands, which are specialized groups of cells, and released into the blood. The blood carries it to tissues or organs, where it delivers its message.

How it delivers that message depends on each hormone's chemistry. Some small, hydrophobic hormones, meaning they do not mix with water, such as steroid and thyroid hormones, pass through the membrane, the cell's outer envelope, and bind to a receptor inside. That receptor acts on the genes and regulates which ones are read. Water-soluble hormones, the ones that do dissolve in water, bind to a receptor on the surface of the cell. It is like the difference between someone who has a key and walks into the house, and someone who rings the doorbell at the front door.

Epinephrine (adrenaline) is an example of the second kind. It binds to a receptor on the surface of skeletal muscle cells, in the muscle we move voluntarily, and levels of a molecule called cyclic AMP rise inside the cell. That internal signal ends up stimulating the breakdown of glycogen, the sugar reserve the muscle keeps on hand.