What defines a hormone?

A hormone is defined as a circulating chemical signal that coordinates bodily functions. Hormones are produced by glands in the endocrine system, such as the pituitary, thyroid, and adrenal glands, and are secreted into the bloodstream. They travel throughout the body to target organs and tissues, where they elicit specific responses that regulate a wide variety of physiological processes, including metabolism, growth and development, mood, and immune response.

This definition underscores the role of hormones as key regulators of bodily functions, differentiating them from other biological molecules. For example, neurotransmitters, while also signaling molecules, are primarily involved in communication between nerve cells and do not circulate throughout the body to exert widespread regulatory effects like hormones do. Similarly, while a specific protein that aids in digestion is crucial for breaking down food, it does not fit the definition of a hormone since it does not travel through the bloodstream to coordinate bodily functions. Lastly, structures involved in reflex actions are related to the nervous system's response mechanism rather than the chemical signaling associated with hormones.

Therefore, the correct description of a hormone emphasizes its nature as a chemical signal that facilitates coordination among various systems in the body.