Which of the following defines hormones?

• A. Local mediators released into the synapse
• B. Long-distance chemical messengers secreted into the blood
• C. Signaling molecules acting on the same cell
• D. Short-range signaling molecules

Answer: (B)

Hormones are defined as long-distance chemical messengers that are secreted into the blood and travel throughout the body to target cells, where they elicit specific physiological responses. This definition highlights the key characteristics of hormones: their ability to act over considerable distances, their presence in the bloodstream, and their role in facilitating communication between different organs and systems within the body. Hormones can regulate a wide variety of bodily functions, including metabolism, growth, and mood, by binding to specific receptors on target cells.

Local mediators are typically released into the synapse and act immediately on nearby cells, which distinguishes them from hormones that function over longer distances. Signaling molecules that act on the same cell are known as autocrine signals, while short-range signaling molecules operate in a paracrine manner, affecting nearby cells without entering the bloodstream. These distinctions underscore the unique role that hormones play in the endocrine system, contrasting with other forms of cellular communication.