Which signaling system uses hormones to coordinate activities across distant organs via the bloodstream?

• A. Nervous system
• B. Endocrine system
• C. Immune system
• D. Digestive system

Answer: (B)

Long-distance chemical signaling via the bloodstream is the hallmark of endocrine communication. Hormones are released into the circulatory system by glands and travel through blood to distant target organs. When a hormone encounters cells that have the appropriate receptors, it binds and triggers changes in their activity, coordinating processes such as metabolism, growth, and stress responses across the body. This mode of signaling is typically slower to start than neural signaling, but its effects tend to last longer and affect multiple organs.

In contrast, the nervous system relies on electrical signals and neurotransmitters to produce rapid, precise responses along specific pathways, usually over short times, and targeted to particular tissues. The immune system uses signaling molecules like cytokines to coordinate defense across the body, but its primary function is coordinated immune responses rather than broad hormonal regulation. The digestive system does involve signaling for digestion, but its signaling is more about local regulation of digestive processes rather than coordinating distant organs through the bloodstream.