Which system uses hormones released into the bloodstream to communicate?

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

Answer: (C)

Hormone-based communication through the bloodstream is a hallmark of endocrine signaling. In this system, glands release chemical messengers called hormones into the blood, and these hormones travel to distant target cells that have specific receptors, triggering responses in areas such as metabolism, growth, and reproduction. This pathway is slower but can affect many parts of the body over time, unlike the nervous system's rapid, direct electrical signaling via neurotransmitters across synapses. The immune system uses signaling molecules too, but they coordinate defense rather than operate through bloodstream-delivered hormones as the main method. The digestive system does regulate digestion with hormones, but the system in which hormones circulate in the blood to communicate with distant organs is the endocrine system.