Which statement best describes the endocrine system?

• A. System of glands that release hormones into the bloodstream
• B. System of glands that release chemicals into ducts
• C. System of nerves that coordinate actions
• D. System of arteries supplying the heart

Answer: (A)

The endocrine system is a network of glands that release hormones directly into the bloodstream, allowing these chemical messengers to travel to distant targets and regulate processes throughout the body. Hormones bind to receptors on or in target cells, prompting responses that can be slower to start but longer-lasting than nerve signals. This bloodstream-based delivery is what sets the endocrine system apart from other signaling systems.

Releasing substances into ducts describes exocrine glands, which secrete onto surfaces or into body cavities rather than into the blood. The nervous system coordinates actions through nerve impulses, not hormones traveling through the bloodstream. Arteries supplying the heart are part of the circulatory system, not signaling glands.