Which system uses glands to release chemical messengers into the bloodstream?

• A. Cardiovascular System
• B. Endocrine System
• C. Respiratory System
• D. Integumentary System

Answer: (B)

Endocrine signaling uses glands to release hormones directly into the bloodstream, so these chemical messengers can travel to distant target tissues and regulate activities like metabolism, growth, and reproduction. Hormones act more slowly than nerve signals but produce longer-lasting effects, which is why this system is built around ductless glands such as the pituitary, thyroid, and adrenal glands that secrete hormones into blood. The other systems don’t primarily release chemical messengers into the bloodstream: the cardiovascular system mainly moves blood; the respiratory system handles gas exchange; and the integumentary system protects and secretes for barrier and skin-related functions rather than endocrine signaling.