Endocrine refers to which primary mechanism?

• A. Internal secretion
• B. External secretion
• C. Nervous signaling
• D. Immune response

Answer: (A)

Endocrine signaling involves internal secretion of chemical messengers (hormones) into the bloodstream to reach distant target cells. Hormones are released by ductless glands and travel through the blood to tissues that have specific receptors, producing slower but longer-lasting effects compared to nerves. This contrasts with external secretion, where glands release substances onto surfaces or into ducts (exocrine glands like salivary or sweat glands). Nervous signaling uses rapid electrical impulses and neurotransmitters at synapses, not circulating hormones, and immune responses involve immune cells and signaling molecules outside this hormonal bloodstream pathway. For example, insulin from the pancreas is released into the blood and acts on liver and muscle tissue, illustrating internal secretion.