Definition and Function

Endocrine glands are ductless glands that secrete hormones directly into the bloodstream. Hormones are chemical substances that act as intercellular messengers and are produced in trace amounts. They are crucial for regulating various physiological processes and maintaining homeostasis.

Classical vs. Current Definition of Hormones

Classical Definition: Hormones are chemicals produced by endocrine glands, released into the blood, and transported to target organs located at a distance.

Current Definition: Hormones are non-nutrient chemicals that act as intercellular messengers and are produced in trace amounts. This definition includes a broader range of molecules beyond those secreted by traditional endocrine glands.

Endocrine Systems in Different Organisms

Invertebrates: Possess simple endocrine systems with few hormones.

Vertebrates: Have a complex endocrine system with numerous chemicals acting as hormones to provide coordination.