Endocrine System Overview

Introduction to Hormones and Endocrine Glands

The endocrine system is a network of glands that secrete hormones to regulate physiological processes throughout the body. Hormones are chemical messengers that travel through the bloodstream to target organs, influencing metabolism, growth, stress response, and homeostasis.

• Hormones: Chemical messengers produced by endocrine glands.

• Endocrine glands: Organs that secrete hormones directly into the bloodstream (e.g., pituitary, adrenal, thyroid).

• Target cells: Cells with specific receptors for a hormone.

Classification of Hormones

Types of Hormones

Hormones are classified based on their chemical structure and mechanism of action. The three main types are peptide hormones, steroid hormones, and amine-derived hormones.

• Peptide hormones: Composed of amino acids; water-soluble; act via membrane receptors.

• Steroid hormones: Derived from cholesterol; lipid-soluble; act via intracellular receptors.

• Amine-derived hormones: Derived from amino acids (mainly tyrosine); include catecholamines and thyroid hormones.

Hormone Release and Half-Life

The method of hormone release and their half-life are important for understanding their physiological effects.

• Exocytosis: Peptide hormones and catecholamines are released from parent cells via exocytosis.

• Half-life: The time required for a hormone to lose half of its physiological activity. Peptide hormones and catecholamines generally have a short half-life, while steroid and thyroid hormones have a longer half-life.

Table: Comparison of Peptide, Steroid, and Amine-Derived Hormones

Endocrine Glands and Hormone Production

Pituitary Gland and Hypothalamus

The pituitary gland, often called the "master gland," works closely with the hypothalamus to regulate many endocrine functions. The hypothalamus produces releasing hormones that control the secretion of pituitary hormones.

• Anterior pituitary: Secretes hormones such as prolactin, growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), and gonadotropins (LH & FSH).

• Hypothalamus: Produces releasing and inhibiting hormones (e.g., corticotropin-releasing hormone, CRH).

• Portal system: Specialized blood vessels connecting hypothalamus and anterior pituitary for hormone transport.

Hormone Pathways: Example of Cortisol

Cortisol is a steroid hormone produced by the adrenal cortex and is essential for the body's response to stress. Its secretion is regulated by the hypothalamic-pituitary-adrenal (HPA) axis.

• Stress (physical or psychological) triggers the release of CRH from the hypothalamus.

• CRH stimulates the anterior pituitary to release ACTH.

• ACTH acts on the adrenal cortex, which releases cortisol.

• Cortisol has many functions, including regulation of metabolism and suppression of inflammation.

• Cortisol exerts negative feedback on the hypothalamus and pituitary to regulate its own levels.

Equation (Negative Feedback):

HPA Axis: Main Organs

• Hypothalamus

• Pituitary gland

• Adrenal gland

Functions and Disorders of Cortisol

• Main function: Reduces inflammation and helps regulate metabolism.

• Excess cortisol: Can lead to Cushing syndrome (weight gain, increased infection risk).

• Deficiency: Addison disease (poor regulation of metabolism and stress response).

Summary Table: Hormone Classification and Key Features

Key Concepts for Exam Preparation

• Understand the classification and mechanisms of hormone action.

• Know the main endocrine glands and their functions.

• Be able to describe the HPA axis and the role of cortisol in stress response.

• Recognize symptoms and causes of hormone imbalances (e.g., Cushing syndrome, Addison disease).

Additional info: Some content and explanations have been expanded for clarity and completeness based on standard physiology textbooks.