hormone and the endocrine system

Topic 14: Hormones and the Endocrine System

Hormones Overview

• Definition: Biological molecules secreted by cells acting as chemical signals regulating activity in other cells.

  • Circulating Hormones: Affect all cells in the body, produced by endocrine glands, travel through the circulatory system, and affect cells with matching receptors.

  • Local Hormones: Affect the cell that secretes them (autocrine) and nearby cells (paracrine).

Hormone Action

• All hormones affect only their target cells, which have specific receptors that bind to these hormones.

  • Target Cells: Specific cells that a hormone travels to produce a specific effect.

  • Receptors: Proteins that bind to hormones, enabling cell response.

  • Non-target Cells: Lack the receptors and do not respond to certain hormones.

Types of Hormones

Hormonal Classification by Structure

• Amino Acid-Based Hormones: Made from amino acids, either a single modified amino acid (amine) or a protein/peptide (3-200 amino acids); generally water-soluble.

• Steroid Hormones: Lipid-based, derived from cholesterol; generally fat-soluble.

Mechanism of Action

Amino Acid-Based Hormones

• Bind to receptor proteins on cell membranes, termed first messengers.

• Activate second messengers inside the cell that initiate changes in the cytoplasm (e.g., enzyme cascades).

  • Example: Glucagon activates a receptor on a target cell, resulting in glycogen breakdown into glucose through a cyclic AMP pathway.

Steroid and Thyroid Hormones

• Fat soluble, allowing passage through cell membranes; can bind directly to receptors in the cytoplasm/nucleus.

• Hormone-receptor complex acts as a transcription factor regulating gene transcription and protein synthesis.

Other Types of Hormones

• Neuropeptides: Hormones from the nervous system influencing cells near their release point.

• Eicosanoids (e.g., prostaglandins): Modified fatty acids affecting tissue response in areas of disturbance/injury.

Endocrine Glands

• Ductless glands: Secrete hormones directly into the bloodstream or surrounding fluids (e.g., pancreas as both an endocrine and an exocrine gland).

• Functions: Regulate metabolism, growth, reproduction, and maintain homeostasis.

The Endocrine System

Major Hormones and Glands

• Glands Include: Hypothalamus, pituitary, thyroid, parathyroid, adrenal, gonads, and pancreas.

• Functions: Control slow functions such as metabolism, growth, and mood; rely on chemical signals through blood.

Hypothalamus and Pituitary Gland

• Hypothalamus controls secretions of the pituitary gland; integrates nervous and endocrine functions.

  • Neurosecretory cells are responsible for releasing hormones affecting the anterior and posterior pituitary functions.

Posterior Pituitary Secretion

• Stores and secretes hormones made by the hypothalamus.

• Examples: ADH (regulates water retention) and oxytocin (milk secretion).

Anterior Pituitary Functions

• Synthesizes and releases hormones controlled by hypothalamic releasing hormones.

• Hormones include FSH, LH, GH, TSH, and ACTH, which regulate various body processes.

Hormonal Regulation of Calcium

• Calcium Levels: Homeostasis maintained by calcitonin (lowers calcium) and parathyroid hormone (raises calcium).

  • Calcitonin: Secreted when calcium levels are high, promotes bone formation.

  • Parathormone: Secreted with low calcium levels; promotes bone resorption and intestinal absorption.

Stress Responses and the Adrenal Glands

Short-term Response (Adrenal Medulla)

• Hormones: Epinephrine and norepinephrine are released, triggering the "fight-or-flight" response, increasing heart rate and glucose levels.

Long-term Response (Adrenal Cortex)

• Hormones: Glucocorticoids (e.g., cortisol) manage glucose and immune function, while mineralocorticoids regulate salt and water balance.

Gonads and Sex Hormones

• Gonads: Include ovaries (females) and testes (males), producing sex hormones regulated by LH and FSH.

  • Hormones: Androgens (e.g., testosterone), estrogens, and progestins aid in secondary sexual traits and reproductive processes.

Pancreas and Blood Sugar Regulation

• Islets of Langerhans: Produce insulin (lowers glucose) and glucagon (raises glucose).

• Diabetes Mellitus: Results when glucose levels are unregulated.

  • Types of Diabetes: Type I (insulin deficiency) and Type II (insulin resistance).

Hormonal Imbalances

• Can disrupt homeostasis, treated by surgery or medication, impacting conditions such as diabetes mellitus.