1 Endocrine System

Endocrine System

Introduction to Endocrinology

• Endocrinology: The study of hormones and hormone-secreting glands in the body.

• The endocrine system is the second major communication system in the body, working alongside the nervous system.

Functions of the Endocrine System

• Hormones serve as messengers controlling numerous functions:

  • Circulation

  • Water and electrolyte balances

  • Digestion and absorption of food

  • Organic metabolism and energy balance

  • Reproduction

  • Responses to stress

Components of the Endocrine System

• Major glands include:

  • Hypothalamus

  • Pituitary gland

  • Thyroid gland

  • Parathyroid glands

  • Adrenal glands

  • Pineal gland

  • Kidneys

  • Pancreas

  • Ovaries

  • Testes

  • GI Tract

  • Liver

  • Thymus

  • Placenta

Endocrine vs. Exocrine Glands

• Endocrine Glands: Secrete hormones into the extracellular space where they diffuse into the bloodstream.

• Exocrine Glands: Specialized cells that secrete products into ducts leading to specific compartments or surfaces.

Hormones

• Hormones: Specialized chemical messengers that alter cell activity by traveling in the bloodstream to target cells.

• Factors: Similar to hormones but not fully identified as distinct compounds.

Target Cells

• Hormones reach all tissues, but only cells with specific receptors (target cells) can respond to hormone signals.

Feedback Mechanisms in Endocrine Regulation

Negative Feedback

• Mechanism where the response opposes the initial stimulus, maintaining homeostasis.

• Hormones from target glands can reduce the secretion of releasing hormones from the hypothalamus.

Positive Feedback

• Control system where an initial disturbance amplifies the effect, leading to a greater response.

• Example: Oxytocin stimulates uterine contractions and milk letdown.

Hypothalamus

• Located below the thalamus, responsible for integrating neural and endocrine functions, particularly in regulating the internal environment.

• Receives stimulatory and inhibitory signals from the CNS and regulates hormone secretion.

Hypothalamic Hormones and Actions

• Corticotropin Releasing Hormone (CRH): Stimulates ACTH secretion.

• Thyrotropin Releasing Hormone (TRH): Stimulates TSH and prolactin secretion.

• Growth Hormone Releasing Hormone (GHRH): Stimulates GH secretion.

• Somatostatin (GH Inhibiting Hormone): Inhibits GH secretion.

• Gonadotropin Releasing Hormone (GnRH): Stimulates LH and FSH secretion.

• Prolactin Releasing Hormone: Stimulates prolactin secretion.

• Prolactin Release Inhibiting Hormone (PIH): Inhibits prolactin secretion.

Regulation of the Pituitary Gland

• Produces hormones that regulate various bodily functions.

• Anterior Pituitary: Produces stimulating hormones.

• Posterior Pituitary: Stores and releases hormones produced by the hypothalamus (e.g., oxytocin, vasopressin).

Anterior Pituitary Gland

• Synthesizes and releases various hormones and receives regulatory input via the hypothalamic-hypophysial portal system.

Key Hormones from the Anterior Pituitary

• Growth Hormone (GH): Affects growth and metabolism.

• Thyroid Stimulating Hormone (TSH): Stimulates thyroid gland.

• Adrenocorticotropic Hormone (ACTH): Stimulates adrenal cortex.

• Prolactin: Stimulates milk production.

• Gonadotropic Hormones: FSH and LH, affect gonadal function.

Posterior Pituitary Gland

• Neurohypophysis, an extension of the hypothalamus, releases hormones like oxytocin and vasopressin.

Thyroid Gland

• Location: Paired gland in the neck.

• Function: Secretes thyroid hormones (T3 and T4) and calcitonin, impacting energy metabolism and growth.

Thyroid Hormones Overview

• Triiodothyronine (T3) and Thyroxin (T4) are the primary hormones, influencing metabolic processes and are derived from iodine.

Calcitonin

• Secreted by C-cells, lowers plasma calcium levels by inhibiting bone resorption.

Adrenal Glands

• Retroperitoneal glands with two main parts: Cortex and Medulla.

• Cortex: Produces hormones like aldosterone, cortisol, and adrenal androgens.

• Medulla: Secretes catecholamines (epinephrine and norepinephrine).

Parathyroid Hormone (PTH)

• Regulated by extracellular calcium levels, increasing bone demineralization and calcium reabsorption in kidneys.

Gonadal Hormones

• Testes: Produce testosterone, responsible for male characteristics.

• Ovaries: Produce estrogens and progesterone, involved in female reproductive functions.

Pancreas

• Functions as both an endocrine and exocrine gland, regulating glucose metabolism through the secretion of insulin, glucagon, and somatostatin.

Kidney Hormones

• Secrete hormones such as renin (regulates blood pressure), erythropoietin (stimulates RBC production), and 1,25-Dihydroxyvitamin D3 (calcium absorption).

Gastrointestinal Tract Hormones

• Produces various hormones (e.g., gastrin, secretin, cholecystokinin) that regulate digestion and metabolic processes.

Liver Hormones

• Releases several hormones including somatomedin (growth-promoting), angiotensinogen (precursor to angiotensin), and thrombopoietin (regulates platelet formation).

Thymus Gland

• A central lymphoid organ important for T-cell maturation, secretes thymosin.

Pineal Gland

• Produces melatonin, which regulates sleep and sexual maturation.

Placental Hormones

• Secretes chorionic gonadotropins, estrogen, and progesterone, maintaining pregnancy and affecting female reproductive health.

Endocrine Disorders

Thyroid Disorders

• Hyperthyroidism: Characterized by excessive hormone production (e.g., Graves' Disease).

• Hypothyroidism: Often due to autoimmune issues or functional defects in the gland.

Adrenal Disorders

• Cushing's Syndrome: Excessive cortisol production leading to metabolic disruptions.

• Addison's Disease: Inadequate adrenal steroid production leading to weakness and hypotension.

Diabetes Mellitus

• Characterized by imbalances affecting blood glucose levels, includes Type I and Type II diabetes.

SIADH & Diabetes Insipidus

• SIADH: Inappropriate secretion of ADH causing water retention.

• Diabetes Insipidus: Insufficient ADH production or response leading to polyuria.

Summary of Endocrine Functions

• The endocrine system coordinates and regulates numerous bodily functions through a complex network of hormones and feedback mechanisms. Understanding these systems enhances comprehension of health and disease states.