Endocrine System

Endocrine System

Introduction

·       The endocrine system is a complex network of glands that secrete hormones directly into the bloodstream to regulate and coordinate various body functions.

·       Unlike the nervous system, which controls body activities rapidly through nerve impulses.

·       The endocrine system provides slower but longer-lasting regulation through chemical messengers (hormones).

Key Features of the Endocrine System

• Works in coordination with the nervous system → termed as neuroendocrine regulation.
• Hormones act on target organs that have specific receptors.
• Maintains homeostasis (internal balance).
• Regulates growth, development, metabolism, reproduction, and response to stress.

Major Endocrine Glands: Anatomical Features & Hormones

1. Hypothalamus

• Location: Base of the brain, below the thalamus and above the pituitary gland.
• Anatomical Feature: Small but highly vascular structure that links nervous and endocrine systems.
• Hormones (Releasing & Inhibiting):
• TRH – Thyrotropin-releasing hormone (stimulates TSH release from anterior pituitary).
• CRH – Corticotropin-releasing hormone (stimulates ACTH release).
• GnRH – Gonadotropin-releasing hormone (stimulates FSH & LH release).
• GHRH – Growth hormone-releasing hormone (stimulates GH release).
• Somatostatin – Inhibits GH & TSH.
• Dopamine – Inhibits prolactin secretion.

2. Pituitary Gland (Hypophysis)

• Location: Small, pea-shaped gland located in the sella turcica of the sphenoid bone, connected to hypothalamus by the infundibulum.
• Divisions:
• Anterior Pituitary (Adenohypophysis)
• Hormones:
• GH – Growth hormone (stimulates growth & metabolism).
• TSH – Thyroid-stimulating hormone (stimulates thyroid).
• ACTH – Adrenocorticotropic hormone (stimulates adrenal cortex).
• FSH & LH – Gonadotropins (control reproductive functions).
• Prolactin – Stimulates milk production.
• Posterior Pituitary (Neurohypophysis) – Stores & releases hypothalamic hormones:
• ADH (Vasopressin) – Regulates water balance by increasing renal water reabsorption.
• Oxytocin – Stimulates uterine contractions & milk ejection.

3. Thyroid Gland

• Location: In the neck, below the larynx, anterior to the trachea; butterfly-shaped with right & left lobes connected by an isthmus.
• Anatomical Features: Richly vascular; composed of spherical follicles lined by epithelial cells.
• Hormones:
• Thyroxine (T4) & Triiodothyronine (T3) – Regulate metabolism, growth, protein synthesis, and energy utilization.
• Calcitonin – Produced by parafollicular (C) cells; lowers blood calcium levels by inhibiting osteoclast activity.

4. Parathyroid Glands

• Location: Four small glands embedded on the posterior surface of the thyroid gland.
• Hormone:
• Parathyroid Hormone (PTH) – Increases blood calcium levels by stimulating bone resorption, enhancing calcium absorption from intestine (via vitamin D activation), and increasing renal calcium reabsorption.

5. Adrenal (Suprarenal) Glands

• Location: On top of each kidney.
• Anatomical Features: Each gland has two parts:
• Adrenal Cortex (outer, glandular part – 3 zones):
• Zona glomerulosa → Secretes Mineralocorticoids (Aldosterone) – regulates sodium & potassium balance.
• Zona fasciculata → Secretes Glucocorticoids (Cortisol) – regulates metabolism, stress response, anti-inflammatory.
• Zona reticularis → Secretes Androgens – weak male sex hormones.
• Adrenal Medulla (inner, neural part):
• Secretes Catecholamines (Adrenaline/Epinephrine, Noradrenaline/Norepinephrine) – increase heart rate, blood pressure, fight-or-flight response.

6. Pancreas (Endocrine Part – Islets of Langerhans)

• Location: Retroperitoneal organ, behind the stomach.
• Anatomical Features: Has both exocrine (digestive enzymes) and endocrine functions.
• Endocrine Hormones:
• Insulin (β-cells) – Lowers blood glucose by promoting glucose uptake.
• Glucagon (α-cells) – Raises blood glucose by glycogenolysis and gluconeogenesis.
• Somatostatin (δ-cells) – Inhibits insulin and glucagon release.
• Pancreatic Polypeptide (PP cells) – Regulates pancreatic secretions.

7. Pineal Gland

• Location: Small, cone-shaped gland in the roof of the third ventricle of the brain.
• Hormone:
• Melatonin – Regulates circadian rhythm (sleep-wake cycle) and seasonal biological rhythms.

8. Thymus Gland

• Location: Behind the sternum, in the anterior mediastinum. Large in children, shrinks after puberty.
• Hormones:
• Thymosins & Thymopoietin – Promote maturation of T-lymphocytes, essential for immune function.

9. Gonads

• Testes (Male): Located in scrotum.
• Hormones:
• Testosterone – Controls spermatogenesis, development of male secondary sexual characteristics.
• Ovaries (Female): Located in pelvic cavity.
• Hormones:
• Estrogen – Regulates female reproductive cycle, secondary sexual characteristics.
• Progesterone – Prepares uterus for implantation, supports pregnancy.
• Inhibin – Inhibits FSH.

10. Other Hormone-Producing Structures

• Kidneys → Erythropoietin (stimulates RBC production), Renin (RAAS system).
• Heart → Atrial Natriuretic Peptide (ANP, lowers blood pressure).
• Stomach & Intestines → Gastrin, Secretin, Cholecystokinin (digestive hormones).
• Placenta → hCG, Estrogen, Progesterone (support pregnancy).

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