Introduction to Endocrine System:
The endocrine system is one of the body’s major control systems. It regulates various physiological processes through the secretion of chemical messengers called hormones.
Unlike the nervous system, which acts quickly and locally, the endocrine system works slowly but has long-lasting effects on growth, metabolism, reproduction, and homeostasis.
Definition
The endocrine system consists of glands that secrete hormones directly into the bloodstream.
These hormones travel to target organs or tissues and regulate their function.
Hence, the endocrine glands are also called ductless glands.
Original Notes:
Major Endocrine Glands and Their Functions
1. Pituitary Gland (Master Gland)
Location: At the base of the brain, below the hypothalamus.
Divisions:
- Anterior Pituitary (Adenohypophysis) – secretes growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and prolactin (PRL).
- Posterior Pituitary (Neurohypophysis) – stores and releases antidiuretic hormone (ADH) and oxytocin.
Function: Controls other endocrine glands, growth, reproduction, lactation, and water balance.
2. Hypothalamus
Location: In the brain, above the pituitary gland.
Function: Acts as a link between the nervous and endocrine systems.
Secretes releasing and inhibiting hormones that control pituitary activity, e.g., TRH (Thyrotropin Releasing Hormone), CRH (Corticotropin Releasing Hormone), and GnRH (Gonadotropin Releasing Hormone).
3. Thyroid Gland
Location: In front of the neck, below the larynx.
Hormones:
- Thyroxine (T4) and Triiodothyronine (T3) – regulate metabolism, growth, and energy production.
- Calcitonin – lowers blood calcium levels.
Disorders: - Hypothyroidism (e.g., Myxedema, Cretinism)
- Hyperthyroidism (e.g., Graves’ disease)
4. Parathyroid Glands
Location: Four small glands behind the thyroid.
Hormone: Parathormone (PTH) – increases blood calcium levels by acting on bones, kidneys, and intestines.
Works opposite to calcitonin for calcium balance.
5. Adrenal Glands
Location: On top of each kidney.
Structure:
- Adrenal Cortex (outer part): Secretes
- Mineralocorticoids (Aldosterone) – regulates sodium and potassium balance.
- Glucocorticoids (Cortisol) – regulates metabolism and stress response.
- Sex Hormones (Androgens) – minor contribution to secondary sexual characteristics.
- Adrenal Medulla (inner part): Secretes
- Adrenaline (Epinephrine) and Noradrenaline (Norepinephrine) – prepare the body for “fight or flight.”
6. Pancreas (Islets of Langerhans)
Location: Behind the stomach.
Hormones:
- Insulin (β-cells) – lowers blood glucose by promoting glucose uptake.
- Glucagon (α-cells) – raises blood glucose by promoting glycogen breakdown.
- Somatostatin – inhibits secretion of both insulin and glucagon.
Disorders: - Diabetes Mellitus (due to insulin deficiency or resistance).
7. Gonads
- Testes (in males): Produce testosterone, which regulates male reproductive organs, sperm production, and secondary sexual characters.
- Ovaries (in females): Secrete estrogen and progesterone, which regulate menstrual cycle, pregnancy, and female secondary sexual characteristics.
8. Pineal Gland
Location: In the brain near the center.
Hormone: Melatonin – regulates sleep–wake cycles and influences reproductive functions.
9. Thymus Gland
Location: Behind the sternum, in front of the heart.
Hormone: Thymosin – promotes maturation of T-lymphocytes and supports immune system development, especially in children.
Functions of the Endocrine System
- Regulates metabolism and energy balance.
- Controls growth and development.
- Maintains reproductive functions.
- Helps in stress response and adaptation.
- Maintains homeostasis by controlling blood sugar, water, and electrolyte levels.
Common Disorders of the Endocrine System
- Diabetes Mellitus: Caused by deficiency or reduced action of insulin.
- Hypothyroidism: Low thyroid hormone leading to slow metabolism and weight gain.
- Hyperthyroidism: Excess thyroid hormone leading to increased metabolism and weight loss.
- Cushing’s Syndrome: Overproduction of cortisol.
- Addison’s Disease: Insufficient secretion of adrenal hormones.
- Gigantism/Acromegaly: Excess growth hormone secretion.
- Dwarfism: Deficiency of growth hormone.
