1.0 Endocrine Physiology: The Glandular Hierarchy
The Endocrine System provides a second level of coordination through Hormones—chemical messengers secreted by Ductless Glands directly into the bloodstream. While the nervous system is built for speed, the endocrine system is optimized for long-term physiological changes like growth, metabolism, and reproduction.
1.1 The Pituitary: The Master Gland
Located at the base of the brain, the Pituitary Gland (Hypophysis) is controlled by the Hypothalamus and regulates most other endocrine glands.
- Growth Hormone (GH): Essential for skeletal growth. Hypersecretion leads to Gigantism (childhood) or Acromegaly (adulthood); hyposecretion leads to Dwarfism.
- TSH (Thyroid Stimulating Hormone): Activates the thyroid gland to release thyroxine.
- ADH (Vasopressin): Regulates water reabsorption in the kidneys.
- Oxytocin: Facilitates uterine contractions during childbirth and milk ejection.
1.2 Metabolic and Glycemic Regulators
The Thyroid and Pancreas are critical for maintaining the body's basal metabolic rate and blood glucose levels.
| Gland | Primary Hormone | Key Function / Disorder |
|---|---|---|
| Thyroid | Thyroxine | Controls BMR. Deficiency causes Simple Goitre or Myxoedema. |
| Pancreas (Islets) | Insulin | Lowers blood sugar. Deficiency causes Diabetes Mellitus. |
| Adrenal Medulla | Adrenaline | "Emergency Hormone"; prepares body for Fight or Flight. |
Exophthalmic Goitre: A condition resulting from the hypersecretion of thyroxine, characterized by an increased metabolic rate, rapid heartbeat, and protrusion of the eyeballs.
1.3 Negative Feedback Mechanism
Hormone secretion is self-regulated. When the concentration of a hormone (e.g., Thyroxine) rises in the blood, it signals the Pituitary and Hypothalamus to inhibit further production of the stimulating hormone (TSH). This ensures a steady-state equilibrium.
Heterocrine Glands: The Pancreas is called a dual gland (Heterocrine) because it has both an Exocrine part (secreting digestive enzymes via ducts) and an Endocrine part (Islets of Langerhans secreting Insulin and Glucagon directly into blood).
Distinguish between Diabetes Mellitus (Insulin deficiency; sugar in urine) and Diabetes Insipidus (ADH deficiency; no sugar in urine, just excess water loss).
2.0 Adrenal Dynamics and Reproductive Coordination
The Adrenal Glands (Suprarenals) and the Gonads represent the final tier of the major endocrine axis. These glands are specialized for immediate stress responses and the long-term orchestration of sexual dimorphism and reproductive cycles.
2.1 The Adrenal Glands: Cortex vs. Medulla
Each adrenal gland sits atop a kidney and is divided into two histologically distinct regions with entirely different functions.
- Adrenal Cortex (Outer): Secretes Corticoids.
• Mineralocorticoids (e.g., Aldosterone): Regulate $Na^+$ and $K^+$ balance.
• Glucocorticoids (e.g., Cortisol): Regulate carbohydrate, protein, and fat metabolism.
• Disorder: Hyposecretion leads to Addison's Disease; Hypersecretion leads to Cushing's Syndrome. - Adrenal Medulla (Inner): Secretes Adrenaline (Epinephrine) and Nor-adrenaline.
• Often called the "Emergency Gland."
• Increases heartbeat, blood pressure, and blood glucose during stress.
2.2 Gonadal Endocrinology
The testes and ovaries are Mixed Glands that produce gametes (exocrine) and hormones (endocrine) responsible for secondary sexual characteristics.
| Hormone | Source | Primary Function |
|---|---|---|
| Testosterone | Leydig Cells (Testes) | Sperm production and male secondary traits (beard, voice). |
| Oestrogen | Graafian Follicle (Ovary) | Regulates menstrual cycle and female traits. |
| Progesterone | Corpus Luteum | The "Pregnancy Hormone"; maintains the uterine lining. |
The Hypothalamic-Pituitary-Endocrine Axis: Most hormone levels are governed by Negative Feedback. For example, high levels of Insulin in the blood signal the Pancreas to stop secreting further Insulin, preventing hypoglycemia.
Local Hormones: Unlike general hormones, these act on neighboring cells. An example is Gastrin, produced by the stomach lining, which stimulates the secretion of gastric juice. These do not travel through the general systemic circulation to distant organs.
Precise Terminology: When discussing the Pancreas, always specify the Islets of Langerhans as the endocrine part. When discussing the Testes, specify Interstitial/Leydig Cells as the site of testosterone production.