ENDOCRINE SYSTEM
The endocrine system consists of glands secreting hormones essential for maintenance of homeostasis throughout the body. Hormones are chemical messengers that act to control and coordinate different functions of tissues and organs.
Classification of Glands
- Endocrine Glands: Ductless glands; release products directly into bloodstream to reach target cells
- Exocrine Glands: Release secretions via ducts to bloodstream or body surfaces
Major Endocrine Glands: Pituitary, pineal, thyroid, parathyroid, adrenal, pancreas, thymus, and gonads (testes in males, ovaries in females). These work with the nervous system as the neuroendocrine system.
PITUITARY GLAND (HYPOPHYSIS)
The pituitary gland is the size of a pea (0.5 g, 1-1.5 cm diameter), located in the hypophyseal fossa of the sphenoid bone. It is attached to the hypothalamus by the infundibulum. Known as the master endocrine gland because it controls other endocrine glands.
Anterior Pituitary Hormones
- Growth Hormone (GH): 191 amino acids; stimulates growth, cell reproduction, mitosis, and metabolism
- Thyroid Stimulating Hormone (TSH): Stimulates T₃ and T₄ production; regulates BMR and body temperature
- Adrenocorticotropic Hormone (ACTH): Stimulates adrenal cortex to secrete glucocorticoids
- Prolactin (PRL): Stimulates mammary gland development and milk production
- Gonadotropins (FSH & LH): Regulate reproduction – in females: ovulation, oestrogen, progesterone; in males: sperm production, testosterone
- Melanocyte Stimulating Hormone (MSH): Regulates melanin production and skin pigmentation
Posterior Pituitary Hormones
- Antidiuretic Hormone (ADH / Vasopressin): Decreases urine output; increases water reabsorption by kidneys; regulates fluid balance
- Oxytocin: Stimulates uterine contractions during childbirth; stimulates milk ejection (milk let-down reflex); facilitates maternal bonding
ADRENAL (SUPRARENAL) GLANDS
Adrenal glands are star-shaped glands located on the anterior portion of kidneys. They regulate stress response by producing corticosteroids and catecholamines.
Adrenal Cortex Hormones (Corticoids)
- Glucocorticoids (e.g., cortisol): Regulate glucose metabolism; gluconeogenesis; anti-inflammatory; suppress immunity; increase plasma glucose during stress
- Mineralocorticoids (e.g., aldosterone): Stimulate Na⁺ and water reabsorption; K⁺ and phosphate excretion; maintain electrolytes, blood volume, and blood pressure
- Androgenic Steroids (e.g., androgens): Develop puberty symptoms (axillary, pubic, facial hair); converted to sex hormones in gonads
Adrenal Medulla Hormones (Catecholamines)
- Epinephrine (Adrenaline): Increases heart rate, BMR, blood sugar, respiration, lipolysis; dilates pupil; “fight or flight” response
- Norepinephrine (Noradrenaline): Maintains blood pressure; enhances alertness; affects sleep-wake cycle, mood, memory
- Dopamine: (small amount)
THYROID GLAND
The thyroid is the largest endocrine gland (5×3 cm, 25 g), brownish-red, highly vascularised (80-120 ml blood/min). Located on the anterolateral side of the cervical trachea, butterfly-shaped with two lobes connected by the isthmus.
Thyroid Hormones
- Thyroxine (T₄ / Tetra-iodothyronine): Regulates BMR; controls urine output; helps in homeothermy; stimulates protein synthesis; regulates mental development
- Triiodothyronine (T₃): More potent form; regulates metabolic rate
- Thyrocalcitonin (TCT / Calcitonin): Lowers blood calcium; increases calcium deposition in bones; decreases calcium reabsorption from urine
PARATHYROID GLANDS
Four pea-sized parathyroid glands embedded on the dorsal surface of the thyroid gland. They secrete Parathyroid Hormone (PTH / Collip’s hormone) which increases blood calcium levels (antagonistic to calcitonin).
Actions of PTH
- Mobilises calcium from bone (stimulates osteoclasts)
- Enhances calcium absorption from small intestine (activates vitamin D)
- Suppresses calcium loss in urine (stimulates calcium reabsorption)
- Stimulates phosphate excretion in urine
PANCREAS
Pancreas is a retroperitoneal gland (12-15 cm long, 2.5 cm thick) located near the duodenum and stomach. It is a mixed gland (exocrine and endocrine). The endocrine part consists of islets of Langerhans (α-cells secrete glucagon; β-cells secrete insulin).
Pancreatic Hormones
- Insulin (β-cells): Lowers blood glucose; increases glycogenesis, protein synthesis, lipogenesis; decreases glycogenolysis and gluconeogenesis
- Glucagon (α-cells): Increases blood glucose; glycogenolytic hormone; stimulates ketogenesis; positive inotropic and chronotropic action on heart
GONADS
Ovaries (Female Gonads)
- Oval-shaped paired organs (2-4 cm long) on either side of uterus
- Produce female gametes (ova)
- Secrete oestrogen, progesterone, and inhibin
Testes (Male Gonads)
- Located in scrotum (4.5×2.5×3 cm)
- Produce sperm
- Leydig cells secrete testosterone
Sex Hormones Summary
- GnRH: Hypothalamus; controls FSH and LH secretion
- FSH: Pituitary; females – follicle growth; males – sperm formation
- LH: Pituitary; females – oestrogen & ovum development; males – testosterone production
- Testosterone: Testes; male secondary sex characteristics, sperm production, bone/muscle growth
- Oestrogen & Progesterone: Ovaries; female secondary sex characteristics, menstrual cycle, pregnancy, menopause
- Prolactin & Oxytocin: Pituitary; milk secretion (prolactin); milk ejection & uterine contraction (oxytocin)
SUMMARY TABLE: ENDOCRINE GLANDS AND HORMONES
| Gland | Hormone | Primary Function |
|---|---|---|
| Pituitary (Anterior) | GH, TSH, ACTH, PRL, FSH, LH, MSH | Growth, metabolism, reproduction, lactation, pigmentation |
| Pituitary (Posterior) | ADH, Oxytocin | Water balance, uterine contraction, milk ejection |
| Thyroid | T₃, T₄, Calcitonin | Metabolism, growth, lowers blood calcium |
| Parathyroid | PTH | Raises blood calcium |
| Adrenal Cortex | Cortisol, Aldosterone, Androgens | Stress response, electrolyte balance, puberty |
| Adrenal Medulla | Epinephrine, Norepinephrine | Fight or flight, blood pressure, alertness |
| Pancreas | Insulin, Glucagon | Lowers/raises blood glucose |
| Ovaries | Oestrogen, Progesterone | Female sex characteristics, reproduction |
| Testes | Testosterone | Male sex characteristics, sperm production |
