Endocrine system — A/L Biology

What must I remember about Endocrine system?

• The endocrine system consists of ductless glands that secrete chemical messengers (hormones) directly into the bloodstream to act on target tissues bearing specific receptor proteins.
• Hormones fall biochemically into water-soluble peptides/proteins (e.g., Insulin, Glucagon, Pituitary hormones; bind to cell-surface receptors and act via second messengers like cAMP) and lipid-soluble steroids/thyroid hormones (e.g., Cortisol, Aldosterone, Estrogen, T3/T4; diffuse across membrane to bind intracellular/nuclear receptors altering gene transcription).
• Hypothalamus and Pituitary axis: Hypothalamus secretes releasing/inhibiting hormones (GnRH, CRH, TRH) into hypophyseal portal system to regulate Anterior Pituitary (secretes GH, TSH, ACTH, FSH, LH, Prolactin); Posterior Pituitary stores and releases neurohormones synthesized in hypothalamus (Oxytocin and ADH / Vasopressin).
• Thyroid and Parathyroid regulation: Thyroid secretes Thyroxine (T4) and Triiodothyronine (T3) regulating basal metabolic rate, and Calcitonin (lowers blood Ca2+); Parathyroid glands secrete PTH (raises blood Ca2+ by stimulating osteoclasts, renal calcium reabsorption, and vitamin D activation).
• Adrenal glands: Adrenal Cortex secretes Glucocorticoids (Cortisol regulating glucose metabolism/stress) and Mineralocorticoids (Aldosterone stimulating renal Na+ reabsorption and K+ excretion); Adrenal Medulla secretes catecholamines (Epinephrine and Norepinephrine for sympathetic emergency responses).

Test yourself (1 minute)

1. Negative feedback loops

   Elevated levels of target hormones (e.g., T3/T4 or Cortisol) inhibit the release of their corresponding trophic hormones from both hypothalamus (TRH, CRH) and anterior pituitary (TSH, ACTH).

2. Calcium homeostasis antagonism

   Calcitonin (from thyroid C cells) deposits Ca2+ in bone; Parathyroid Hormone (PTH) stimulates osteoclasts to dissolve bone matrix and activates vitamin D to calcitriol in kidneys, elevating blood Ca2+.

3. Islets of Langerhans dual control

   Beta cells secrete Insulin (hypoglycemic: stimulates glycogenesis, GLUT4 translocation, inhibits gluconeogenesis); Alpha cells secrete Glucagon (hyperglycemic: promotes glycogenolysis and gluconeogenesis).

Practise Endocrine system questions today

Source: NIE G.C.E. (A/L) Biology Syllabus & Resource Book, Unit 5

What should I study first?

What comes next?

Next in the syllabus

Where does Endocrine system fit in the syllabus?

Year 2 · Term 1