Chemical Coordination and Integration

The endocrine system consists of ductless glands that secrete hormones directly into the bloodstream. Hormones act on specific target organs via specific receptors and regulate growth, metabolism, reproduction, and homeostasis.

Chemical nature of hormones:

• Peptide/protein hormones — water-soluble; act via surface receptors and second messengers (e.g., insulin, FSH, LH, ADH).
• Steroid hormones — lipid-soluble; diffuse into cell and bind nuclear receptors to regulate gene expression (e.g., cortisol, testosterone, oestrogen).
• Amino acid derivative hormones — e.g., adrenaline, thyroxine.

Pituitary gland (master gland) — located in the sella turcica of sphenoid bone, connected to hypothalamus by infundibulum.

• Anterior pituitary (adenohypophysis) secretes: GH (growth hormone), TSH (thyroid stimulating), ACTH, FSH, LH, Prolactin, MSH.
• Posterior pituitary (neurohypophysis) — releases ADH (antidiuretic hormone / vasopressin) and Oxytocin, which are actually synthesised in the hypothalamus.

Thyroid gland — bilobed; follicle cells produce T₃ (triiodothyronine) and T₄ (thyroxine); parafollicular C-cells produce calcitonin.

• T₃ and T₄ regulate basal metabolic rate, protein synthesis, and growth.
• Require iodine; deficiency → goitre (simple goitre due to iodine deficiency; endemic goitre).
• Calcitonin lowers blood calcium (antagonist to PTH).

Parathyroid gland — 4 glands on posterior thyroid; secretes PTH (parathyroid hormone / parathormone) which raises blood calcium by stimulating bone resorption, renal Ca²⁺ reabsorption, and intestinal Ca²⁺ absorption.

Pancreas — both exocrine and endocrine. Islets of Langerhans:

• α cells — secrete glucagon; raises blood glucose (glycogenolysis, gluconeogenesis).
• β cells — secrete insulin; lowers blood glucose (glycogenesis, glucose uptake); deficiency → Diabetes mellitus.
• δ cells — secrete somatostatin (inhibits both insulin and glucagon).

Adrenal glands — sit atop kidneys; two distinct regions:

• Adrenal cortex (outer) — secretes three corticosteroids:
  — Glucocorticoids (cortisol) — stress response, anti-inflammatory, raises blood glucose.
  — Mineralocorticoids (aldosterone) — retains Na⁺ and water; raises blood pressure.
  — Small amounts of sex hormones (androgens).
• Adrenal medulla (inner) — secretes epinephrine (adrenaline) and norepinephrine (noradrenaline) — fight-or-flight response: raises heart rate, BP, blood glucose.

Testes — Leydig cells secrete androgens (mainly testosterone); regulate spermatogenesis, secondary male characters, anabolic effects.

Ovaries — follicles secrete oestrogens; corpus luteum secretes progesterone; regulate female secondary characters, menstrual cycle, and pregnancy maintenance.

Pineal gland — secretes melatonin; regulates circadian rhythms, sleep-wake cycle, and seasonal reproduction.

Thymus — secretes thymosins; stimulates T-lymphocyte differentiation and immune response. It is large in young and degenerates in old age.

Other endocrine tissues:

• Heart — ANF (atrial natriuretic factor) reduces blood pressure.
• Kidney — erythropoietin stimulates RBC formation.
• GI tract — gastrin, secretin, cholecystokinin (CCK), GIP.
• Placenta — hCG, oestrogens, progesterone, relaxin, human placental lactogen (hPL).

Most endocrine glands are regulated by negative feedback loops involving the hypothalamus and pituitary. Rising hormone levels inhibit further secretion of releasing factors and tropic hormones.