19.1 Endocrine System â Overview
Neural coordination is fast but short-lived; chemical (endocrine) coordination is slower but longer-lasting. Together they integrate body functions.
| Feature | Exocrine gland | Endocrine gland |
|---|
| Duct | Present | Absent (ductless) |
| Secretion | Enzymes, sweat, milk | Hormones |
| Transport | Through ducts | Through blood |
| Example | Salivary, sweat gland | Thyroid, pituitary |
Hormones â properties
- Non-nutrient chemicals acting as intercellular messengers, secreted in trace amounts.
- Act only on target tissues that bear specific receptors (membrane-bound or intracellular).
- The vertebrate endocrine system: hypothalamus, pituitary, pineal, thyroid, parathyroid, thymus, adrenals, pancreas and gonads.
Hormone chemistry: peptide/protein (insulin, glucagon, pituitary), steroids (cortisol, aldosterone, sex hormones), iodothyronines (thyroid) and amino-acid derivatives/amines (adrenaline).
19.2 Hypothalamus & Pituitary Gland
The hypothalamus contains neurosecretory cells producing releasing and inhibiting hormones that regulate the anterior pituitary (e.g. GnRH stimulates gonadotropin release).
The pituitary (hypophysis) sits in the bony sella turcica. It has an anterior adenohypophysis (pars distalis + pars intermedia) and a posterior neurohypophysis (pars nervosa).
Hypothalamus (releasing hormone)âAnterior pituitary (trophic hormone)âTarget glandâHormone acts; feedback inhibits
Anterior pituitary hormones
| Hormone | Main action |
|---|
| GH (somatotropin) | Body growth; excessâgigantism/acromegaly, deficiencyâdwarfism |
| TSH | Stimulates thyroid |
| ACTH | Stimulates adrenal cortex |
| LH & FSH (gonadotropins) | Gamete formation & sex hormones |
| Prolactin (PRL) | Milk production & mammary growth |
| MSH (pars intermedia) | Melanin / skin pigmentation |
Posterior pituitary
- Stores & releases oxytocin (uterine contraction during birth, milk ejection) and vasopressin/ADH (water reabsorption in kidney).
- Both are actually synthesised by the hypothalamus. ADH deficiency â diabetes insipidus.
đ§ Memory Hook
"FLAT PiG" â anterior pituitary
FSH, LH, ACTH, TSH, Prolactin, GH. Posterior just stores Oxytocin & ADH.
19.3 Pineal Gland
The pineal gland, on the dorsal forebrain, secretes melatonin.
- Regulates the 24-hour (circadian) rhythm â the sleepâwake cycle, body temperature and pigmentation.
- Also influences metabolism, reproduction and defence capability.
Melatonin secretion rises in the dark â making it the bodyâs "hormone of darkness/sleep".
19.4 Thyroid Gland
The bilobed thyroid gland uses dietary iodine to make the hormones Tâ (thyroxine / tetraiodothyronine) and Tâ (triiodothyronine).
Functions of thyroid hormones
- Regulate the basal metabolic rate (BMR) and tissue Oâ consumption / heat production.
- Support red blood cell formation, growth, and development of the nervous system.
- Maintain waterâelectrolyte and carbohydrateâproteinâfat metabolism.
| Disorder | Cause | Feature |
|---|
| Goitre | Iodine deficiency | Enlarged thyroid |
| Cretinism | Hypothyroidism in children | Stunted growth, mental retardation |
| Myxoedema | Hypothyroidism in adults | Low BMR, oedema, weight gain |
| Gravesâ disease | Hyperthyroidism | Exophthalmos (bulging eyes) |
The thyroid also secretes thyrocalcitonin (TCT), which lowers blood calcium.
19.5 Parathyroid & Thymus
Four parathyroid glands lie on the back of the thyroid and secrete parathyroid hormone (PTH / parathormone).
- PTH is hypercalcemic: it raises blood CaÂČâș by bone resorption and â CaÂČâș reabsorption in the kidney & gut.
- PTH and thyrocalcitonin act antagonistically to balance calcium.
Thymus
- A lobular gland between the lungs that secretes thymosins.
- Thymosins drive differentiation of T-lymphocytes (cell-mediated immunity) and promote antibody production.
- The thymus is large in children and degenerates with age, weakening immunity.
đ§ Memory Hook
"PTH PLUS calcium · Calci-TONE-down"
Parathormone raises blood calcium; calcitonin tones it down.
19.6 Adrenal Glands
Each adrenal gland sits atop a kidney and has an outer cortex and inner medulla.
Adrenal medulla
- Secretes adrenaline (epinephrine) and noradrenaline â catecholamines, the emergency / fight-or-flight hormones.
- They â heart rate, â blood pressure, â alertness, dilate pupils, and raise blood glucose (glycogenolysis).
Adrenal cortex (three zones)
| Zone | Hormone class | Action |
|---|
| Glomerulosa | Mineralocorticoids (aldosterone) | Naâș & water retention; Kâș excretion |
| Fasciculata | Glucocorticoids (cortisol) | Gluconeogenesis; anti-inflammatory |
| Reticularis | Small amounts of androgens | Secondary sexual features |
Hypofunction of the cortex causes Addisonâs disease.
đ§ Memory Hook
"Salt · Sugar · Sex â deeper = sweeter"
GlomerulosaâSalt (aldosterone), FasciculataâSugar (cortisol), ReticularisâSex (androgens). Medulla = Adrenaline (emergency).
19.7 Pancreas â Islets of Langerhans
The pancreas is a heterocrine (mixed) gland; its endocrine portion is the Islets of Langerhans.
| Cell | Hormone | Effect on glucose |
|---|
| α (alpha) | Glucagon | Raises (hyperglycemic) |
| ÎČ (beta) | Insulin | Lowers (hypoglycemic) |
| ÎŽ (delta) | Somatostatin | Inhibits insulin & glucagon |
- Insulin promotes glucose uptake by cells and conversion of glucose â glycogen (glycogenesis).
- Glucagon stimulates glycogenolysis & gluconeogenesis in the liver.
- Insulin deficiency â diabetes mellitus (hyperglycaemia, glucosuria, ketone-body build-up).
đ§ Memory Hook
"Insulin IN, Glucagon UP"
Insulin moves glucose into cells (lowers sugar); Glucagon pushes sugar up. ÎČ-Below, α-Above.
19.8 Gonads & Other Hormone Sources
The gonads are endocrine as well as reproductive glands.
| Gland / cells | Hormone | Role |
|---|
| Testis â Leydig (interstitial) cells | Androgens (testosterone) | Spermatogenesis & male secondary characters |
| Ovary â follicle | Estrogen | Female secondary characters |
| Ovary â corpus luteum | Progesterone | Maintains pregnancy (endometrium) |
Hormones from non-endocrine organs
- Heart (atrial wall): Atrial Natriuretic Factor (ANF) â lowers blood pressure (vasodilation).
- Kidney (JG cells): Erythropoietin (stimulates RBC formation) and renin (controls blood pressure).
- GI tract: Gastrin, Secretin, Cholecystokinin (CCK) and Gastric Inhibitory Peptide (GIP) regulate digestion.
19.9 Mechanism of Hormone Action
A hormone acts only where its receptor is present. Two broad routes depend on whether the hormone can cross the membrane.
| Peptide / protein / amine | Steroid / thyroid (lipid-soluble) |
|---|
| Examples | Insulin, glucagon, adrenaline | Cortisol, testosterone, Tâ/Tâ |
| Receptor | On cell membrane | Inside the cell (cytoplasm/nucleus) |
| Action | Generate second messenger (cAMP) | Hormoneâreceptor complex binds DNA |
| Result | Activate enzymes (fast) | Regulate gene expression (slower) |
Steroid enters cellâBinds intracellular receptorâComplex enters nucleusâSwitches genes on/offâNew proteins made
đ§ Memory Hook
"Water-loving knocks · Fat-loving enters"
Water-soluble (peptide) hormones knock at membrane receptors â cAMP. Lipid-soluble (steroid/thyroid) hormones enter and act on genes.
19.10 Feedback Control & Quick Reference
Most hormones are governed by negative feedback: a rising hormone level inhibits its own releasing/trophic hormones, keeping the body in balance (homeostasis).
Antagonistic hormone pairs
| Effect | Raises | Lowers |
|---|
| Blood glucose | Glucagon | Insulin |
| Blood calcium | PTH | Calcitonin (TCT) |
| Blood pressure | Renin / ADH | ANF |
Master gland = pituitary, but it is itself controlled by the hypothalamus â making the hypothalamus the true integrator of nervous & endocrine systems.