Endocrine System & Hormones

• The body has two coordination systems that make all organs work together at the right time.
• The nervous system sends fast electrical signals along nerves; the effect is instant but short-lasting.
• The endocrine system sends slow chemical signals through the blood; the effect is delayed but long-lasting and reaches distant organs.
• A useful analogy: nervous = phone call (instant, ends at once); endocrine = letter (slow, travels far, lasts long).
• Exam point: nervous = fast and short; endocrine = slow but long-lasting.
• Signal — Electrical — Chemical (hormone)
• Pathway — Nerves — Blood
• Speed — Fast — Slow
• Effect — Short-lasting — Long-lasting

The body has two coordination systems that make all organs work together at the right time.

The nervous system sends fast electrical signals along nerves; the effect is instant but short-lasting.

The endocrine system sends slow chemical signals through the blood; the effect is delayed but long-lasting and reaches distant organs.

A useful analogy: nervous = phone call (instant, ends at once); endocrine = letter (slow, travels far, lasts long).

Exam point: nervous = fast and short; endocrine = slow but long-lasting.

• NCERT defines a hormone as a non-nutrient chemical that acts as an inter-cellular messenger.
• Hormones are produced in trace (tiny) amounts, yet are extremely powerful.
• "Non-nutrient" means a hormone is not food and gives no energy - it only carries an instruction.
• A hormone acts only on its target organ or target cells, which have matching receptors.
• Hormones are poured directly into the blood, which carries them throughout the body.

NCERT defines a hormone as a non-nutrient chemical that acts as an inter-cellular messenger.

Hormones are produced in trace (tiny) amounts, yet are extremely powerful.

"Non-nutrient" means a hormone is not food and gives no energy - it only carries an instruction.

A hormone acts only on its target organ or target cells, which have matching receptors.

Hormones are poured directly into the blood, which carries them throughout the body.

• A gland is a cell or organ that makes and releases a useful substance (a secretion).
• Exocrine glands release products through tubes called ducts (e.g. salivary, sweat, oil, mammary, tear glands).
• Endocrine glands have no ducts; they are ductless glands that pour hormones directly into blood.
• Memory hook: Exo = exit through a tube; Endo = enters the blood directly.
• The pancreas is a composite (mixed) gland - exocrine part makes digestive juice, endocrine part makes insulin and glucagon.
• Duct — Has ducts — Ductless
• Releases into — Surface/cavity — Blood
• Product — Enzymes, sweat, milk — Hormones
• Example — Salivary gland — Thyroid gland

A gland is a cell or organ that makes and releases a useful substance (a secretion).

Exocrine glands release products through tubes called ducts (e.g. salivary, sweat, oil, mammary, tear glands).

Endocrine glands have no ducts; they are ductless glands that pour hormones directly into blood.

The pancreas is a composite (mixed) gland - exocrine part makes digestive juice, endocrine part makes insulin and glucagon.

• The hypothalamus is a small region at the base of the brain that links the brain to the endocrine system.
• It regulates body temperature, hunger, thirst, sleep and emotions.
• It directly controls the pituitary gland using releasing and inhibiting hormones.
• Releasing hormones tell the pituitary to secrete; inhibiting hormones tell it to stop.
• Example: GnRH stimulates release of gonadotrophins; somatostatin inhibits growth hormone.

The hypothalamus is a small region at the base of the brain that links the brain to the endocrine system.

It directly controls the pituitary gland using releasing and inhibiting hormones.

Releasing hormones tell the pituitary to secrete; inhibiting hormones tell it to stop.

Example: GnRH stimulates release of gonadotrophins; somatostatin inhibits growth hormone.

• The pituitary is a pea-sized gland sitting in a bony pocket called the sella turcica, located in the brain.
• It is called the master gland because its hormones control many other endocrine glands.
• It has two parts: anterior (adenohypophysis) and posterior (neurohypophysis).
• The anterior pituitary makes six hormones: GH, PRL, TSH, ACTH, LH and FSH.
• The posterior pituitary makes nothing itself - it only stores and releases oxytocin and vasopressin (ADH).
• Pars intermedia makes MSH (controls skin pigment); in humans it is nearly merged with the anterior part.

The pituitary is a pea-sized gland sitting in a bony pocket called the sella turcica, located in the brain.

It is called the master gland because its hormones control many other endocrine glands.

It has two parts: anterior (adenohypophysis) and posterior (neurohypophysis).

The posterior pituitary makes nothing itself - it only stores and releases oxytocin and vasopressin (ADH).

Pars intermedia makes MSH (controls skin pigment); in humans it is nearly merged with the anterior part.

• GH (Growth Hormone) controls growth of body tissues.
• Too much GH in children causes gigantism; too little causes pituitary dwarfism.
• Excess GH in an adult causes acromegaly (enlarged face, jaw and hands).
• TSH tells the thyroid to make thyroxine; ACTH tells the adrenal cortex to make glucocorticoids.
• LH and FSH (gonadotrophins) control the gonads - sperm/androgens in males, ovulation/follicle growth in females.
• PRL (Prolactin) develops mammary glands and triggers milk formation.
• GH — Body growth
• PRL — Milk formation
• TSH — Stimulates thyroid
• ACTH — Stimulates adrenal cortex
• LH/FSH — Control gonads

GH (Growth Hormone) controls growth of body tissues.

Too much GH in children causes gigantism; too little causes pituitary dwarfism.

Excess GH in an adult causes acromegaly (enlarged face, jaw and hands).

TSH tells the thyroid to make thyroxine; ACTH tells the adrenal cortex to make glucocorticoids.

LH and FSH (gonadotrophins) control the gonads - sperm/androgens in males, ovulation/follicle growth in females.

PRL (Prolactin) develops mammary glands and triggers milk formation.

• Both hormones are actually made by the hypothalamus and only stored here.
• Oxytocin causes uterine contraction during birth and milk ejection.
• Vasopressin / ADH makes the kidney reabsorb water, so it is anti-diuretic.
• Shortage of ADH causes diabetes insipidus (excess dilute urine).
• Memory aid: oxytocin = birth and milk; ADH = saves water.

Both hormones are actually made by the hypothalamus and only stored here.

Oxytocin causes uterine contraction during birth and milk ejection.

Vasopressin / ADH makes the kidney reabsorb water, so it is anti-diuretic.

Shortage of ADH causes diabetes insipidus (excess dilute urine).

• The pineal gland is a tiny gland located in the brain.
• It secretes the hormone melatonin.
• Melatonin regulates the body's 24-hour (diurnal) rhythms like the sleep-wake cycle.
• It is often called the sleep hormone because it rises in darkness and falls in daylight.
• Think of it as the body's internal clock-keeper.

The pineal gland is a tiny gland located in the brain.

It secretes the hormone melatonin.

Melatonin regulates the body's 24-hour (diurnal) rhythms like the sleep-wake cycle.

It is often called the sleep hormone because it rises in darkness and falls in daylight.

• The thyroid is a butterfly-shaped gland with two lobes beside the trachea, joined by the isthmus.
• It makes thyroxine (T4) and triiodothyronine (T3), both of which need the element iodine.
• These hormones control the Basal Metabolic Rate (BMR) and support red blood cell formation.
• Iodine deficiency causes hypothyroidism and goitre (swollen neck) - reason we use iodised salt.
• Too little thyroid hormone in a baby causes cretinism (stunted growth, low IQ).
• The thyroid also makes thyrocalcitonin (TCT), which lowers blood calcium.
• Goitre — Iodine deficiency (hypothyroid)
• Cretinism — Low thyroid in infants
• Hyperthyroidism — Excess thyroid hormone

The thyroid is a butterfly-shaped gland with two lobes beside the trachea, joined by the isthmus.

It makes thyroxine (T4) and triiodothyronine (T3), both of which need the element iodine.

These hormones control the Basal Metabolic Rate (BMR) and support red blood cell formation.

Iodine deficiency causes hypothyroidism and goitre (swollen neck) - reason we use iodised salt.

Too little thyroid hormone in a baby causes cretinism (stunted growth, low IQ).

The thyroid also makes thyrocalcitonin (TCT), which lowers blood calcium.

• There are four parathyroid glands, embedded on the back of the thyroid lobes.
• They secrete parathyroid hormone (PTH), a peptide hormone.
• PTH raises blood calcium (Ca2+), doing the opposite of thyrocalcitonin (TCT).
• Memory aid: PTH pushes calcium UP; TCT brings it DOWN.
• Together they keep blood calcium steady - called calcium homeostasis.
• Calcium is needed for bones, muscle contraction, nerve signals and blood clotting.
• PTH — Raises (UP)
• TCT (Calcitonin) — Lowers (DOWN)

There are four parathyroid glands, embedded on the back of the thyroid lobes.

PTH raises blood calcium (Ca2+), doing the opposite of thyrocalcitonin (TCT).

Together they keep blood calcium steady - called calcium homeostasis.

• The thymus lies in the chest behind the breastbone and secretes thymosin, which develops immunity (T-cells).
• The adrenal glands sit on top of each kidney and have two parts: cortex and medulla.
• The adrenal medulla secretes adrenaline and noradrenaline - the emergency "fight-or-flight" hormones.
• Adrenaline raises heart rate, blood pressure and blood glucose during stress or fear.
• The adrenal cortex secretes glucocorticoids (e.g. cortisol) and mineralocorticoids (e.g. aldosterone).
• Aldosterone controls salt and water balance; cortisol manages stress and metabolism.
• Medulla — Adrenaline — Fight-or-flight
• Cortex — Cortisol — Stress/metabolism
• Cortex — Aldosterone — Salt-water balance

The thymus lies in the chest behind the breastbone and secretes thymosin, which develops immunity (T-cells).

The adrenal glands sit on top of each kidney and have two parts: cortex and medulla.

The adrenal medulla secretes adrenaline and noradrenaline - the emergency "fight-or-flight" hormones.

The adrenal cortex secretes glucocorticoids (e.g. cortisol) and mineralocorticoids (e.g. aldosterone).

Aldosterone controls salt and water balance; cortisol manages stress and metabolism.

• The endocrine part of the pancreas is the Islets of Langerhans.
• Beta cells secrete insulin, which lowers blood glucose by helping cells absorb it.
• Alpha cells secrete glucagon, which raises blood glucose.
• Insulin and glucagon together maintain blood sugar homeostasis.
• Lack of insulin causes diabetes mellitus (high blood glucose, sugar in urine).
• Blood glucose is commonly expressed in mg/dL (milligrams per decilitre); normal fasting is about 70- 110 mg/dL.
• Beta cell — Insulin — Lowers
• Alpha cell — Glucagon — Raises

The endocrine part of the pancreas is the Islets of Langerhans.

Beta cells secrete insulin, which lowers blood glucose by helping cells absorb it.

Alpha cells secrete glucagon, which raises blood glucose.

Insulin and glucagon together maintain blood sugar homeostasis.

Lack of insulin causes diabetes mellitus (high blood glucose, sugar in urine).

Blood glucose is commonly expressed in mg/dL (milligrams per decilitre); normal fasting is about 70- 110 mg/dL.

• The testes (in males) secrete testosterone, the main male sex hormone (androgen).
• Testosterone controls male secondary characters like beard, deep voice and sperm production.
• The ovaries (in females) secrete estrogen and progesterone.
• Estrogen develops female secondary characters; progesterone maintains pregnancy.
• After menopause, the ovaries stop producing estrogen and progesterone.
• Gonads are controlled by the pituitary gonadotrophins LH and FSH.
• Testes — Testosterone — Male
• Ovaries — Estrogen — Female
• Ovaries — Progesterone — Female

The testes (in males) secrete testosterone, the main male sex hormone (androgen).

The ovaries (in females) secrete estrogen and progesterone.

Estrogen develops female secondary characters; progesterone maintains pregnancy.

After menopause, the ovaries stop producing estrogen and progesterone.

Gonads are controlled by the pituitary gonadotrophins LH and FSH.

• Diabetes mellitus - insulin deficiency, high blood sugar (pancreas).
• Goitre / cretinism - iodine/thyroxine deficiency (thyroid).
• Gigantism, dwarfism, acromegaly - growth hormone imbalance (pituitary).
• Diabetes insipidus - ADH deficiency (posterior pituitary).
• Tetany - low calcium from low PTH (parathyroid).
• Diabetes mellitus — Insulin / Pancreas
• Goitre — Thyroxine / Thyroid
• Gigantism — GH / Pituitary
• Diabetes insipidus — ADH / Pituitary
• Tetany — PTH / Parathyroid

Diabetes mellitus - insulin deficiency, high blood sugar (pancreas).

Goitre / cretinism - iodine/thyroxine deficiency (thyroid).

Diabetes insipidus - ADH deficiency (posterior pituitary).

Tetany - low calcium from low PTH (parathyroid).