Science · Medicine & the body

The endocrine system

The word 'hormone' coined 1905

Medicine — hormones and the ductless glands

The body's chemical signalling network: a set of ductless glands that release hormones directly into the bloodstream to act on distant tissues. It works more slowly than the nervous system but its effects last far longer, and the two meet at the hypothalamus, which controls the pituitary hanging beneath it. The principal glands are the pituitary, pineal, thyroid, parathyroids, thymus, adrenals, pancreatic islets and the gonads. The word "hormone" was coined by Ernest Starling in 1905, three years after he and William Bayliss discovered the first one, secretin.

How to spot them

▸Ductless glands secreting into the blood, as opposed to exocrine glands with ducts
▸Pituitary in the sella turcica, called the master gland but itself controlled by the hypothalamus
▸Anterior pituitary makes its own hormones; the posterior only stores two made in the hypothalamus
▸Adrenal cortex outside for cortisol and aldosterone, medulla inside for adrenaline
▸Nearly everything runs on negative feedback loops

Key facts

Secretin, found by Bayliss and Starling in 1902, and the word Starling coined for such substances in 1905 (1905)

'Hormone', from the Greek for 'I arouse'; secretin was the first one identified

A pea-sized gland in a bony saddle at the base of the skull, in two lobes with quite different origins

The pituitary, in the sella turcica; often called the master gland

A butterfly-shaped gland straddling the windpipe that cannot work without iodine

The thyroid, making thyroxine and triiodothyronine

An outer layer making cortisol and aldosterone wrapped round an inner core making adrenaline

The adrenal cortex and medulla, sitting on top of the kidneys

Oxytocin and vasopressin: manufactured in one structure, released from another

Made in the hypothalamus, stored and released by the posterior pituitary

Melatonin from the small gland Descartes called the seat of the soul

The pineal, which governs the circadian rhythm

Acromegaly, Cushing's syndrome, Addison's disease and Graves' disease

Excess growth hormone, excess cortisol, adrenal failure and an overactive thyroid

Four tiny glands behind a larger one in the neck, raising blood calcium

The parathyroids and their hormone, PTH

Key ideas

Exocrine glands send their products down a duct to a surface; these glands have no ducts and post their messages into the bloodstream, so only cells with the right receptor respond.
Most axes run on negative feedback: the hypothalamus releases a releasing hormone, the pituitary a stimulating hormone, the target gland its own hormone, and that hormone switches the first two off.

Quick-fire facts

•Ernest Starling introduced the word 'hormone' in 1905; he and William Bayliss had discovered secretin, the first known hormone, in 1902
•The hypothalamus links the nervous and endocrine systems and controls the pituitary below it
•The pituitary sits in the sella turcica of the sphenoid bone; the anterior lobe makes growth hormone, ACTH, TSH, FSH, LH and prolactin
•The posterior pituitary releases oxytocin and antidiuretic hormone (vasopressin), both manufactured in the hypothalamus
•The thyroid is the largest purely endocrine gland; it makes thyroxine (T4) and triiodothyronine (T3), which require iodine, and calcitonin, which lowers blood calcium
•The four parathyroid glands secrete parathyroid hormone, which raises blood calcium — the opposite of calcitonin
•The adrenal cortex produces aldosterone, cortisol and androgens; the medulla, derived from neural crest, produces adrenaline and noradrenaline
•The pancreas is both exocrine (digestive enzymes) and endocrine (insulin, glucagon, somatostatin)
•The pineal gland secretes melatonin; Descartes called it the seat of the soul
•The thymus, where T cells mature, shrinks after puberty
•Addison's disease is adrenal insufficiency — John F. Kennedy had it; Cushing's syndrome is cortisol excess, named after Harvey Cushing
•Excess growth hormone causes gigantism in children and acromegaly in adults
•Diabetes insipidus, caused by lack of antidiuretic hormone, is unrelated to diabetes mellitus except in producing copious urine

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Sources

OpenStax, Rice University — Anatomy and Physiology 2e — An Overview of the Endocrine System
Henry Gray (Susan Standring, ed.) — Gray's Anatomy