Endocrine glands and their secretions

30 min

The gland that is not the master

The pituitary gland, no bigger than a pea, was once called the "master gland" because its hormones control the thyroid, the adrenal cortex and the ovaries and testes. But the pituitary takes its orders from the hypothalamus just above it, a part of the brain. The hypothalamus is where the nervous system and the endocrine system meet: it receives information from the whole brain and turns it into hormonal signals. This lesson follows that link and then surveys the main glands, their hormones and their effects.

Definition

Neurosecretory cell: a neurone that secretes a hormone into the blood instead of a neurotransmitter into a synapse.

Releasing hormone: a hormone from the hypothalamus that makes the anterior pituitary release one of its hormones, such as thyrotropin-releasing hormone (TRH).

Tropic hormone: a hormone whose target is another endocrine gland, such as thyroid-stimulating hormone (TSH).

Portal vessel: a blood vessel that carries blood from one capillary network to another, here from the hypothalamus to the anterior pituitary.

The hypothalamus and the pituitary gland

Posterior pituitary: ADH and oxytocin are made in neurosecretory cells of the hypothalamus. They pass down the axons in the stalk and are stored in the posterior pituitary, which releases them into the blood when the neurones fire. This route is fast: a nerve impulse releases a hormone.
Anterior pituitary: its own glandular cells make six hormones. The hypothalamus controls them by releasing hormones (and some inhibiting hormones) carried to the anterior pituitary in the portal vessel.

The main endocrine glands

Gland	Hormone	Target	Main effects
Hypothalamus	Releasing hormones, for example TRH, CRH, GnRH	Anterior pituitary	Stimulate release of TSH, ACTH, FSH and LH
Anterior pituitary	Growth hormone (GH)	Most cells, especially bone and muscle	Protein synthesis, growth of bones and muscles
	Thyroid-stimulating hormone (TSH)	Thyroid gland	Secretion of thyroxine
	Adrenocorticotropic hormone (ACTH)	Adrenal cortex	Secretion of cortisol
	FSH and LH	Ovaries and testes	Development of gametes; secretion of sex hormones (unit 8)
	Prolactin	Mammary glands	Milk production after birth
Posterior pituitary	Antidiuretic hormone (ADH)	Collecting ducts of the kidney	More water reabsorbed; less, more concentrated urine
Posterior pituitary	Oxytocin	Uterus; mammary glands	Contractions of the uterus in birth; release of milk
Thyroid	Thyroxine (needs iodine)	Most cells	Raises metabolic rate; needed for normal growth and brain development
Thyroid	Calcitonin	Bone, kidney	Lowers blood calcium
Parathyroids	Parathyroid hormone (PTH)	Bone, kidney, gut	Raises blood calcium
Adrenal medulla	Adrenaline	Heart, liver, muscles, blood vessels	"Fight or flight": faster heart, more glucose in the blood, wider airways
Adrenal cortex	Cortisol (a steroid)	Liver, muscle, immune cells	Long-term stress response; raises blood glucose; reduces inflammation
Adrenal cortex	Aldosterone (a steroid)	Kidney	More Na⁺ reabsorbed, so more water retained
Pancreas (islets of Langerhans)	Insulin, from β (beta) cells	Liver, muscle, fat cells	Lowers blood glucose
Pancreas (islets of Langerhans)	Glucagon, from α (alpha) cells	Liver	Raises blood glucose
Ovaries	Oestrogen, progesterone	Uterus, breasts and other organs	Female secondary sexual characteristics; menstrual cycle; pregnancy (unit 8)
Testes	Testosterone	Many organs	Sperm production; male secondary sexual characteristics
Pineal gland	Melatonin	Brain	Daily sleep–wake rhythm; secreted mainly in darkness
Thymus	Thymosins	T lymphocytes	Maturation of T cells, mainly in childhood

Adrenaline in action. Adrenaline from the adrenal medulla is released within seconds of a fright, because the gland is stimulated by sympathetic nerves. It increases heart rate and the force of each beat, makes the liver break down glycogen to glucose, widens the airways, and diverts blood from the gut and skin to the muscles. Cortisol from the adrenal cortex is the slower hormone of long-lasting stress.

Worked example 1: following a hormone cascade

A student has been worried about exams for several weeks, and her cortisol level is raised. Trace the chain of signals from the brain, naming each gland and hormone.

The hypothalamus secretes corticotropin-releasing hormone (CRH) into the portal vessel.
CRH stimulates the anterior pituitary to release adrenocorticotropic hormone (ACTH), a tropic hormone, into the general circulation.
ACTH stimulates the adrenal cortex to secrete cortisol.
Cortisol raises blood glucose by making the liver produce glucose from amino acids and fats.

Note: each step is a separate gland, so the signal is amplified at each level, and each level can be regulated by feedback, as lesson 3 shows.

Worked example 2: adrenaline and the heart

Illustrative data: at rest, a person's heart rate is 72 beats per minute and each beat pumps 70 cm³ of blood. After an injection of adrenaline, the rate is 126 beats per minute and each beat pumps 90 cm³. Calculate (a) the percentage increase in heart rate, (b) the cardiac output before and after, and (c) by what factor the output rises.

(a) $\frac{126 - 72}{72} \times 100 = 75\%$ .
(b) Cardiac output = heart rate × stroke volume. Before: $72 \times 70 = 5\,040\,\mathrm{cm^3\,min^{-1}} \approx 5.0\,\mathrm{dm^3\,min^{-1}}$ . After: $126 \times 90 = 11\,340\,\mathrm{cm^3\,min^{-1}} \approx 11\,\mathrm{dm^3\,min^{-1}}$ .
(c) $\frac{11\,340}{5\,040} = 2.25$ : more than double.

Interpretation: adrenaline raises both the rate and the force of the heartbeat, so muscles receive far more oxygen and glucose for "fight or flight".

Warning

Common mistakes

"ADH is made in the posterior pituitary." It is made in the hypothalamus and only stored and released by the posterior pituitary.
"Adrenaline comes from the adrenal cortex." Adrenaline is from the adrenal medulla; the cortex makes steroids such as cortisol and aldosterone.
"Insulin and glucagon come from the same cells." Insulin is from β cells, glucagon from α cells, in the islets.

CrashCourse: "Endocrine System, Part 2 - Hormone Cascades: Crash Course Anatomy & Physiology #24". Watch for how the hypothalamus and pituitary control other glands.

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Practice: endocrine glands

Name the gland that secretes (a) thyroxine, (b) glucagon, (c) melatonin, (d) aldosterone.
Name two hormones released by the posterior pituitary and state where they are made.
Explain how the hypothalamus controls the anterior pituitary.
What is a tropic hormone? Give two examples.
State three effects of adrenaline that prepare the body for action.
Heart rate rises from 64 to 112 beats per minute. Calculate the percentage increase.
Compare the hormones that regulate blood calcium.
Suggest why the anterior pituitary is supplied by a portal vessel instead of by an ordinary artery.

Show answersHide answers

(a) Thyroid. (b) Pancreas (α cells of the islets). (c) Pineal gland. (d) Adrenal cortex.
ADH and oxytocin; they are made in neurosecretory cells of the hypothalamus.
Neurosecretory cells in the hypothalamus secrete releasing (or inhibiting) hormones into capillaries; a portal vessel carries them straight to the anterior pituitary, where they stimulate (or inhibit) the release of its hormones.
A hormone whose target is another endocrine gland; for example TSH (thyroid), ACTH (adrenal cortex), FSH and LH (gonads).
Any three: faster and stronger heartbeat; glycogen broken down to glucose in the liver; airways widened; blood diverted from gut and skin to muscles; pupils dilated.
$\frac{112 - 64}{64} \times 100 = 75\%$ .
Parathyroid hormone from the parathyroid glands raises blood calcium (release from bone, more reabsorption in the kidney, more absorption from the gut); calcitonin from the thyroid lowers it. They are antagonistic.
The releasing hormones are produced in tiny amounts. Carried directly to the pituitary, they arrive concentrated and quickly; in the general circulation they would be diluted in the whole blood volume before reaching it.

Key points

The hypothalamus links the nervous and endocrine systems: it makes ADH and oxytocin (released by the posterior pituitary) and releasing hormones that reach the anterior pituitary through a portal vessel.
The anterior pituitary secretes GH, TSH, ACTH, FSH, LH and prolactin; several are tropic hormones acting on other glands.
Thyroid: thyroxine and calcitonin; parathyroids: PTH; adrenal medulla: adrenaline; adrenal cortex: cortisol and aldosterone; islets: insulin (β) and glucagon (α); gonads: sex hormones; pineal: melatonin.
Adrenaline acts within seconds in "fight or flight"; cortisol manages longer stress.

Sources and credits

7.5.6: Endocrine Glands and 7.5.4: Regulation of Body Processes, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for the hypothalamus–pituitary link and the glands and hormones in the table. Adapted by Ganzaa.
Video: "Endocrine System, Part 2 - Hormone Cascades: Crash Course Anatomy & Physiology #24", CrashCourse, embedded from YouTube.

Figure 1, the table, the worked examples (with illustrative data) and the practice questions were written by Ganzaa.

1The gland that is not the master

2The hypothalamus and the pituitary gland

3The main endocrine glands

The gland that is not the master

The hypothalamus and the pituitary gland

The main endocrine glands