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Endocrine system
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Hormones and target cells
Hormones are chemical signals produced by endocrine cells
that can be grouped in endocrine glands.
Hormones circulate in the blood stream and affect the activity
of target cells that exhibit specific receptors.
There are peptide, steroid and amino acid derived hormones.
They can be water-soluble (they bind to membrane receptors)
or lipid-soluble (they interact with cytoplasmic and nuclear
receptors).
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Hypothalamus
The hypothalamus is a link between the nervous system
and the endocrine system.
Depending on the stimuli coming from the external or internal
environment, the hypothalamus sends nervous signals and
releases specific hormones to control physiological activities.
The hypothalamus is connected with the pituitary gland.
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Pituitary gland
The pituitary gland, or hypophysis, is divided in two parts.
The posterior pituitary (or neurohypophysis) secretes two
hormones in the blood stream which are produced by the
hypothalamus:
• oxytocin, which stimulates lactation and uterine
contraction;
• ADH, which increases water absorption in the kidney.
The anterior pituitary (or adenohypophysis) produces and
secretes 9 hormones:
o tropic hormones (TSH, FSH, LH and ACTH), that act
on other glands;
o hormones that act on target tissues.
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Thyroid and parathyroid glands
The thyroid produces two hormones:
• thyroid hormone, which regulates cell metabolism;
• calcitonin, which controls the concentration of calcium in
the blood stream (it has the opposite effect of the
parathyroid hormone).
The thyroid is controlled by the TSH hormone.
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Adrenal glands and pineal gland
The adrenal glands (or suprarenal glands) have an outer
cortex and an inner medulla.
The medulla produces and releases adrenaline and
noradrenaline which are neurotransmitters secreted during
conditions of stress.
The adrenal cortex produces and releases glucocorticoids,
mineralocorticoids and sexual steroids.
The epiphysis, or pineal gland, produces melatonin which
influences the circadian rhythms of physiological functions,
including sleep.
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Endocrine pancreas
The endocrine cells of the pancreas are grouped in the islets
of Langerhans.
The alpha cells secrete glucagon when the concentration of
glucose in the blood stream is too low.
The beta cells secrete insulin when the concentration of
glucose in the blood stream is too high.
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Gonads
Gonads, or sex glands, produce sexual hormones.
Ovaries in females produce progesterone and estrogen.
Testicles in males produce androgens.
The production of sexual hormones is regulated by LH and
FSH (gonadotropins), produced by the adenohypophysis.
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