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Endocrine system 2
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3. Hormones and target cells
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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). 3
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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. 4
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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:
tropic hormones (TSH, FSH, LH and ACTH), that act on other glands;
hormones that act on target tissues. 5
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6. Thyroid and parathyroid glands
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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. 6
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7. Adrenal glands and pineal gland
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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. 7
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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. 8
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9. Gonads Gonads, or sex glands, produce sexual hormones.
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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. 9
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