Summary

This chapter has covered the human endocrine system as the body's second, chemical coordination system, working alongside the faster nervous system through hormones — chemical messengers secreted directly into the blood by ductless (endocrine) glands and acting only on target cells bearing a matching receptor. The hypothalamus, though part of the brain, doubles as a true endocrine structure, sending releasing/inhibiting hormones to the anterior pituitary via a local portal blood system and sending oxytocin/vasopressin directly, by nerve axon, to the posterior pituitary for storage and release. The pituitary gland, the 'master gland', secretes six anterior hormones (GH, TSH, ACTH, FSH, LH, prolactin), one pars-intermedia hormone (MSH), and releases the two hypothalamus-made posterior hormones (oxytocin, vasopressin). The pineal gland secretes melatonin, regulating the sleep-wake (circadian) rhythm according to the light-dark cycle. The thyroid gland secretes T3/T4, governing basal metabolic rate and growth, and calcitonin, which lowers blood calcium; the parathyroid glands secrete PTH, which raises blood calcium by acting on bone, kidney and (via vitamin D) the intestine, directly antagonising calcitonin. The adrenal gland's cortex secretes mineralocorticoids (aldosterone, salt/water balance), glucocorticoids (cortisol, metabolism and stress) and minor sex corticoids, while its medulla secretes adrenaline and noradrenaline, mediating the rapid fight-or-flight response. The pancreas's islets of Langerhans secrete insulin (beta cells, lowers blood glucose) and glucagon (alpha cells, raises blood glucose) as an antagonistic pair, alongside somatostatin and pancreatic polypeptide. The gonads — testis (testosterone) and ovary (estrogen, progesterone) — combine gamete production with secretion of the sex hormones responsible for secondary sexual characters and reproductive cycling. …