Chemical Coordination and Integration: The Endocrine System

Neural Control and Coordination already covers the nervous system's own, faster, electrical-and-chemical signalling route. Chemical coordination via hormones works on the same basic principle, a signal produced in one place acting on a distant target, but travels through the bloodstream instead, trading speed for a broader, longer-lasting reach.

The pituitary: two lobes with two different relationships to the hypothalamus

The pituitary gland, sitting just beneath the hypothalamus, is split into two lobes with genuinely different relationships to the hypothalamus controlling them. The anterior pituitary actually synthesises its own hormones, growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinising hormone (LH) and prolactin, under the hypothalamus's own regulatory control, delivered via a dedicated portal blood vessel system connecting the two. The posterior pituitary, by contrast, does not manufacture its own hormones at all: oxytocin and ADH (vasopressin), already covered in this site's Excretory Products note for ADH's specific role in water reabsorption, are actually synthesised in the hypothalamus itself and merely transported down a direct nerve tract to be stored in, and released from, the posterior pituitary, which functions as a release site rather than a true production site for either hormone.

Thyroid and parathyroid: opposing control of blood calcium

The thyroid gland produces thyroxine (requiring iodine as a raw material), which regulates the body's basal metabolic rate, and calcitonin, which lowers blood calcium levels when they run too high. The parathyroid glands produce parathyroid hormone (PTH), which does the exact opposite, raising blood calcium levels when they run too low, most directly by promoting the release of calcium stored in bone. Calcitonin and PTH thus form a genuine push-pull pair regulating blood calcium from opposite directions, the same kind of antagonistic-hormone-pair logic that recurs across the endocrine system.

Adrenal glands: cortex and medulla, two glands in one

The adrenal gland is functionally two distinct glands fused together. The outer adrenal cortex produces steroid hormones including cortisol (a stress and metabolism-regulating hormone) and aldosterone, already covered in this site's Excretory Products note as the RAAS hormone that promotes sodium and water reabsorption. The inner adrenal medulla produces adrenaline (epinephrine) and noradrenaline (norepinephrine), the hormones behind the rapid "fight or flight" response, raising heart rate and redirecting blood flow to muscle in response to acute stress or danger.

Pancreas: a single gland with two opposing hormones

The pancreas's hormone-producing cells, clustered in the islets of Langerhans, contain two distinct cell types producing directly opposing hormones. Beta cells produce insulin, which lowers blood glucose by promoting its uptake into cells. Alpha cells produce glucagon, which raises blood glucose by promoting the breakdown of stored glycogen, another antagonistic hormone pair maintaining blood glucose within a narrow working range from both directions at once.

How hormones actually act: two different mechanisms by chemical type

Hormones act on their target cells through one of two fundamentally different mechanisms, depending on the hormone's own chemical nature. Protein and peptide hormones (such as ACTH, TSH, FSH and LH), unable to cross the target cell's membrane directly, instead bind to a receptor on the cell surface, which triggers a second messenger system inside the cell, commonly involving cyclic AMP (cAMP), generated by the enzyme adenylyl cyclase, which then activates a cascade of further proteins inside the cell to produce the hormone's actual effect. Steroid hormones (such as cortisol and the sex hormones), being lipid-soluble, instead cross the cell membrane directly and bind a receptor inside the cell itself, with the resulting hormone-receptor complex acting directly on the cell's DNA to switch specific genes on or off, a fundamentally different, more direct route to the same broad goal of altering the target cell's behaviour.

Quick revision points

• Anterior pituitary: synthesises its own hormones (GH, TSH, ACTH, FSH, LH, prolactin) under hypothalamic control via a portal blood system. Posterior pituitary: does not make its own hormones; oxytocin and ADH are made in the hypothalamus and merely stored/released from the posterior pituitary via a direct nerve tract.
• Thyroid: thyroxine (needs iodine, regulates metabolic rate) and calcitonin (lowers blood calcium). Parathyroid: PTH (raises blood calcium) is calcitonin's direct functional opposite.
• Adrenal cortex: cortisol (stress/metabolism), aldosterone (sodium/water reabsorption, part of RAAS). Adrenal medulla: adrenaline and noradrenaline (fight-or-flight response).
• Pancreas (islets of Langerhans): beta cells produce insulin (lowers blood glucose), alpha cells produce glucagon (raises blood glucose), another antagonistic pair.
• Hormone action mechanisms: protein/peptide hormones use a cell-surface receptor and a second messenger system (e.g. cAMP); steroid hormones are lipid-soluble, cross the membrane directly, and act via an intracellular receptor directly on gene expression.