Endocrine system and major glands overview
Homeostasis and response • Hormonal coordination in humans
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Key concepts
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Definition and principles of hormonal coordination
Hormones are chemicals secreted by glands directly into the bloodstream and carried to target organs where they produce specific effects. Hormonal signals act more slowly than nerve impulses but last longer and influence processes across the whole body. Hormonal coordination uses receptors to detect changes, glands to release hormones, and target organs to correct the change. Homeostasis commonly uses negative feedback: a change in a level triggers hormone release that moves the level back toward normal.
Pituitary gland as the 'master gland'
The pituitary gland sits below the hypothalamus in the brain and measures about the size of a pea. It secretes several hormones into the blood that control growth, water balance (ADH), and reproductive hormones (FSH, LH), and it produces thyroid-stimulating hormone (TSH) that acts on the thyroid. The pituitary therefore 'turns on' and partly controls other endocrine glands. Pituitary hormones often act on other glands: for example, TSH from the pituitary stimulates thyroxine release from the thyroid, and FSH and LH control ovarian function. The pituitary–hypothalamus connection links nervous and endocrine control.
Major endocrine glands and their positions
Major glands include the pituitary (brain), thyroid (neck), adrenal glands (above the kidneys), pancreas (behind the stomach), ovaries (lower abdomen in females) and testes (in scrotum in males). A labelled diagram shows these positions relative to the body. Each gland produces characteristic hormones: thyroid produces thyroxine; adrenal glands produce adrenaline; pancreas produces insulin and glucagon; ovaries produce oestrogen and progesterone; testes produce testosterone. Each hormone targets specific organs to produce defined effects.
Thyroxine and basal metabolic rate (higher-tier)
Thyroxine is produced by the thyroid gland and controls basal metabolic rate by regulating how quickly cells use energy and make proteins. Higher thyroxine increases metabolic rate, affecting growth and energy use across many tissues. Insufficient iodine reduces thyroxine production, which can stimulate increased TSH and cause an enlarged thyroid (goitre). Thyroxine secretion is regulated by the pituitary via TSH. The interaction of thyroid and pituitary forms a negative feedback loop that keeps thyroxine concentration near a set point.
Adrenaline and the 'fight or flight' response (higher-tier)
Adrenaline is secreted by the adrenal glands in response to stress or danger. Adrenaline increases heart rate and blood pressure, raises blood glucose by stimulating glycogen breakdown, and redirects blood to muscles to prepare for rapid physical activity. These short-term effects provide an immediate energy boost. Adrenaline supports a rapid systemic response that complements slower hormonal adjustments; it does not operate primarily by negative feedback but by immediate release in response to stimuli.
Negative feedback control for thyroxine (higher-tier)
Negative feedback for thyroxine follows a sequence: a drop in blood thyroxine is detected by the hypothalamus/pituitary, the pituitary increases TSH secretion, TSH stimulates the thyroid to release more thyroxine, thyroxine level rises and the pituitary reduces TSH release, restoring balance. The opposite sequence occurs if thyroxine is too high. Interpretation of simple diagrams requires identification of receptor (detects thyroxine level), pituitary (secretes TSH), thyroid (releases thyroxine) and the negative feedback arrow that reduces TSH when thyroxine is high.
Key notes
Important points to keep in mind