CoachA gland whose product is released into the blood (rather than down a duct, like saliva’s) is an endocrine gland, and its products are the hormones. The main writers: the brain-base control gland — puberty’s starter, and the conductor of most of the others; the thyroid in the neck — its hormone sets the whole body’s running pace; the adrenal caps on the kidneys — the alarm hormone of frights and sprints; the pancreas — its hormone manages the blood’s glucose; and the ovaries and testes — the cycle and rebuild hormones of Chapter 57.
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Example 61.3 (One messenger’s tour: the alarm hormone)
A near-miss on the bicycle: the adrenal glands dump their alarm hormone into the blood, and within seconds — one circulation lap (Example 27.2) — every fitted receiver responds at once: the heart’s pace jumps, the airways widen, the liver releases banked glucose (Example 50.9’s bank, called), muscle vessels widen and the digestive tract’s narrow (Proposition 49.4’s rerouting, hormone-held), and the skin goes pale and prickling. One letter, six answers, one coordinated state: ready.
Example 61.5 (When feedback fails: diabetes)
In diabetes, the storage-hormone signal fails — the pancreas stops writing it, or the receivers stop answering. Glucose piles up in the blood after every meal; the kidney’s reclaiming is overwhelmed and sugar appears in the urine — Example 53.5’s classic signal, explained at last. Treatment is applied chapter: measured doses of the missing hormone, timed to meals — the letter written by syringe. Millions live well on exactly this arrangement.
Example 61.8 (The method on the thyroid’s pace-setter)
Writer: the thyroid, conducted by the control gland. Receivers: nearly every cell — the hormone sets the rate of Proposition 46.5’s slow fire. Feedback: the control gland reads the level and trims its conducting. Failure: too little, and the whole engine idles — tiredness, cold, slowed everything; too much, and it races. Use: the missing hormone is a daily tablet — among medicine’s oldest letters-by-post.