In short
A hormone is a chemical messenger made by a gland and carried by the blood to a target organ, where it changes how that organ works.
Hormones work more slowly than nerve impulses, but their effects last longer. Insulin from the pancreas lowers blood sugar. Adrenaline from the adrenal glands prepares the body for danger. Thyroxine from the thyroid controls the rate of metabolism, and it needs iodine. In plants, hormones such as auxin control growth towards light. A fall or rise of a hormone is corrected by negative feedback.
After a meal, blood glucose rises and the pancreas releases insulin. This makes the liver and muscles take up glucose, and the blood level falls.
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