CoachA hormone is a molecule secreted into the blood by a gland and acting, at a distance, on the cells that carry a receptor for it. A hormonal system is regulated by feedback: the hormone produced at the end of a chain acts back on the glands at its start. Negative feedback — the product reduces the command — holds a level constant; positive feedback — the product increases the command — produces a surge.
Examples
Example 20.8 (Reading the pill’s effect)
On the chart of a woman taking the pill, the four curves are flat: oestrogens and progesterone at the low, steady level the pills supply, LH and FSH below their cycle minimum, no peak anywhere. Miss the pills for a few days early in the pack and FSH escapes: a follicle may start, and the rest can follow. The pill works because the feedback works, and a forgotten pill is a hole in the feedback.