glands

A single drop of blood carries signals from the brain to the pancreas, the adrenal glands, the thyroid, and the gonads — all simultaneously. Hormones travel in concentrations so tiny that a teaspoon of testosterone spread across 500 Olympic swimming pools would still be detectable by a target cell. Here is how the endocrine system orchestrates the body without a single wire.

A single nanogram of adrenaline — one-billionth of a gram — is enough to trigger a full fight-or-flight response. Hormones are the body's most powerful chemical messengers.