Two kinds of gland
A gland is simply an organ whose job is secretion — making and releasing a substance. The key split is which way it points. An exocrine gland secretes outward through a duct: sweat onto the skin, saliva into the mouth, digestive juice into the gut. An endocrine gland has no duct at all; it secretes inward, releasing its hormone directly into the blood that flows past it.
All the endocrine glands together — and the scattered hormone-making cells that are not organized into obvious organs — make up the endocrine system. Later guides will tour the named glands (pituitary, thyroid, adrenals, pancreas, gonads). For now, hold onto the simple picture: ductless senders dropping messages into the river of the blood.
The receptor decides who listens
We said in guide one that the blood reaches all cells but only target cells respond. The reason is the receptor — a protein shaped to grip one particular hormone. A target cell is, by definition, any cell that carries the receptor for a given hormone. No receptor, no response. This is why one hormone can leave most of the body untouched while transforming a chosen few tissues.
Receptors sit in one of two places, and the location depends on the hormone's chemistry. A water-loving (hydrophilic) hormone cannot slip through the oily cell membrane, so it must knock from outside — it binds a cell-surface receptor and triggers signals indoors without ever entering. A fat-loving (lipophilic) hormone dissolves straight through the membrane and binds an intracellular receptor inside the cell, often acting directly on the genes.
Putting the pieces together
- An endocrine gland makes a hormone and releases it, ductless, into the blood.
- The blood carries the hormone past every cell in the body.
- Only target cells — those carrying the matching receptor — bind it.
- Where the receptor sits (surface or inside) shapes how fast and how long the cell responds.