endocrine
Meaning of endocrine: Signals released straight into the bloodstream reach every tissue, so a gland the size of a pea can change what the whole body does.
Definition of endocrine
Endocrine describes tissue that makes hormones and releases them directly into the blood, with no duct to carry them anywhere. Because the bloodstream reaches everywhere, a hormone released this way can act on organs far from where it was made. The endocrine tissues include the pituitary, thyroid, parathyroid and adrenal glands, the ovaries and testes, and cell clusters within the pancreas.
How does endocrine signaling differ from other kinds?
| Type | Where the signal goes | Example |
|---|---|---|
| Endocrine | Into the blood, acting on distant organs | Thyroid hormone released into the circulation |
| Exocrine | Through a duct onto a surface or into a cavity | Digestive enzymes delivered into the intestine |
| Paracrine | To neighboring cells, without entering the circulation | Local signals coordinating cells within one tissue |
| Autocrine | Back onto the cell that released it | A cell adjusting its own activity |
The endocrine route is slow and broad. A hormone takes seconds to minutes to reach its targets and its effects can last hours or longer, which suits it to tasks such as growth, metabolism and reproduction. Nerve signaling covers the opposite ground, arriving in thousandths of a second and stopping just as fast. Only cells carrying the matching receptor respond to a given hormone, and that is how a signal broadcast to the entire body produces a specific effect in a few places.
Can one organ do both?
The pancreas is the standard example and does both at once using separate tissue. Most of it is exocrine, producing digestive enzymes that travel down a duct into the digestive tract. Scattered through it are small islands of endocrine cells releasing insulin and glucagon straight into passing blood vessels. Damage to one part does not necessarily affect the other, which is why a person can lose digestive enzyme production while insulin release continues, or the reverse. The ovaries and testes work along similar lines, releasing hormones into the blood while producing eggs and sperm that leave by an entirely different route.
How is the output controlled?
Almost every endocrine gland sits inside a feedback loop. The hypothalamus signals the pituitary, the pituitary signals a target gland such as the thyroid or the adrenal cortex, and the hormone that gland produces circulates back and damps the two levels above it. Output therefore holds within a range without anything having to monitor it deliberately. The loop also explains what a pair of blood tests can reveal: a low hormone with a raised pituitary signal points to failure of the gland itself, while a low hormone with a low pituitary signal points higher up the chain. Disorders across the endocrine system come down to too much hormone, too little, resistance to a hormone present in normal amounts, or a growth in a gland that either floods the body with hormone or produces none at all.
Used in a sentence
Endocrine testing was ordered to evaluate the function of the patient's thyroid and adrenal glands.
Good to know
The pancreas contains both endocrine tissue, which releases insulin directly into the blood, and exocrine tissue, which releases digestive enzymes through a duct into the intestine, so the same organ performs two distinct kinds of secretion.
Word origin
Where “endocrine” comes from
Formed from Greek endo-, meaning within, and krinein, meaning to separate or secrete.