endocrine system
Meaning of endocrine system: Ductless glands drop their messengers straight into the blood, so a signal made in one place acts on organs far away.
Definition of endocrine system
The endocrine system is the network of glands and scattered cells that make hormones and release them directly into the bloodstream, where they travel to act on tissues elsewhere in the body. Its main members are the pituitary, thyroid, parathyroid and adrenal glands, the pancreas, and the ovaries or testes, coordinated largely from the brain.
How does it differ from the nervous system?
Both carry information around the body, and they work as a pair rather than in competition. The hypothalamus, a small region of the brain, sits at the junction and converts nerve signals into hormone signals.
| Feature | Endocrine | Nervous |
|---|---|---|
| Signal | Hormone released into the blood | Electrical impulse along a nerve fiber |
| Route | The circulation, reaching everywhere | A wired path to one destination |
| Speed | Seconds to hours | Immediate |
| How long the effect lasts | Minutes to years | Brief |
A hormone reaches every tissue the blood reaches, yet only cells carrying the matching receptor respond, which is how a message broadcast to the whole body produces a specific effect in one organ. That difference is also why the nervous system is suited to pulling a hand off a hot surface and the endocrine system to running puberty.
What does it control?
Growth and sexual development, metabolism and the use of food, the balance of salt and water, the cycle of sleep and waking, appetite, the response to stress and the maintenance of pregnancy all run on hormones. The classic glands are not the whole system either. Fat tissue, the heart, the kidneys, the lining of the gut and the placenta all release hormones, and the pancreas does two jobs at once, sending digestive enzymes down a duct while its islets release insulin and glucagon straight into the blood. That duct is the dividing line: an endocrine gland has none, whereas an exocrine gland such as a sweat or salivary gland delivers its product through one.
How is hormone output kept in check?
Mostly by feedback. The hypothalamus signals the pituitary, the pituitary signals a target gland, and the hormone that gland releases travels back and quiets both, so levels settle within a range instead of climbing without limit. Blood testing exploits this arrangement. Measuring the pituitary signal alongside the hormone itself shows where a fault lies: a low thyroid hormone with a high signal points at the thyroid, while a low hormone with a low signal points higher up, at the pituitary. The pituitary is often called the master gland, which overstates its independence, since it works largely on instructions from the hypothalamus above it.
Used in a sentence
Blood tests were ordered to evaluate the function of several endocrine system glands.
Good to know
Hormonal signaling through the endocrine system is generally slower and more sustained than the electrical signaling of the nervous system, so its effects tend to develop and resolve more gradually.
Word origin
Where “endocrine system” comes from
Formed from Greek endon, meaning within, and krinein, meaning to separate or secrete, reflecting how these glands release hormones internally into the bloodstream.