# Endocrine
## Overview
**Endocrine** describes glands and processes that secrete hormones directly into the bloodstream, without using ducts. The endocrine system — including the pituitary, thyroid, adrenal glands, and pancreas — regulates metabolism, growth, reproduction, and mood through chemical signaling.
## Etymology
The term was coined in 1914 from two Greek elements: *endon* ('within, inside') and *krinein* ('to separate, distinguish, secrete'). The compound literally means 'separating within' — that is, secreting substances into the interior of the body (the bloodstream) rather than onto external surfaces.
The term was developed to contrast with **exocrine** (*exo-* 'outside' + *krinein*), which describes glands that secrete through ducts onto body surfaces — sweat glands, salivary glands, and the portion of the pancreas that releases digestive enzymes into the gut.
## The Krinein Family
Greek *krinein* ('to separate, distinguish, decide') is one of the most intellectually productive roots in Western vocabulary. From PIE **\*krey-** ('to sieve, separate'), it generated a constellation of English words united by the concept of separation and judgment:
- **Crisis**: from *krisis* ('turning point, decision') — the moment when outcomes separate - **Criterion**: from *kriterion* ('standard for judging') — the means by which things are separated - **Critic**: from *kritikos* ('able to judge') — one who separates good from bad - **Crime**: through Latin *crimen* ('judgment, accusation, offense') from *cernere* ('to sift'), the Latin cognate of *krinein* - **Discern**: from Latin *discernere* ('to separate, distinguish') — *dis-* + *cernere* - **Secret**: from Latin *secretus* ('set apart') — past participle of *secernere* (*se-* 'apart' + *cernere*) - **Certain**: from Latin *certus* ('settled, sure') — something that has been 'sifted out' as definite
The biological sense of 'secretion' — a gland separating substances from blood and releasing them — preserves the most literal meaning of the Greek root.
## Historical Context
The concept of internal secretion predates the word. In 1855, Claude Bernard described the liver's release of glucose into the blood as an 'internal secretion.' In 1902, William Bayliss and Ernest Starling discovered secretin, the first identified hormone, demonstrating that chemical messengers travel through the blood to regulate distant organs. Starling coined the word **hormone** in 1905 (from Greek *hormān*, 'to set in motion').
The term **endocrine** was formalized as the field coalesced into a distinct medical discipline. By the 1920s, endocrinology was established as a specialty, and the endocrine system was recognized as a regulatory network comparable in importance to the nervous system.
## Endo- and Exo-
The Greek prefixes *endo-* ('within') and *exo-* ('outside') form a productive pair in scientific English: endothermic/exothermic (absorbing/releasing heat), endogenous/exogenous (arising from within/without), endoskeleton/exoskeleton (internal/external skeleton). In each case, the prefix specifies the direction of the process relative to the boundary of the system.
## Related Forms
The word family includes **endocrinology** (the medical specialty), **endocrinologist** (a specialist), and **endocrinopathy** (disease of the endocrine system). The informal shortening 'endo' is used in medical jargon.