endocrine gland
Summary
An endocrine gland is a ductless gland that secretes hormones into the surrounding fluid and blood, which carry them to distant target cells. Only cells with the matching receptor respond, so target specificity depends on receptors, not on delivery.
Detail
Meaning: Endocrine glands release hormones by exocytosis or diffusion into interstitial fluid, and the hormones then enter nearby capillaries. Examples are the pituitary, thyroid, parathyroids, adrenals, pineal, gonads and the pancreatic islets. Exocrine glands (sweat, salivary, digestive) instead send secretions through ducts to a surface or lumen. Hormones act on target cells that carry the right receptor. Peptide hormones are water soluble and bind surface receptors, often triggering second messengers. Steroid hormones are lipid soluble, bind intracellular receptors, and usually change gene transcription.
On the MCAT: Passages usually describe a feedback axis, such as hypothalamus → anterior pituitary → target gland, and ask you to predict hormone levels after a gland is removed, overactive or blocked. Negative feedback is the usual pattern: the final hormone inhibits the earlier steps. You may also be asked to classify a hormone as peptide or steroid and to predict how fast or how long it acts.
Common confusion: Students label an organ as only endocrine or only exocrine. The pancreas is both: its islets secrete insulin and glucagon into blood, while its acinar cells send digestive enzymes through a duct. Also, the posterior pituitary does not make its hormones. It stores and releases oxytocin and ADH made by hypothalamic neurons.
Sources
- Campbell Biology
- Principles of Anatomy and Physiology
- Human Physiology: An Integrated Approach
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.