Encyclopedia of Health and Medicine

E–F
endocrine gland A structure, sometimes called a
ductless gland, within the body that produces
chemicals, called hormones, it secretes directly
into the bloodstream. Hormones influence the
function of cells that contain receptors for them.
The PITUITARY GLAND, ADRENAL GLANDS, and THYROID
GLAND are examples of endocrine glands. An
exocrine gland, by contrast, secretes the chemicals
it produces into ducts (specialized channels) for
release into body structures. The SALIVARY GLANDS
and SWEAT GLANDS are examples of exocrine glands.
The KIDNEYS, gastrointestinal tract, and PLACENTA in
a pregnant woman also contain endocrine structures.
Three mechanisms can trigger endocrine activity.
They are
• humoral, in which the endocrine system
responds to chemicals in the bloodstream such
as calcium (triggering CALCITONIN release from
the thyroid gland or PARATHYROID HORMONE from
the PARATHYROID GLANDS)
• hormonal, in which the hormones from one
endocrine gland direct activity from other
endocrine glands such as the STRESS RESPONSE
HORMONAL CASCADE
• neurologic, in which NERVE impulses stimulate
endocrine action such as from the HYPOTHALA-
MUS to the posterior lobe of the pituitary gland
Sometimes the neurologic system and the
endocrine system secrete the same chemicals,
such as EPINEPHRINE and NOREPINEPHRINE. When
endocrine structures synthesize these chemicals,
they are hormones and they travel to their target
cells through the blood circulation. When the
neurologic system synthesizes these structures,
they are neurotransmitters, and they travel to
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their target cells through interstitial fluid (fluid
between cells). Neurotransmitters travel to their
destinations, elicit reactions, and dissipate more
rapidly than hormones.
THE ENDOCRINE GLANDS
ADRENAL GLANDS HYPOTHALAMUS ISLETS OF LANGERHANS
OVARIES PARATHYROID GLANDS PINEAL GLAND
PITUITARY GLAND PLACENTA TESTES
THYMUS
THYROID GLAND
For further discussion of the endocrine glands
within the context of the endocrine system’s
structure and function please see the overview
section “The Endocrine System.”
See also DIGESTIVE HORMONES; NEUROTRANSMITTER.
epinephrine A chemical the adrenal medulla of
the ADRENAL GLANDS and the synaptic vesicles of
the NERVE endings produce. Epinephrine, also
called adrenaline, functions in the body as a peptide
HORMONE when synthesized by the adrenal
medulla and as a NEUROTRANSMITTER when synthesized
in the BRAIN or nerve endings. Among the
hormones activated in the STRESS RESPONSE HOR-
MONAL CASCADE, epinephrine
• constricts peripheral BLOOD vessels to centralize
blood flow and raise BLOOD PRESSURE
• dilates bronchial structures in the LUNGS to
increase air flow
• initiates rapid conversion of glycogen to GLU-
COSE in the LIVER to raise the blood glucose level
and increase energy to the cells
• intensifies the HEART’s contractions to increase
CARDIAC OUTPUT (the amount of blood the heart
pumps out with each CARDIAC CYCLE)
• accelerates the HEART RATE