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, andTHYROID
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
andSWEAT GLANDSare examples of exocrine glands.
The KIDNEYS, gastrointestinal tract, and PLACENTAin
a pregnant woman also contain endocrine struc-
tures.
Three mechanisms can trigger endocrine activ-
ity. They are

• humoral, in which the endocrine system
  responds to chemicals in the bloodstream such
  as calcium (triggering CALCITONINrelease from
  the thyroid gland or PARATHYROID HORMONEfrom
  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 NERVEimpulses stimulate
  endocrine action such as from the HYPOTHALA-
  MUSto 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

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 OFLANGERHANS

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 GLANDSand the synaptic vesicles of

the NERVE endings produce. Epinephrine, also

called adrenaline, functions in the body as a pep-

tide HORMONEwhen synthesized by the adrenal

medulla and as a NEUROTRANSMITTERwhen synthe-

sized in the BRAINor nerve endings. Among the

hormones activated in the STRESS RESPONSE HOR-

MONAL CASCADE, epinephrine

• constricts peripheral BLOODvessels 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-
  COSEin the LIVERto 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

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