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Ireland 3e Chapte Podcast, Chapter 17
The Endocrine System
The endocrine system is composed of a series of glands, connected by
the cardiovascular system. Endocrine glands secrete hormones onto
their surface, rather than through ducts like exocrine glands.
Hormones are chemical messengers that allow the glands of the
endocrine system to communicate with other parts of the body. The
cells that hormones act upon are called target cells. One hormone
may have a series of different target cells, causing different but related
effects in each target cell.
The two main classes of hormones are steroid hormones, which are
structurally related to cholesterol and nonsteroid hormones which are
mainly composed of amino acids. Depending on the type of hormone,
its action on the target cell is slightly different. The receptors for
steroid hormones lie within or near the DNA, and the hormone either
causes an increase or decrease in transcription of a gene.
These hormones cannot pass the cell membrane, therefore they are
picked up by receptors on the cell’s surface. Binding the hormone on
the surface begins a cascade of activity that results in the modification
of a protein already present in the cell cytosol.
The pituitary gland lies below the hypothalamus in the sella turcica of
the sphenoid bone, and releases hormones that activate other
endocrine glands. The hypothalamus releases factors that stimulate
the pituitary gland. The posterior pituitary gland secretes two
hormones, oxytocin and antidiuretic hormone. These are actually
produced by neurons found in the hypothalamus whose axons extend
into the posterior pituitary. The anterior pituitary is glandular in
nature and secretes seven hormones. Over- or under-secretion of any
of these hormones causes odd growth patterns or ion imbalances
resulting in impaired functioning.
Each of the endocrine glands secretes specialized hormones in
response to drifting blood chemistry levels that threaten homeostasis.
The adrenal glands secrete glucocorticoids, mineralocorticoids and
androgens in response to ACTH released from the anterior pituitary
gland. ACTH is adrenocorticotropic hormone. The pancreas performs
both endocrine and exocrine functions. The islets of Langerhans
secrete insulin and glucagons, the hormones that are responsible for
maintaining blood glucose levels. Basal metabolic rate is governed by
the thyroid gland. If it is not working properly, a goiter or
exopthalmos and other signs of Grave’s disease may develop. Four
small parathyroid glands lie buried, behind the thyroid gland. These
glands are responsible for maintaining blood calcium levels. There are
other endocrine organs in the body as well. The heart, kidneys and
stomach produce hormones, as does the pineal gland in the brain.
The development of the body is under the control of the endocrine
system. Humans pass through recognized stages as they mature.
Newborns, or neonates, are totally dependent on adults. The majority
of their time is spent sleeping and gaining weight. Infancy is the stage
from 2 to 15 months. During this time, body proportions begin to
shift, teeth erupt, eating and sleep patterns begin to match the rest of
the family, and the immune system begins to function. The years
from age 2 to approximately age 12 are referred to as childhood.
Growth occurs, muscle coordination improves, language skills develop,
and socialization patterns appear. Once reproductive hormones begin
to be produced by the anterior pituitary, puberty has begun. This is a
relatively short stage of concentrated growth and development of the
reproductive organs. Adulthood follows puberty, lasting until
senescence and death. The inevitable result of reaching adulthood is
aging.