The Endocrine System, Overview, Animation

the endocrine system is one of the two
systems that are responsible for communication and integration between
various body tissues the other being the nervous system endocrine communication
is achieved by means of chemical messengers called hormones hormones are
produced in endocrine glands and secreted into the bloodstream to reach
body tissues a hormone can travel wherever the blood goes but it can only
affect cells that have receptors for it these are called target cells there are
two major types of hormones steroid hormones derived from cholesterol and
are lipid soluble and non steroid hormones derived from peptides or amino
acids and are water-soluble lipid soluble steroid hormones can cross the
cell membrane to bind to their receptors inside the cells either in the cytoplasm
or nucleus steroid hormone receptors are typically transcription factors upon
forming the hormone receptor complex binds to specific DNA sequences to
regulate gene expression and thus mediating cellular response on the other
hand water-soluble non steroid hormones are unable to cross the lipid membrane
and therefore must bind to receptors located on the surface of the cell the
binding triggers a cascade of events that leads to production of cAMP, a
second messenger that is responsible for cellular response to hormone it does so
by changing enzyme activity or ion channel permeability major endocrine
glands include the hypothalamus pituitary gland pineal gland thyroid and
parathyroid glands thymus adrenal gland islets of the pancreas and testes in men
or ovaries in women the endocrine system also includes
hormone secreting cells from other organs such as kidneys and intestines
except for the hypothalamus and the pituitary different endocrine glands are
involved in different more or less independent processes for example the
pancreas produces insulin and glucagon that keep blood sugar levels in check
the parathyroid glands produce hormones that regulate calcium and phosphorus
thyroid hormones control metabolic rates while the ovaries and testes are
involved in reproductive functions on the other hand the hypothalamus and
pituitary gland play a more central integrative role the hypothalamus is
also part of the brain it secretes several hormones called neuro hormones
which control the production of other hormones by the pituitary thus the
hypothalamus links the nervous system to the endocrine system the pituitary is
known as the master gland because it controls the functions of many other
endocrine glands a major role of the endocrine system is
to maintain the body’s stable internal conditions or homeostasis such as blood
sugar levels or serum calcium levels to do this it utilizes negative feedback
mechanisms which work very much like a thermostat the heater is on when the
temperature is low off when it’s high for example when blood glucose level is
high such as after a meal glucose induces insulin release from the
pancreas insulin helps body cells consume glucose clearing it from the
blood low blood glucose can no longer act on the pancreas which now stops
releasing insulin another example is the regulation of thyroid hormone levels
which are induced by a pituitary hormone called thyroid stimulating hormone TSH
TSH in turn is under control of thyrotropin-releasing hormone trh from
the hypothalamus when thyroid hormone levels are too high they suppress the
secretion of TSH and trh consequently inhibiting their own production


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