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