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 2 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 cell, 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 intestine. 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 hormones 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.