The human endocrine system
The endocrine system secretes hormones into the bloodstream from glands throughout
the body. Hormones produce an effect on specific target organs in the body.
The endocrine system, like the nervous system, is a communication network. While
the nervous system uses electrical impulses to transmit signals between the brain
and body, the endocrine system uses chemical messengers called hormones that
travel through the circulatory system to affect target organs. So, one messenger
molecule might affect many different types of cells, all over the body.
The word endocrine comes from the Greek words endon, meaning "inside" or
"within" and "exocrine," from the Greek word krīnō, meaning "to separate or
distinguish." The body has both an endocrine system and an exocrine system to
secrete hormones. The difference between them is that the exocrine system
secretes hormones through ducts that diffuse a short distance to their target, while
the endocrine system is ductless, secreting hormones into the circulatory system for
distribution throughout the entire organism.
Textbooks cite variable numbers of endocrine glands, largely because many cells
groups can secrete hormones. The primary endocrine system glands are:
Hypothalamus
Pituitary gland
Pineal gland
Thyroid gland
Parathyroid glands
Adrenal gland
Pancreas
Ovary (in females)
Testis (in males)
However, other groups of cells may secrete hormones, including the placenta
(estrogen and progesterone) and stomach (ghrelin). Older sources may cite the
thymus as a member of the endocrine system, but it is excluded from modern texts
because it doesn't actually secrete any hormones.
Question:
How does the hypothalamus control the endocrine system?
Endocrine System:
The endocrine system is a group of gland distributed throughout the body that help regulate
many functions in our body. Some of the main functions include metabolism, growth, sexual
development, body temperature, breastfeeding, and sleep cycles.
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Human endocrine system
Hormones and nerves
A hormone is a chemical substance, produced by a gland and carried in the
bloodstream, which alters the activity of specific target organs. An example of this is the
release of the hormone adrenaline, which is released by the adrenal gland. One of its
target organs is the heart, where it increases the heart rate.
Once a hormone has been used, it is destroyed by the liver.
Hormones can control the body, and the effects are much slower than the nervous
system, but they last for longer.
Contraceptive pills contain hormones to reduce the chances of becoming pregnant
There are important differences between nervous and hormonal control.
Nervous Hormonal
Electrical (chemical at
Type of signal Chemical
synapses)
Transmission of
By nerve cells (neurones) By the bloodstream
signal
Effectors Muscles or glands Target cells in particular tissues
Muscle contraction or
Type of response Chemical change
secretion
Speed of response Very rapid Slower
Short (until nerve impulses Long (until hormone is broken
Duration of response
stop) down)
Master gland
The pituitary gland in the brain is known as a 'master gland'. It secretes several
hormones into the blood in response to the body's condition, such as blood water levels.
These hormones can also act on other glands to stimulate the release of different types
of hormones and bring about effects.
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Different hormones
The body produces a range of different chemical hormones that travel in the
bloodstream and affect a number of different organs or cells in the body. The diagram
below shows this in detail.
Important hormones released into the bloodstream include ADH (anti-diuretic hormone),
adrenaline and insulin.
Source Organ(s) Role Effects
Controlling the Increases reabsorption
Pituitary
ADH Kidneys water content of water by the
gland
of the blood collecting ducts
Several targets Increases breathing
including the rate, heart rate, flow of
Adrenal Preparation for
Adrenaline respiratory and blood to muscles,
glands 'fight or flight'
circulatory conversion of glycogen
systems to glucose
Controlling Increases conversion of
Insulin Pancreas Liver blood glucose glucose into glycogen
levels for storage
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Negative feedback systems in hormonal control
Homeostatic control
In animals, conditions such as water concentration, temperature, and glucose
concentration must be kept as constant as possible. Control systems that keep such
conditions constant are examples of homeostasis; this is the maintenance of constant
internal conditions in an organism.
A negative feedback mechanism is an important type of control that is found in
homeostasis. A negative feedback control system responds when conditions change
from the ideal or set point and returns conditions to this set point. There is a continuous
cycle of events in negative feedback.
General stages in negative feedback
In general this works by:
if the level of something rises, control systems reduce it again
if the level of something falls, control systems raise it again
Negative feedback flowchart
An example of negative feedback is the control of body temperature. Body temperature
is controlled by the hypothalamus in your brain, and if your body gets too hot, your body
begins to sweat to try and reduce it. Conversely if the body gets too cold, it begins to
shiver to try and raise the temperature.
