system helps regulate and maintain various body functions by synthesizing
(making) and releasing hormones, chemical messengers. The major
areas of control and integration include responses to stress and
injury, growth and development, absorption of nutrients, energy
metabolism, water and electrolyte balance, reproduction, birth,
and lactation. The endocrine system is composed of glands that release
their hormones directly into the bloodstream for chemical signaling
of target cells. These glands include the pituitary gland, the pineal
gland, the hypothalamus, the thyroid gland, the parathyroid glands,
the thymus, the adrenal glands, the ovaries (in females) or testes
(in males), and the pancreas.
body synthesizes hormones in one part and transports it to another
through the bloodstream or lymph. Endocrine glands have a rich blood
supply through which hormones travel to reach their target organs.
Hormones alter the metabolism of target organs by increasing or
decreasing their activity. These changes in activity are strictly
balanced to maintain homeostasis (a stable internal environment).
and neural components
Glands are of
two types. Endocrine glands do not have a duct system and are called
ductless glands. These glands release hormones directly into the
blood or lymph. Exocrine glands such as the sudoriferous (sweat)
glands contain ducts. Ducts are tubes leading from a gland to its
system and the nervous system are so closely associated that they
are collectively called the neuroendocrine system. Neural control
centers in the brain control endocrine glands. The main neural control
center is the hypothalamus, also known as the "master switchboard."
Suspended from the hypothalamus by a thin stalk is the pituitary
gland. The hypothalamus sends messages to the pituitary gland; the
pituitary gland, in turn, releases hormones that regulate body functions.
glands are linked to neural control centers by homeostatic feedback
mechanisms. The two types of feedback mechanisms are negative feedback
and positive feedback. Negative feedback decreases the deviation
from an ideal normal value, and is important in maintaining homeostasis.
Most endocrine glands are under the control of negative feedback
mechanisms act like a thermostat in the home. As the temperature
rises (deviation from the ideal normal value), the thermostat detects
the change and triggers the air-conditioning to turn on and cool
the house. Once the temperature reaches its thermostat setting (ideal
normal value), the air conditioning turns off.
An example of
negative feedback is the regulation of the blood calcium level.
The parathyroid glands secrete parathyroid hormone, which regulates
the blood calcium amount. If calcium decreases, the parathyroid
glands sense the decrease and secrete more parathyroid hormone.
The parathyroid hormone stimulates calcium release from the bones
and increases the calcium uptake into the bloodstream from the collecting
tubules in the kidneys. Conversely, if blood calcium increases too
much, the parathyroid glands reduce parathyroid hormone production.
Both responses are examples of negative feedback because in both
cases the effects are negative (opposite) to the stimulus.
mechanisms control self-perpetuating events that can be out of control
and do not require continuous adjustment. In positive feedback mechanisms,
the original stimulus is promoted rather than negated. Positive
feedback increases the deviation from an ideal normal value. Unlike
negative feedback that maintains hormone levels within narrow ranges,
positive feedback is rarely used to maintain homeostatic functions.
An example of
positive feedback can be found in childbirth. The hormone oxytocin
stimulates and enhances labor contractions. As the baby moves toward
the vagina (birth canal), pressure receptors within the cervix (muscular
outlet of uterus) send messages to the brain to produce oxytocin.
Oxytocin travels to the uterus through the bloodstream, stimulating
the muscles in the uterine wall to contract stronger (increase of
ideal normal value). The contractions intensify and increase until
the baby is outside the birth canal. When the stimulus to the pressure
receptors ends, oxytocin production stops and labor contractions
pituitary gland is called the "master gland" because it regulates
many key functions. The pituitary gland has an adenohypophysis (anterior
lobe) and a neurohypophysis (posterior lobe). The adenohypophysis
produces and secretes seven hormones in response to commands from
- Thyroid Stimulating hormone (TSH)
- Adrenocorticotropic hormone (ACTH)
- Follicle Stimulating hormone (FSH)
- Luteinizing hormone (LH)
- Prolactin (PRL)
- Growth hormone (GH)
- Melanocyte-stimulating hormone (MSH)
The TSH, ACTH,
FSH, and LH hormones are tropic hormones that simulate other endocrine
glands. In response, the other endocrine glands produce hormones
that affect metabolism. For example, TSH from the pituitary gland
stimulates the thyroid gland to produce thyroid hormones. In turn,
thyroid hormones inhibit the release of calcium in the blood.
hormones have unique effects upon metabolism. ACTH acts upon the
cortex (outer area) of the adrenal gland to produce steroid hormones.
FSH and LH act upon women and men in regulating various sexual characteristics.
growth hormone act upon certain body tissues; they do not affect
specific organs. Prolactin travels to the breast tissue glands of
nursing mothers, causing milk production. Growth hormone stimulates
protein synthesis and cell division in cartilage and bone tissue.
Gigantism results when excessive amounts of growth hormone are produced
during childhood. Pituitary dwarfism occurs when too little growth
hormone is produced. Acromegaly occurs when too much GH is produced
bodies of the hypothalamus produce two hormones: antidiuretic hormone
(ADH) and oxytocin. These hormones are transported along the axons
to the axon terminals in the pituitary posterior lobe. Both hormones
are stored in the terminals until they are released into the blood
vascular network surrounding the posterior pituitary gland.
ADH acts upon
the kidney tubules to help maintain a constant level of body water.
This level is accomplished by increasing the water reabsorption
amount when body water levels are low. Oxytocin triggers milk release
from breast tissue when infants nurse and causes muscle contractions
in the uterus during labor.
gland has two lobes connected by an isthmus (small connecting stalk)
and is in the lower part of the neck just below the larynx. The
thyroid gland produces three hormones:
- Thyroxine (T4)
- Triiodothyronine (T3)
T3 and T4 are
collectively called thyroid hormone and are produced in the follicles
(hollow spherical structures) of the thyroid gland. Thyroid hormone
affects body growth, metabolic rates, and the development of bones
and skeletal muscle. Thyroid hormone also increases the sensitivity
of the cardiovascular system to sympathetic nervous activity. This
effect helps maintain a normal heart rate.
cells (C cells) between the thyroid gland follicles produce calcitonin.
Calcitonin lowers blood calcium levels.
glands are embedded in back of the thyroid gland and secrete PTH
(parathyroid hormone). PTH increases blood calcium by stimulating
bone calcium release into the bloodstream and by increasing the
calcium absorption rate in the gastrointestinal tract and kidneys.
glands are on top of each kidney. Each gland has a cortex (outer
region) and a medulla (inner region). The cortex secretes glucocorticoids
such as cortisol, mineralocorticoids, and small amounts of androgens
and estrogens responsible for some secondary sex characteristics.
Glucocorticoids raise blood sugar levels by increasing gluconeogenesis
(synthesis of glucose from amino acid). This action ensures glucose
supplies for the body when it is under stress. Mineralocorticoids
such as aldosterone promote sodium (salt) reabsorption by stimulating
the kidneys to absorb more sodium from the blood.
"emergency gland" develops from nervous tissue; the autonomic nervous
system controls its secretions. The medulla secretes epinephrine
(adrenaline) and norepinephrine (noradrenaline), chemicals that
raise the blood levels of sugar and fatty acids. These hormones
also increase the heart rate and force of contraction. These effects
prepare the body for the "Fight or Flight" response (instant physical
activity), enabling the individual to think quicker, fight harder,
and run faster. These hormones also constrict the blood vessels
supplying the skin, kidneys, gastrointestinal tract, and other areas
of the body not needed for the response.
The ovary is
the site of estrogen and progesterone synthesis. Estrogen is required
to form the ovum (egg) during oogenesis and prepares the uterus
for implanting a fertilized egg. Progesterone prepares the breasts
for lactation during pregnancy and works with estrogen to regulate
the menstrual cycle.
The testes produce
the hormone testosterone. Testosterone is required for sperm formation
during spermatogenesis, the development of male external genitalia,
and secondary sexual traits such as beard growth, chest hair, and
enlarged thyroid cartilage.