What Is the Endocrine System?
Hormones transfer information and instructions from one set of cells to another. Although many different hormones circulate throughout the bloodstream, each one affects only the cells that are genetically programmed to receive and respond to its message. Hormone levels can be influenced by factors such as stress, infection, and changes in the balance of fluid and minerals in blood.
The foundations of the endocrine system are the hormones and glands.
As the body's chemical messengers, hormones transfer information and
instructions from one set of cells to another. Although many different hormones
circulate throughout the bloodstream, each one affects only the cells that are
genetically programmed to receive and respond to its message. Hormone levels can
be influenced by factors such as stress, infection, and changes in the balance
of fluid and minerals in blood.
A gland is a group of cells that produces and secretes, or gives off,
chemicals. A gland selects and removes materials from the blood, processes them,
and secretes the finished chemical product for use somewhere in the body. Some
types of glands release their secretions in specific areas. For instance,
exocrine (pronounced: ek-suh-krin) glands, such as the sweat and salivary
glands, release secretions in the skin or inside of the mouth. Endocrine glands,
on the other hand, release more than 20 major hormones directly into the
bloodstream where they can be transported to cells in other parts of the body.
Glands
The major glands that make up the human endocrine system are the:
The pancreas is also part of this hormone-secreting system, even though it is also associated with the digestive system because it also produces and secretes digestive enzymes. Although the endocrine glands are the body's main hormone producers, some non-endocrine organs - such as the brain, heart, lungs, kidneys, liver, thymus, skin, and placenta - also produce and release hormones.
What Does the Endocrine System Do?
Once a hormone is secreted, it travels from the endocrine gland through the
bloodstream to the cells designed to receive its message. These cells are called
target cells. Along the way to the target cells, special proteins bind to some
of the hormones. The special proteins act as carriers that control the amount of
hormone that is available to interact with and affect the target cells. Also,
the target cells have receptors that latch onto only specific hormones, and each
hormone has its own receptor, so that each hormone will communicate only with
specific target cells that possess receptors for that hormone. When the hormone
reaches its target cell, it locks onto the cell's specific receptors and these
hormone-receptor combinations transmit chemical instructions to the inner
workings of the cell.
When hormone levels reach a certain normal or necessary amount, further
secretion is controlled by important body mechanisms to maintain that level of
hormone in the blood. This regulation of hormone secretion may involve the
hormone itself or another substance in the blood related to the hormone. For
example, if the thyroid gland has secreted adequate amounts of thyroid hormones
into the blood, the pituitary gland senses the normal levels of thyroid hormone
in the bloodstream and adjusts its release of thyrotropin, the pituitary hormone
that stimulates the thyroid gland to produce thyroid hormones. Another example
is parathyroid hormone, which increases the level of calcium in the blood. When
the blood calcium level rises, the parathyroid glands sense the change and
decrease their secretion of parathyroid hormone. This turnoff process is called
a negative feedback system.
Things That Can Go Wrong With the Endocrine System
Type 1 diabetes. When the pancreas fails to produce
enough insulin, type 1 diabetes (pronounced: dy-uh-be-teez and previously known
as juvenile diabetes) occurs. Symptoms include excessive thirst, hunger,
urination, and weight loss. In children and teens, the condition is usually an
autoimmune disorder in which specific immune system cells and antibodies
produced by the child's immune system attack and destroy the cells of the
pancreas that produce insulin. The disease can cause long-term complications
including kidney problems, nerve damage, blindness, and early coronary heart
disease and stroke.
Type 2 diabetes. Unlike type 1 diabetes, in which
the body can't produce normal amounts of insulin, in type 2 diabetes the body is
unable to respond to insulin normally. Children and teens with the condition
tend to be overweight, and it is believed that excess body fat plays a role in
the insulin resistance that characterizes the disease. In fact, the rising
prevalence of this type of diabetes in children has paralleled the dramatically
increasing rates of obesity among children and teens in recent years. The
symptoms and possible complications of type 2 diabetes are basically the same as
those of type 1.
Hyperthyroidism. Hyperthyroidism (pronounced:
hi-per-thigh-roy-dih-zum) is a condition in which the levels of thyroid hormones
in the blood are excessively high. Symptoms may include weight loss,
nervousness, tremors, excessive sweating, increased heart rate and blood
pressure, protruding eyes, and a swelling in the neck from an enlarged thyroid
gland (goiter). The disease may be controlled with medications or by removal or
destruction of the thyroid gland through surgery or radiation treatments.
Hypothyroidism. Hypothyroidism (pronounced: hi-po-thigh-roy-dih-zum)
is a condition in which the levels of thyroid hormones in the blood are
abnormally low. Thyroid hormone deficiency slows body processes and may lead to
fatigue, a slow heart rate, dry skin, weight gain, constipation, and, in
children, slowing of growth and delayed puberty. The condition can be treated
with oral thyroid hormone replacement.