Imagine your body as a massive megacity, and like any other megacity, it has a crisscrossing maze of
communication networks to prevent chaos. In our bodies, one key network is the nervous system,
running like telephone wires throughout the body, rapidly sending messages
from the brain to a body part and back. There is also the slower “courier service”
that takes messages on foot, so to speak. This so-called courier service is the endocrine system.
The endocrine system is composed of glands that produce hormones
to affect changes in distant target organs. A gland, in this case, is the primary message
sender. A gland is a group of cells that produce and secrete chemical messengers.
There are two types of glands, differentiated based on how they release their chemicals.
Those that release secretions directly through a duct are exocrine glands.
Your sweat glands are exocrine glands, secreting sweat directly onto your skin.
Those that release their secretions into the bloodstream are called the endocrine glands.
There are seven major endocrine glands scattered throughout the body: the pituitary gland,
the pineal gland, the thyroid, the thymus, the pancreas, the adrenal glands
and the gonads (testes or ovaries). The chemical messengers released by
endocrine glands are called hormones. These hormones run the show and carry
critical messages to other parts of the body. They control everything, how you respond to
temperature changes, when you feel the urge to eat and the urge to stop eating,
when you feel sleepy, when you feel awake, the mineral balance in your fluids, and even the
Now, how do all these glands and secretions come together to make the endocrine system function?
To clarify, let’s take the example of a single hormone: insulin.
Imagine taking a break from studying to eat a peanut butter sandwich.
Your digestive system will break down the sandwich and absorb the nutrients into the bloodstream.
This causes your blood sugar levels to rise,
something the pancreas senses through its chemical receptors—little proteins jutting out from cells.
The pancreas produces and secretes a hormone called insulin. One of insulin’s functions is
to tell other body cells to take up the freshly consumed sugar and use it up.
Insulin basically increases our sugar metabolism. The pancreas releases insulin into the
circulatory system, which carries it to different body parts and target cells.
The “target cells” are any cell that can “sense” the insulin, meaning that it has
an insulin receptor. Muscles, adipose cells or fat cells, and liver cells are some cell types
that have insulin receptors. The insulin binds to the receptor and the receptor signals the
cell to change its metabolism pattern. In short, a change in the body occurs,
the endocrine system responds by releasing one or more hormones into the bloodstream,
and then those hormones travel through the body and affect change in a target organ.
Unlike the nervous system, hormones act slow. An impulse in the nervous system can move as fast as
1 to 120 meters per second, whereas insulin remains in the bloodstream for 4 to 6 mins.
Even more extreme, the thyroid hormone remains in the bloodstream for 7 to 10 days!
Over the next few hours, as the insulin circulates through the bloodstream, the blood sugar level
will drop. Low blood sugar levels will tell the pancreas to reduce or stop producing insulin.
This is called negative feedback, when the feedback from the body stops the
gland from producing more hormones. There is also positive feedback, when
the hormone levels need to be raised higher and higher or lower and lower to continue a certain
physiological response. This form of feedback is much rarer than a negative feedback loop.
An example of positive feedback is what happens during the birth of a child.
During labor, the posterior pituitary gland increases oxytocin secretion to induce
muscle contractions in the uterus. Oxytocin levels continue to increase to strengthen the
contractions, far beyond normal optimum ranges. The increasing intensity of the contractions
help push the baby out of the uterus. Once the birth occurs, the levels of oxytocin decrease.
If this was a negative feedback loop, oxytocin secretions would cease when the muscle
contractions reached their optimal range. However, in a positive feedback scenario, the oxytocin
keeps rising as the contractions intensify. The endocrine system is a finely balanced machine,
and if the levels of hormones get wonky, it can cause disease, disorder and discomfort.
If there’s too much of a hormone, it is denoted by “hyper,” such as hyperthyroidism,
when the thyroid produces too much of the thyroid hormone thyroxine.
This throws the body’s metabolism into chaos;
some symptoms include uncharacteristic weight loss, sleeping problems, and muscle weakness.
The other side of the spectrum is when there is too little of a hormone, denoted by the suffix
“hypo”. Type 1 diabetes occurs when the pancreas produces too little insulin. Without insulin,
cells find it difficult to use the sugar in the bloodstream.
Individuals often take frequent insulin shots to compensate for their depleted insulin level.
As they say, “Too much of a good thing can be bad,” and that’s definitely true for the
endocrine system. The entire system is largely dedicated to keeping your body in homeostasis,
and this delicate balance is what keeps you alive!