Professor Dave here, I wanna tell you about the endocrine system.
We’ve already looked at a few systems, those being the skeletal system, muscular system,
and nervous system, so we are starting to comprehend how the human body works.
And with that, we are bridging the gap between a single cell and a complex multicellular organism.
But we still have a number of systems to go, and the next one up is the endocrine system.
This interacts with the nervous system in order to control the activity within all the
cells in the body, so let’s take a look at how this works now.
The endocrine system is comprised primarily of glands, which are organs that secrete a
particular compound.
This compound acts as a messenger, traveling throughout the bloodstream and delivering
that message to cells all over the body.
These compounds are called hormones, and the messages they carry may tell cells to start
dividing, or to start synthesizing a particular protein, whether their target is a particular
tissue, or if they simply act as intermediaries that communicate with other endocrine glands.
We already examined how these compounds interact with receptors in the plasma membrane of a
cell, and the chemistry that occurs as a result, which will elicit an appropriate response
within the cell, so for more information on that, check out some of my biochemistry tutorials.
Here, we will focus on the organs of the endocrine system and their specific functions.
The organs of the endocrine system are very small, and scattered around the body.
These endocrine glands differ from the exocrine glands that produce sweat and saliva in that
these ones produce hormones, which are typically released into the bloodstream.
These glands include the pituitary and pineal glands, and the hypothalamus, all of which
are parts of the brain.
Then there are the thyroid and parathyroid glands, found in the neck in front of the trachea.
Next we have the adrenal glands and the pancreas, which are in the lower torso, as well as the
gonads, which are called testes for a male and ovaries for a female.
Other organs also contain clusters of endocrine cells, like the small intestine, stomach,
kidneys, and heart, as these also secrete hormones at certain times, though it is not
their primary function.
These structures map out the major hormonal axes of the body, those being the hypothalamic-pituitary-adrenal
axis, which regulates cortisol, and the renin-angiotensin-aldosterone axis, which helps to regulate sodium, potassium,
and water balance.
Those are the organs of the endocrine system, so what are the hormones they produce?
We mentioned a few of them just now when describing the hormonal axes, and these typically belong
to one of just two categories.
The first kind are amino acid-derived or small proteins, and the second kind are steroids.
Both of these are types of molecules we learned about in the biochemistry series, in case
you need a little review.
Any of these hormones are secreted by glands when they encounter certain stimuli, and these
can be of three varieties.
Humoral stimuli involve changes in the concentrations of certain components in the bloodstream,
like particular ions or nutrients.
Neural stimuli involve stimulation by nerve fibers.
And hormonal stimuli involve interaction with other hormones, so sometimes a hormone will
interact with a gland, and as a result the gland will secrete some other hormone.
These hormones can elicit the intended effect in even very low concentration, and they circulate
in the blood either free, or bound to a protein carrier, depending on their solubilities.
So let’s go through each gland now, and highlight key aspects of their function, starting
with the pituitary gland.
This is a small sac comprised of a posterior lobe and an anterior lobe, which hangs from
the hypothalamus via the infundibulum.
This gland secretes at least eight hormones.
In the posterior lobe, these are neurohormones that arrive ready-made from the hypothalamus,
while the anterior lobe secretes its own hormones.
The neural connection between the pituitary gland and the hypothalamus is called the hypothalamic-hypophyseal
tract, and the two main hormones that travel through here to be secreted by the posterior
pituitary are called oxytocin and and antidiuretic hormone, or ADH, also called vasopressin.
Oxytocin is released during childbirth, and is also involved in the release of milk during
breastfeeding.
In addition, it is involved in sexual and affectionate behavior between mates, promoting
couple bonding.
ADH is completely different, inhibiting urine formation during times of high blood osmolarity
or low blood pressure, thus helping the body avoid dehydration.
The anterior pituitary has a different role, manufacturing and secreting a variety of hormones
that in turn regulate other endocrine glands.
These hormones are growth hormone, which promotes tissue growth and directs cells to metabolize
fat, thyroid-stimulating hormone, which stimulates the thyroid gland as one might expect, adrenocorticotropic
hormone which stimulates the adrenal cortex, follicle-stimulating hormone and luteinizing
hormone, which regulate the gonads, and prolactin, which stimulates milk production in females.
Any of these can be released when a particular stimulus is received from the hypothalamus.
Next let’s look at the thyroid gland.
This is comprised of two lobes connected by the isthmus, and it produces thyroid hormone,
which is actually two hormones, thyroxine and triiodothyronine, also known as T4 and
T3 respectively.
This is a hormone that affects almost every cell in the body by increasing metabolism,
regulating tissue growth, and maintaining blood pressure.
Next up are the parathyroid glands, of which there are typically four.
These secrete parathyroid hormone, which regulate calcium and phosphate levels in the blood.
Then there are the adrenal glands, which are each comprised of an inner adrenal medulla
and an outer adrenal cortex.
The cortex synthesizes a wide variety of steroid hormones called corticosteroids.
Mineralocorticoids like aldosterone regulate ion concentrations like sodium and potassium.
Glucocorticoids like cortisol and cortisone influence metabolism by keeping blood glucose
levels consistent, and they also maintain blood pressure by influencing vasoconstrictors.
Cortisol also triggers gluconeogenesis, where glucose is made from fats and amino acids.
And gonadocorticoids are sex hormones like androgens, which are typically converted into
testosterone.
The adrenal medulla, on the other hand, synthesize catecholamines like epinephrine and norepinephrine.
These are important in emergency situations, so the perception of danger triggers their
release, upon which they induce high levels of glucose in the blood and a faster heartbeat,
which are good for evading danger.
Finally there is the pineal gland, found in the diencephalon of the brain.
This produces melatonin, which helps us sleep.
That wraps up the glands that are devoted solely to endocrine function.
There are a few other organs to mention, however, starting with the pancreas.
This is found in the abdomen, and it is responsible for making the hormones glucagon and insulin.
These regulate blood glucose levels.
Glucagon is a polypeptide made of twenty nine amino acid residues which will stimulate an
increase of glucose in the blood.
Much of its activity occurs in the liver, where it triggers breakdown of glycogen, and
glucose synthesis by other means, as well as the release of the resulting glucose.
Insulin is a larger peptide, made of fifty one residues, qualifying as a small protein.
This will stimulate a decrease of glucose in the blood by inhibiting glycogen breakdown
and glucose synthesis.
So glucagon and insulin have nearly opposite actions in the body, as glucagon release is
triggered by fasting, and insulin release is triggered by eating.
Finally, the gonads produce sex hormones just like the adrenal cortical cells.
The ovaries produce estrogens and progesterone, which control the development of reproductive
organs and secondary sex characteristics in females.
The testes produce testosterone, which controls the development of reproductive organs and
secondary sex characteristics in males.
The placenta is a temporary endocrine organ which exists during fetal development in a
pregnant woman.
It secretes steroid and protein hormones that influence pregnancy, and support the development
and growth of the fetus.
Lastly, there are other organs that also produce hormones.
Adipose cells release leptin, which communicates how much fat is stored in the body, as well
as resistin and adiponectin, which are related to response to insulin.
Enteroendocrine cells in the gastrointestinal tract release peptide hormones that regulate
digestion.
The heart has cells that secrete atrial natriuretic peptide, or ANP, which regulates blood volume
and blood pressure, as well as B-type natriuretic peptide, or BNP, in response to stretching
caused by increased blood volume.
The kidneys secrete erythropoietin, which promotes red blood cell production, as well
as renin, which is associated with aldosterone release.
Bones can secrete osteocalcin, which promotes the release of insulin.
The skin produces cholecalciferol when exposed to a type of ultraviolet light, which is an
intermediate in vitamin D production.
And with that we have completed a basic introduction to the endocrine system, and all the glands
and hormones that are part of it.
This remarkable system of communication is critical for human development, and our bodily
functions depend on the ability of hormones to travel everywhere they need to go.
As we said, hormones travel through the bloodstream, but what is blood, exactly, and how does blood
circulate around the body?
Let’s move forward and learn about this incredible system next.