Endocrine System

I remember when I first heard the word “hormones. ” I was a little kid – had no idea what it meant -but I recall the word when someone made a phrase “it’s just hormones” but then over the years, the word “hormones” came up a lot more and it became clear to me: whatever these hormones are – they must be pretty important – because they’re mentioned for a lot of different things. And, indeed, today, I can tell you that, yes, hormones are a very big deal.

As well as the system that secretes them. You know what system I’m talking about. The endocrine system.

The endocrine system consists of structures that secrete hormones: that could be at the cellular level or at the organ level. But a big focus for the system and for this video is on glands. Now some of the major glands in the endocrine system also have non-endocrine duties that we won’t be going into, and some of these glands are also classified as organs in their roles with other body systems.

Ok so endocrine glands: We’ll start with in the brain. Hypothalamus, Pineal gland, pituitary gland. Those are all found in the brain.

In the neck area, thyroid and parathyroid. And farther down in the upper chest, the thymus. Above your kidneys, the adrenal glands.

Near the stomach, the pancreas. Gonads: The gonads are reproductive glands - ovaries which are also part of the female reproductive system. Testes, which are part of the male reproductive system.

Now two things to point out about all of these glands: Number 1: We’re focusing on endocrine function and not exocrine in this video. What’s the difference? Well, endocrine gland function means they are going to release hormones right into their surroundings.

They have no special ducts to carry things away. Exocrine glands have ducts that carry whatever is secreted into a body opening or body surface. Like sweat glands.

Or mammary glands, which produce milk. Sometimes a gland that has endocrine function can also have exocrine function. The pancreas for example!

The pancreas is using its endocrine function when it’s releasing insulin or glucagon, which deals with blood sugar – more on that later- but it has an exocrine function when it secretes pancreatic enzymes through accessory ducts into the small intestine. And number 2: Yes, these are major endocrine glands that we mentioned, but there are also cells within organs that can have endocrine functions. Want an example?

Ok, remember when we had talked about the stomach in the digestive system? We presented some examples of enzymes and hydrochloric acid as part of the gastric juice in the stomach, but we didn’t discuss a hormone that helps make the secretion of stomach acid possible. Gastrin is one of those hormones, and there are specific cells in the stomach that can secrete it.

So now that we’ve mentioned that and we’ve given a little tour of the glands, it’s time to talk about hormones. There are different classes of hormones and while that’s not our focus: realize that they can be derived from different biomolecules such as amino acids or a full chain of amino acids (otherwise known as a polypeptide), or lipids – like in the case of steroids. And it’s important to realize that because it turns out that what hormones are made of – meaning what makes up the hormone structure - is going to influence the hormone function.

By function, that includes the receptors the hormones bind to. Hormones generally bind to specific target cells. And then the target cells will do some kind of action: maybe they’ll increase the rate of mitosis or prepare certain enzymes for action.

There’s a lot of responses a target cell may take. So we’re going to go through the glands again but this time, we’re going to mention some major hormones associated with each gland and some general functions. Alright in the brain: so you can consider the hypothalamus-pituitary region to be a sort of complex because they are the boss of pretty much the rest of the endocrine system.

The pituitary gland – and you can divide that into the anterior [meaning towards the front] and posterior [meaning here towards the back]– so anterior and posterior pituitary stems off of the hypothalamus. The hypothalamus makes several hormones and the posterior pituitary holds on to them and then secretes those hormones. The posterior pituitary doesn’t make its own hormones.

The amazing anterior pituitary on the other hand can make its own hormones! But it’s still very regulated by the hypothalamus. The posterior pituitary will secrete oxytocin – important for uterine contractions when one is having a baby and antidiuretic hormone (ADH) – this stimulates the kidneys to reabsorb water.

As for the anterior pituitary? Here’s a few. Growth hormone (GH) -as its name suggests it promotes growth, prolactin (PRL)- promotes producing milk from mammary glands, Thyroid-stimulating hormone (TSH) – stimulates the thyroid to release thyroid hormone, Follicle stimulating hormone (FSH) – stimulates gonads to make gametes, Luteinizing hormone (LH) -stimulates gonads to make androgens, and adrenocorticotropic hormone (ACTH) –a very long name for a hormone - that one stimulates the adrenal cortex to release hormones.

So you can see why this hypothalamus-pituitary complex is an important one. What about the little pineal gland? Don’t underestimate its importance: it secretes melatonin, which helps regulate circadian rhythm – which involves our sleep-wake cycle.

The thyroid. Shaped like a beautiful butterfly that sits just under the larynx and wraps around the trachea, sitting in front of the trachea. It makes T4 (thyroxine) and T3 (triiodothyronine).

These hormones have major roles in metabolic processes. The thyroid also makes calcitonin which can lower the blood calcium level down. The Parathyroid glands sit posterior – in the back – here.

The parathyroid gland releases parathyroid hormone (PTH) which actually raises blood calcium levels. What’s with blood calcium? Calcium levels are involved in cell signaling among many other things.

Ok, now down to the thymus. You know, some endocrine diagrams don’t include the thymus. It may be because in adulthood, the thymus tends to shrink down and is not as active as the person ages.

But before puberty when T cell development is especially active, the thymus is very important as an organ for the site of T cell maturity. And as a gland, the thymus does make hormones that impact immune system function: thymosin, which can stimulate the making of T cells, is an example. Ok, now take a look at these adrenal glands above the kidneys.

Adrenal glands have two major parts: the adrenal medulla and the adrenal cortex. The adrenal medulla secretes epinephrine and norepinephrine which are involved in several body responses including the fight or flight response. The adrenal cortex secretes hormones known as glucocorticoids – the hormone cortisol is an example – cortisol can increase blood glucose levels and plays major roles with stress and inflammation.

The adrenal cortex can also release mineralocorticoids -the hormone aldosterone is an example- it’s critical for the reabsorption of sodium and the excretion of potassium when we were talking about the nephrons in the kidneys. Pancreas. So after your food is digested, you need to get the glucose (that’s a sugar) from the blood INTO your body cells.

The hormone insulin is made by your pancreas and insulin tells the body cells to take in the glucose. Without it, your cells couldn’t have the glucose they need to survive – a reason that a person with diabetes may need to take insulin if their pancreas is not functioning normally. The pancreas also makes glucagon – this helps raise the blood glucose level – one way is that it can stimulate the liver to convert its glycogen to glucose.

And last, the gonads. Ovaries make estrogen. Generally estrogen promotes uterine lining growth and female secondary sex characteristics.

Ovaries also makes progesterone which also promotes uterine lining growth and keeps it maintained which is critical for fetal development. The testes make androgens – an example of an androgen is testosterone. Androgens help with making sperm cells and the development of male secondary sex characteristics.

We do want to point out that estrogen, progesterone, and testosterone: are generally present and serve functions in all individuals. However, estrogen and progesterone are generally in higher concentrations with major functions in females whereas androgens are generally in higher concentration with major functions in males. So, we toured through some endocrine glands and the hormones they make.

Those hormones act as chemical signals so the target cells can perform some sort of action. The response time can be slow or fast and the distance the hormones travel to reach their target cells might be short or long. We want to just give a little reminder that hormones aren’t the only form of signaling in the body.

The nervous system, which needs its own video, uses electrical signaling and chemical signaling in the form of neurotransmitters. Generally nervous system signaling will be for short distances and has a very fast response time. Finally, in talking about why the endocrine system is so important: let’s consider an example of when it is not functioning normally.

In hypothyroidism, the thyroid gland is underactive. This can have many causes which can impact how it is treated, but generally in hypothyroidism, the thyroid isn’t producing enough thyroid hormones. Recall that the thyroid hormones have major roles in metabolic processes.

Therefore, someone with hypothyroidism may experience extreme fatigue, a slowed heart rate, and other symptoms. Sometimes an individual with this condition can also have a higher-than-normal level of thyroid stimulating hormone (TSH), because their anterior pituitary is trying to stimulate their underactive thyroid. An endocrinologist is a physician that specializes in conditions of the endocrine system, and their treatment plan for hypothyroidism may include receiving replacement thyroid hormone.

See our further reading suggestions for more examples. Well, that’s it for the Amoeba Sisters, and we remind you to stay curious.