Endocrine system anatomy and physiology

Definitions & Key takeaways

The endocrine system comprises all endocrine organs, which produce various hormones in the body. Hormones are like chemical messengers, which travel in the blood to arrive at their receptors, where they initiate action.

Organs of the endocrine system include the hypothalamus, pituitary gland, thyroid gland, parathyroid gland, adrenal glands, pancreas, and Gonads (ovaries and testes). Each gland releases specific hormones that help regulate everything from hunger and thirst to blood pressure and reproduction.

Chapters:

Watch video only

Introduction0:00–0:28

The endocrine system is made up of various endocrine glands that each secrete hormones into the bloodstream. When hormones reach their target cell, they bind to a receptor on the cell’s membrane or within that cell, and in response the target cell changes what it’s doing.
So at the end of the day, the endocrine system helps establish homeostasis - a sense of balance even when there are changes in the external environment.

Steroid hormones0:28–1:14

Now, structurally, hormones can be either steroids or non-steroids. Steroid hormones are made from cholesterol, and they’re made by the adrenal glands, which sit above each kidney, and the gonads - either the testes or ovaries.
Steroid hormones are hydrophobic or non-polar - meaning that they hate watery environments, so they travel through the bloodstream bound to transport proteins to reach their target cells.
But because steroid hormones are relatively small, and non-polar, they are also able to diffuse right across phospholipid membrane of target cells.
Once inside the cell, they bind to a receptor that goes on to activate certain genes in the nucleus. Non-steroid hormones, on the other hand, are either peptides or proteins - so chains of amino-acids, or they can derive from a single amino acid.

Non-steroid hormones1:14–2:03

Peptidic hormones, like insulin and glucagon, are hydrophilic - meaning they love coursing through our blood. However, when they reach the cell membrane of a target cell, they can’t pass through the phospholipid bilayer.
Instead, they bind to cell surface receptor proteins. Once the receptors bind to a non-steroid hormone, they change shape, and that activates various proteins and enzymes that go on to create changes in gene expression within the cell.
So ultimately, once the non-steroid hormone binds to the receptor, there’s a change in the cell even though the hormone never actually enters the cell.

Amino-acid hormones2:03–3:07

Finally, there are amino-acid hormones that derive from the amino acid, tyrosine, which are the thyroid hormones, as well as adrenaline and noradrenaline - also called epinephrine and norepinephrine.
Now, these hormones are synthesized differently, so the molecular tweaks here and there make them behave differently; either more like steroids, or like peptides.
Thyroid hormones for example, behave more like steroid hormones: they travel the bloodstream bound to a transport protein, and cross the cell membrane to bind to an intracellular receptor, and signal changes in gene expression in the nucleus.
Adrenaline and noradrenaline, on the other hand, behave more like peptide hormones - they travel through blood unbound, and bind to cell surface receptors on cells, which then set off intracellular changes.
In fact, that’s partly responsible for the increased blood flow to the heart and muscles, that occurs during a fight-or-flight response, when you’re fighting with an airline so you can catch a flight.

Hypothalamus-Pituitary Axis3:07–5:27

Now, the endocrine glands are scattered throughout the body, much like a remote work environment - so let’s get acquainted with our crew here.
All the way up into the brain, there’s the hypothalamus - which is like the CEO, and right below it, the first officer, the pituitary gland.
The hypothalamus and pituitary are physically connected by a thin stalk, and they work closely together to make hormones that help control the production of other endocrine glands, like the thyroid, the adrenal glands, and the gonads.
The hypothalamus is made up of several nuclei which are clusters of neurons with various roles, including secretion of hormones.
The pituitary gland is made up of two lobes - the anterior lobe, which is made up of glandular tissue, and the posterior lobe, which is made up of the axons of neurons coming down from the supraoptic and paraventricular nucleus in the hypothalamus.
Now, the hypothalamus is the link between the nervous and the endocrine system - it receives information from the entire body regarding all sorts of things - such as body temperature, blood osmolarity, or even if there’s some sort of danger - and it responds by producing hormones that are stored in the posterior pituitary, to be released later, or hormones that act on the anterior pituitary, making it secrete some hormones of its own.
So the hypothalamus gives the order, and the pituitary enforces it. This is possible because there are anatomical connections between the hypothalamus and both the anterior and posterior pituitary.
Between the hypothalamus and the anterior lobe of the pituitary, there’s the hypothalamo-hypophyseal-portal system. This is a system of tiny capillaries that moves hormones quickly from the hypothalamus to the anterior pituitary.
These hypothalamic hormones can be stimulatory or inhibitory. Let’s start with the stimulatory, or releasing, hormones.
These include thyrotropin releasing hormone, or TRH; corticotropin releasing hormone, or CRH; gonadotropin releasing hormone, or GnRH and growth hormone releasing hormone, or GHRH.

Stimulatory hormones5:27–7:26

These stimulatory hormones make the anterior pituitary synthesize its own hormones in response. TRH leads to the production of thyroid stimulating hormone, or TSH, which reaches the thyroid and tells it to make some more thyroid hormones.
When plasma thyroid hormone levels increase, this sends a negative feedback signal to the pituitary to make less TSH, keeping thyroid hormone levels in an optimal range.
Next, there’s CRH, which makes the pituitary produce adrenocorticotropic hormone - or ACTH - which goes to the adrenal glands and makes them secrete more of a hormone called cortisol.
As before, high levels of cortisol inhibit the production of ACTH through a negative feedback mechanism. Next, there’s GnRH which makes the pituitary secrete gonadotropins - follicle-stimulating hormone, or FSH, and luteinizing hormone, or LH.
Gonadotropins act on the gonads and regulate the production and maturation of gametes - sperm for the testes and oocytes for the ovaries, as well as the production of sex hormones - testosterone, estrogen and progesterone.
As a general rule, sex hormones also send a negative feedback mechanism back to the pituitary. The exception is that in females, right before ovulation, estrogen levels get really high, and they make the pituitary even more sensitive to hypothalamic GnRH.
So this acts as a positive feedback signal, leading to a massive surge of FSH and LH that leads to ovulation. Finally, GHRH makes the anterior pituitary secrete more growth hormone - or GH - which has a direct effect on the long bones and other tissues in our body, making them, well… grow.

Inhibitory hormones7:26–8:33

So those were the stimulatory hypothalamic hormones. The inhibitory hypothalamic hormones are much easier to remember; there are only 2: growth hormone inhibiting hormone, or GHIH, also known as somatostatin, and prolactin inhibiting factor, which is also called dopamine.
GHIH is also synthesized by other organs in our body, like our digestive tract, and it tells the pituitary to secrete less growth hormone.
Now, with prolactin inhibiting factor, things are a bit trickier. Because prolactin increases milk production in the breasts, it’s only needed during breastfeeding.
So outside breastfeeding, the hypothalamus continually secretes prolactin inhibiting factor, which goes to the anterior pituitary and inhibits prolactin production so no milk is produced.
However, during breastfeeding, when the baby starts suckling, this sends a signal to the hypothalamus to halt production of prolactin inhibiting factor, basically inhibiting the inhibitor - and this allows the anterior pituitary to make prolactin.

Posterior pituitary8:33–10:02

The hypothalamus is connected to the posterior pituitary through the pituitary stalk, which is made up of the axons of hypothalamic neurons coming specifically from the paraventricular and supraoptic nuclei.
Both these nuclei secrete antidiuretic hormone - also known as ADH, or vasopressin - and oxytocin, which travel down the axons of these neurons and reach the posterior lobe of the pituitary gland.
Down the length of these axons, there are small dilations called Herring bodies, which store the hormones until they get a signal to release them.
When the time is right, the axons release ADH or oxytocin into the posterior pituitary capillaries, and from there, into the system circulation.
For ADH, the signal is either a high blood osmolarity or a low blood volume. ADH helps retain water from the urine and also causes vasoconstriction of blood vessels, which helps decrease osmolarity and increase blood pressure.
The other hormone, oxytocin, dilates the cervix and stimulates uterine contractions during childbirth, and makes the muscle cells in the breasts contract to eject the milk during breastfeeding.
So aside from motherhood, its levels are generally pretty low - but they do increase a little bit during pleasant social interactions, hugs and physical contact, and even after an orgasm - hence, that lovely “afterglow”.
Ok now, let’s look at the other glands on our endocrine crew. Also in our head, behind the hypothalamus and the pituitary, there is the tiny pineal gland.

Pineal gland10:02–10:27

The pineal gland is made up of cells called pinealocytes, which synthesize and release melatonin. Melatonin is mostly secreted during the night, and it regulates our body’s circadian rhythm - or the “inner clock” that tells us when we should be sleeping and when we should be awake.
Next, there’s the thyroid gland is at the front of the neck, and it’s made up of a left and a right lobe - like two wings of a butterfly.

Thyroid gland10:27–11:28

The thyroid gland is made up of thousands of follicles that make triiodothyronine or T3, and thyroxine or T4. Once inside the cell T­4 is mostly converted into T3, and it can exert its effect.
T3 speeds up the basal metabolic rate - so the thyroid gland is like the “operations manager” helping to boost productivity.
In between the thyroid follicles, there are also parafollicular cells, OR C- cells - which secrete a hormone called calcitonin.
On the back of each thyroid lobe, there are also two parathyroid glands - one above, and one below- so 4 in total. They make parathyroid hormone.
Both calcitonin and parathyroid hormone are involved in calcium, phosphate and bone metabolism, and are regulated by calcium levels in the blood.

Adrenal gland11:28–12:22

Next up, are the adrenal glands which just above each kidneys. Each adrenal gland is made up of an outer layer called the cortex, surrounding a core called the medulla.
The cortex of the adrenal gland can further be divided in three zones that produce steroid hormones - the zona glomerulosa, that makes aldosterone, the zona fasciculata, which produces cortisol, and the zona reticularis, which makes small amounts of sex hormone precursors.
Aldosterone is secreted when there is low pressure, or too much potassium in the blood. Upon release, aldosterone prevents loss of water and sodium in the urine, and increases the elimination of potassium.
Cortisol is also known as the “stress hormone”, and it’s secreted during “fight or flight situations”, along with adrenaline and noradrenaline, which are secreted nearby in the adrenal medulla.

Pancreas12:22–13:37

Finally, there’s the pancreas - which has an exocrine part, that helps with digestion, and an endocrine part, that secretes insulin and glucagon.
So the pancreas is like the guy on the team who’s working two full time jobs! The exocrine part of the pancreas secretes digestive enzymes directly into the duodenum, which help break down food into nutrients the small intestine can absorb.
The endocrine part is made up of beta and alpha cells that secrete insulin and, respectively, glucagon in response to blood sugar levels.
So, when there’s high blood sugar, like right after a meal, beta cells secrete insulin into the blood, which binds to specific receptors on all the cells in our body, helping them take in glucose, and use it for energy - therefore lowering our blood sugar levels after meals.
With low blood sugar, on the other hand, the alpha cells take charge and secrete glucagon. Glucagon knows the liver has a glucose “stash” called glycogen - so it binds to a receptor on liver cells, which respond by activating enzymes that break down glycogen and release glucose into the bloodstream.
And that helps us make it between meals. Alright, as a quick recap.

Review13:37–14:29

There are steroid hormones, which are hydrophobic and diffuse across cell membranes and bind intracellular receptors, peptide hormones, which are hydrophilic and bind to cell surface receptors, and tyrosine derived hormones that behave either like steroid hormones or like peptide hormones.
Homeostasis begins with the hypothalamus which secretes stimulatory and inhibitory hormones that affect the anterior pituitary as well as hormones that diffuse directly into the blood at the posterior pituitary.
Ultimately these hormones affect other endocrine glands, like the thyroid, the adrenal glands, and the gonads. Other endocrine glands that are not under direct control of the hypothalamus are the parathyroids, which make parathyroid hormone, and the pancreas, which secretes insulin and glucagon.