The Endocrine System is basically made up of a group of glands that secrete hormones directly into the circulatory system. These hormones are then carried along the body until they reach their Target Cell (aka. their finish line). Here the hormones bind to the cell’s receptor, and once they are securely attached to it, they give the cell specific instructions.
So essentially, it’s a bunch of glands, all working behind the scenes to control the organ’s functions. Think of them as the masterminds, that instruct the cells on how to work.
What’s important to keep in mind is that all of these glands work together in harmony. They are also very flexible and change according to the external environment to allow the body to function smoothly. This balance is known as Homeostasis.
The Endocrine Glands include:
Pineal Gland
Pituitary Gland
Pancreas
Ovaries / Testes
Thyroid Gland
Parathyroid gland
Hypothalamus
Gastrointestinal Tract
Adrenal Glands
Great, now that you have a good overview of what the Endocrine System is, let’s start moving in more in-depth and try to understand what hormones actually are.
The Endocrine System Anatomy & Physiology – Part 1
Hormones
To make our lives easier, we will divide hormones into two groups: Steroids and Non-Steroids.
Steroid Hormones
Steroid Hormones are made up of Cholesterol. They are built inside the Adrenal Glands which can be found on top of the Kidney, or on top of the Gonads (i.e. Testes/Ovaries).
These steroid hormones are Hydrophobic which means they hate water (remember phobia = scared, phobic = scared of water) 😉 So because blood is relatively “watery”, steroid hormones bind with Transport Proteins to get through the bloodstream and reach their Target Cell.
It’s like when you were a little kid, and you played “The Floor is Lava”, you would hop onto the carpets or furniture to get from one place to another without touching your feet to the floor. Steroids do the same thing; they hop onto Transport Proteins, so they don’t have to feel anything in the bloodstream.
Eventually, once they arrive at the Target Cell, the steroid hormones diffuse through the Phospholipid Membrane and bind to the Receptor. When they are tight with the receptor, the steroids can wake specific Genes in the Nucleus, and change the Gene Expression.
Non-Steroid Hormones
Non-Steroid Hormones are made up of Peptides or Amino-Acids (aka. Protein).
Insulin and Glucagon are Peptide Hormones. These hormones are Hydrophilic, which means they LOVE water, and they can race through the bloodstream without any problems. But when they arrive at the Target Cell, they can NOT diffuse through the membrane as the steroid hormones do. So they bind with the Cell Surface Receptors instead.
Once the non-steroid hormones are tightly attached, they change the shape of the cell surface receptors. The cell senses this change and activates a bunch of proteins and enzymes, in turn, changing the Gene Expression.
Thyroid hormones, Adrenaline and Noradrenaline, are Amino-Acid Hormones.
The thyroid hormones act just like Steroid Hormones. First, they go through the bloodstream with the help of transport proteins. Then, they go into the cell membrane and bind with a receptor inside the cell. Which again creates a change in Gene Expression.
On the other hand, Adrenaline and Noradrenaline act like Peptide Hormones. Which means they are not scared to travel through the bloodstream alone. But when they reach the cell, they bind to the cell surface receptors because they can not enter it. This binding to the cell surface receptor activates some proteins and enzymes and changes the Gene Expression.
Ok Let us Recap because this has given me a headache already,
There are two types of hormones: Steroid and Non-Steroid
Steroid Hormones are made up of Cholesterol in the Adrenal Glands. They are scared of water, ie. Hydrophobic and so they need Transport Proteins to travel through the bloodstream.
When steroid hormones arrive at the cell, they diffuse through the membrane, bind with a receptor inside and change the Gene Expression
Non-Steroid Hormones are made of Peptides or Amino-Acids.
The Peptide Hormones are Hydrophilic, so they don’t need help to travel through the bloodstream. But when they arrive at the cell, they can NOT diffuse in it. So they bind to a Cell Surface Receptor and change the Gene Expression.
The Amino-Acid Hormones don’t know what they want, and so they act as they please. The Thyroid Hormones function like Steroid Hormones, while the Adrenaline and Noradrenaline function like the Peptide Hormones.
Perfect! So now you should be able to understand what the endocrine system is, and what hormones are. If you’re still not sure, then go back and read it again.
If you feel like you have understood them, we can move onto the 2nd part of the Endocrine System Anatomy & Physiology.
The Endocrine System Anatomy & Physiology – Part 2
The next step is to learn about all the different Endocrine Glands found around the body.
The Hypothalamus and Pituitary Gland
Starting at the top, the Hypothalamus and Pituitary Gland are both found in the brain. These two are connected with a very thin stalk. And together, they produce hormones that regulate the work of the other endocrine glands.
Kind of ironic isn’t it? But look at it as if the Hypothalamus and the Pituitary Glands are the managers of endocrine glands, overseeing everything. The role of the Hypothalamus as a manager does not end there. In fact, the Hypothalamus is the one that receives all the information of complaints from the whole body. So if the body is complaining of overheating, or a dangerous situation, it’s the hypothalamus’ job to deal with it.
When the Hypothalamus receives a complaint, it releases hormones which command the pituitary gland to take action, and so the pituitary gland releases hormones. Almost as if the Hypothalamus is the responsible employee, while the Pituitary is just lazy. So when there is action, the Hypothalamus has to nudge the Pituitary to get it’s life together and work.
Makes sense?
Good, let’s move on cause there’s a lot more info about these two.
The Hypothalamic Hormones
So far, we’ve mentioned that the Hypothalamus secretes hormones. But that’s way too easy for you, so we are going in more detail to learn the different types of hormones that are secreted.
First off, these hormones can be either Stimulatory or Inhibitory. Meaning they either stimulate/encourage the production of something, or they inhibit/stop the production of something.
The Stimulatory hormones are:
Thyrotropin Releasing Hormone (TRH)
Corticotropin-Releasing Hormone (CRH)
Growth Hormone Releasing Hormone (GHRH)
Gonadotropin-Releasing Hormone (GnRH)
These stimulatory hormones will stimulate the pituitary gland to produce its own hormones.
Now take a deep breath, because this is where the chaos begins.
The Stimulatory Hypothalamic Hormones
The TRH triggers the production of Thyroid Stimulating Hormone (TSH). The TSH makes its way to the Thyroid and instructs it to make Thyroid Hormones. Eventually, when the body senses that there is an increase of Plasma Thyroid Hormone, it sends a Negative Feedback signal to the Pituitary telling it to produce less TSH. This whole cycle keeps the thyroid hormone levels balanced.
The CRH triggers the production of Adrenocorticotropic Hormone (ACTH). The ACTH makes its way to the Adrenal Glands and instructs them to make Cortisol Hormone. The increase of Cortisol Hormones sends negative feedback to the Pituitary, telling it to produce less ACTH.
The GHRH triggers the production of Growth Hormone (GH). The GH makes its way to the long bones, and body tissues and helps them to grow. Eventually, when they’ve grown enough, they will send negative feedback to stop the production of GH.
The GnRH triggers the production of Gonadotropins. These Gonadotropins produce Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH). Both hormones target the Gonads, which control the production and growth of Gametes (aka. sperm or oocytes). They also produce sex hormones like Testosterone, Oestrogen and Progesterone.
Typically all the sex hormones send negative feedback when they start to rise. However, there is an exception to the Oestrogen.
Just before Ovulation, oestrogen levels will increase a lot. This increase makes the pituitary gland highly sensitive to Hypothalamic GnRH. Now if the Pituitary is highly sensitive to GnRH, it means that it will produce a lot of FSH and LH. This loop is known as Positive Feedback, and it is the reason why Ovulation occurs.
The Inhibitory Hypothalamic Hormones
The inhibitory hormones are a whole lot easier to remember because there’s only two.
Growth Hormone Inhibitory Hormone (GHIH)
Prolactin Inhibiting Factor (PIH)
GHIH is sometimes Somatostatin. As a hormone, its job is to stop the Pituitary Gland from secreting Growth Hormones.
The PIH is a bit more complicated. Firstly, Prolactin is responsible for producing breast milk when a woman is breastfeeding; however, this milk is not needed if the woman is not pregnant. So as a result, the Hypothalamus is continuously producing PIH, which keeps the body from producing milk.
If the woman eventually gets pregnant and starts breastfeeding, the PIH will be inhibited, and Prolactin will produce breastmilk.
The Antidiuretic Hormone and Oxytocin
If you remember correctly, we said that the Hypothalamus is linked to the Pituitary Gland with a thin stalk. This tiny stalk is made up of Axons of the Hypothalamic Neurons.
Now, in these Axons, the body stores two hormones:
Antidiuretic Hormone (ADH) aka. Vasopressin.
Oxytocin
ADH is typically released when the body has high blood osmolarity or a low blood volume (i.e. low blood pressure). It deals with the increased blood osmolarity by keeping in water from the urine, and it fixes the blood pressure by constricting the blood vessels. This is known as Vasoconstriction.
Oxytocin is secreted during childbirth, as it dilates the cervix and induces uterine contractions. It also makes the breast muscle cells contract and pushes out breast milk.
Interesting Fact: Oxytocin is also released during an orgasm 😉 That’s why women get an “Afterglow”.
And that was the second part of the Endocrine System Anatomy & Physiology! Don’t celebrate just yet; there’s plenty more to go. Let’s move on the 3rd part.
The Endocrine System Anatomy & Physiology – Part 3
The Rest of the Endocrine Glands
The Pineal Gland
Up until now, we’ve only spoken about the Hypothalamus and Pituitary Gland. But, there is another little gland hiding in the brain called the Pineal Gland.
The Pineal Gland is formed by a bunch of Pinealocyte cells, which create and secrete Melatonin. Typically, Melatonin is released at night, and it controls your body’s Circadian Rhythm (the biological clock).
The Thyroid and Parathyroid Glands.
This butterfly-like gland is located at the front of your neck. And just like a butterfly has a left and right-wing, the thyroid gland has a left and a right lobe. The Thyroid Gland produces Triiodothyronine (T3) and Thyroxine (T4), two hormones that will increase the Basal Metabolic Rate, and affect your weight.
The Thyroid Gland also has Parafollicular Cells which secrete Calcitonin Hormone.
Behind the thyroid lobes, there are four Parathyroid Glands that produce Parathyroid Hormone. Together, the Calcitonin and Parathyroid Hormone work to metabolise bone, calcium and phosphate.
The Adrenal Glands
The Adrenal Glands are found on top of each Kidney, and they consist of the Cortex (outer layer) and the Medulla (inner core).
Lucky for us (# sarcasm), someone discovered that the Cortex is split into three different areas that make steroid hormones:
Zona Glomerulosa: produces Aldosterone
Zona Fasciculata: makes Cortisol
Zona Reticularis: creates a small amount of sex hormone precursors
Aldosterone steals the water and sodium away from the urine and pushes potassium out of the body. So it is released when the body has low blood pressure or if the blood has too much potassium.
Cortisol gives you the “Fight or Flight” feeling when you are in stressful situations.
The Pancreas
Finally, the last part of the Endocrine System is the Pancreas.
Even though it’s often forgotten, the Pancreas does a fantastic job. Two jobs actually!
It has both an Exocrine function and an Endocrine function. The Exocrine function produces digestive enzymes straight into the Duodenum, which make the break down of food more accessible. The Endocrine function is supported by Alpha & Beta Cells that make Insulin, and Glucagon that controls the Blood Sugar Levels.
So when you eat food, your blood sugar levels increase, and the Beta Cells push Insulin into the bloodstream. Once in the blood, Insulin instructs the cells to absorb the glucose and use it as energy. This will, in turn, reduce the blood sugar levels.
But if you’ve been starved for a while, your blood sugar level decreases and the Alpha Cells start secreting Glucagon.
Now the Glucagon is smart because it knows that the Liver has a Glucose Reserve known as Glycogen. With this in mind, the Glucagon attaches to the receptor cells on the Liver. Here the Liver activates enzymes to break down the Glycogen into glucose and release it into the bloodstream. This gives us the right blood glucose level between meals.
And that’s it! The entire Endocrine System is done!
Did you find these notes helpful? Make sure to read the rest of the Anatomy & Physiology Series
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