Sign In | Subscribe

Enter your Sign on user name and password.

Forgot password?
  • Follow us on:
Start learning today, and be successful in your academic & professional career. Start Today!
Loading video...
This is a quick preview of the lesson. For full access, please Log In or Sign up.
For more information, please see full course syllabus of AP Biology
  • Discussion

  • Study Guides

  • Download Lecture Slides

  • Table of Contents

  • Transcription

  • Related Books & Services

Lecture Comments (11)

0 answers

Post by Quazi Niloy on December 15, 2014

Hello Dr. Eaton

I hope all is well

Around 62:80 you put grey matter is inside and white matter is outside. This is a contradiction to your earlier statements

I hope you can fix the problem.

Quazi Niloy

1 answer

Last reply by: Dr Carleen Eaton
Tue Mar 12, 2013 1:07 AM

Post by Joao Carlos Gomes Neto on March 10, 2013

Hi, I cant watch from the point where you start talking about the thyroid. The video always goes back to the introduction again.


1 answer

Last reply by: Dr Carleen Eaton
Thu Jun 21, 2012 12:22 PM

Post by Vinh Dong on June 1, 2012

Thank you so much for these lectures. I'm using these lectures as a basic foundation for MCAT study, and so far I'm doing better on my practice tests. These lectures are hopefully my secret weapon into getting into medical school.

1 answer

Last reply by: Dr Carleen Eaton
Sat Feb 4, 2012 4:34 PM

Post by Dorine Lantimo on February 3, 2012

Do men produce oxcytocin?

1 answer

Last reply by: Basil Khuder
Sat Nov 24, 2012 8:04 PM

Post by Senghuot Lim on June 12, 2011

i know that she's smart and all. however, she speaks and write fast...i'm okay with it...but i would suggest her to slow down a bit because most people that pay money here are usually have trouble in class or need more resource other than the teachers or books.

1 answer

Last reply by: Dr Carleen Eaton
Sun Apr 17, 2011 4:46 PM

Post by Billy Jay on April 17, 2011

I don't see the Reproduction Lecture. Any idea when it'll be posted? Thanks.

The Endocrine System

  • Endocrine glands secrete hormones into the bloodstream. Hormones exert effects on target cells.
  • The pancreas produces insulin and glucagon. Insulin decreases the level of glucose in the blood, whereas glucagon increases the level of glucose in the blood.
  • The secretion of hormones by the anterior pituitary gland is regulated by the hypothalamus. The hypothalamus also produces oxytocin and antidiuretic hormone (ADH), which are stored in and secreted by the posterior pituitary.
  • The anterior pituitary secretes hormones, called tropic hormones, that act on other endocrine glands as well as hormones with direct physiological effects.
  • The adrenal cortex secretes mineralocorticoids and glucocorticoids. The adrenal medulla secretes epinephrine and norepinephrine, which trigger the fight or flight response.
  • The thyroid produces thyroxine, which increases the metabolic rate, and calcitonin, which lowers blood calcium levels.
  • Parathyroid hormone is released by the parathyroid glands in response to decreased blood calcium levels. PTH stimulates the release of stored calcium from the bones.
  • The ovaries produce estrogen and progesterone. Estrogen is responsible for the development of secondary sex characteristics in females. Progesterone maintains pregnancy.
  • The testes secrete testosterone, which stimulates the development of male secondary sex characteristics.

The Endocrine System

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • The Endocrine System Overview 0:07
    • Thyroid
    • Exocrine
    • Pancreas
    • Paracrine Signaling
    • Pheromones
  • Mechanisms of Hormone Action 6:06
    • Reception, Transduction, and Response
    • Classes of Hormone
    • Negative Feedback: Testosterone Example
  • The Pancreas 15:11
    • The Pancreas & islets of Langerhan
    • Insulin
    • Glucagon
  • The Anterior Pituitary 19:25
    • Thyroid Stimulating Hormone
    • Adrenocorticotropic Hormone
    • Follide Stimulating Hormone
    • Luteinizing Hormone
    • Growth Hormone
    • Prolactin
    • Melanocyte Stimulating Hormone
  • The Hypothalamus and Posterior Pituitary 25:45
    • Hypothalamus, Oxytocin, Antidiuretic Hormone (ADH), and Posterior Pituitary
  • The Adrenal Glands 31:20
    • Adrenal Cortex
    • Adrenal Medulla
  • The Thyroid 35:54
    • Thyroxine
    • Calcitonin
  • The Parathyroids 41:44
    • Parathyroids Hormone (PTH)
  • The Ovaries and Testes 43:32
    • Estrogen, Progesterone, and Testosterone
  • Example 1: Match the Following Hormones with their Descriptions 45:38
  • Example 2: Pancreas, Endocrine Organ & Exocrine Organ 47:06
  • Example 3: Insulin and Glucagon 48:28
  • Example 4: Increased Level of Cortisol in Blood 50:25

Transcription: The Endocrine System

Welcome to

In this section of animal physiology, we will be focusing on the endocrine system.0002

Hormones are molecules that are released by endocrine glands, and they travel via the blood stream to illicit a response from distant target tissues.0010

To understand endocrine glands, we use an example, and an example would be the thyroid gland.0023

The thyroid, which we will talk about in more detail later, secretes thyroxin or thyroid hormone into the blood stream.0028

And thyroxin goes on to affect target tissues and organs.0039

And the effect that it has on a tissue or organ maybe different depending on what type of tissue it is, so the results are overall an increase in metabolism.0050

And that results in an increase in temperature, heart rate and changes in other physiological processes.0062

So, one hormone can have multiple effects in the body.0071

Endocrine glands secrete substances, secrete hormones, into the extracellular fluid.0076

From the extracellular fluid, the hormone diffuses into the blood stream.0084

So, here, I am talking about thyroxin being secreted into the blood stream, but to get a little more subtle about it,0090

what happens is that the hormone is secreted by the thyroid cells into the fluid surrounding the cells0095

and then, from there, can diffuse into the blood stream and then, be delivered throughout the body.0102

Although, the hormone will go throughout the body via the blood stream,0108

it is only going to affect target tissues that have a receptor for that particular hormone.0111

Exocrine organs or exocrine glands secrete substances via ducts. They secrete them via ducts- secrete substances into ducts.0119

So, it is exocrine, whereas, endocrine glands secrete substances into the blood stream.0133

Recall that we talked about the pancreas when we covered the GI tract, when we covered the digestive system.0145

And I said that the pancreas is both an exocrine organ and an endocrine organ.0151

So, to give you another example of an exocrine gland- sweat glands, salivary glands. They secrete substances into ducts.0157

Now, the pancreas, as we will talk about shortly, is both an exocrine gland and an endocrine gland.0164

And its exocrine function is that it secretes digestive enzymes via the pancreatic duct into the small intestine such as lipases and amylase.0181

However, it also secretes - so this is digestive enzymes via ducts - hormones -0192

we will talk specifically about these hormones, insulin and glucagon - into the blood stream.0203

So, the pancreas is actually both an exocrine gland and an endocrine gland.0208

Hormones have many different effects. Growth hormone regulates growth, as the name suggests.0213

Estrogen and testosterone affect development and reproduction. They stimulate the formation of gametes.0221

Other hormones like insulin and glucagon regulate blood glucose level. Calcium levels, metabolism as I mentioned, these are all regulated by hormones.0228

Before we go on and talk in detail about hormones and the endocrine system, I want to mention a couple of other terms you should be familiar with.0239

Endocrine system is one way of signaling. The nervous system is another method that the body has for signaling and coordinating activity.0248

A third mechanism is what is called paracrine signaling, and in paracrine signaling, a substance is released by an organ; but it affects only nearby cells.0257

So, this is local regulation, so a substance secreted and affects nearby cells. It diffuses over and affects only cells nearby.0269

This is endocrine signaling where the substance is released, but it diffuses into the blood stream and can go all over the body. That is endocrine.0286

Whereas, in paracrine, I have an organ with cells, and it is secreting substances; and these substances only affect other cells that are nearby.0296

They are not going to go out into the blood stream and go all over the body.0308

Finally, pheromones also are involved in signaling, and these are a means of communication, but often, it is to signal members of the same species.0314

For example, pheromones may be used to mark territory for an animal or to attract a mate.0334

So, these are method of communication that is not just within the body.0342

But, it can also signal another member of the species that it is that individual's territory, or that individual is available for mating, to attract a mate.0348

So, this is another type of signaling.0359

OK, now, I am going to review mechanisms of hormone action, and we talked about this in detail in the lecture on intercellular communication.0365

If you have not already watched that, I suggest that you watch that.0378

However, I am going to review the major points so that you can understand endocrine action.0382

In this lecture though, we are going to focus more on specific endocrine organs, the hormones they secrete and the effects of those hormones.0386

But just looking at the molecular mechanism of hormone action, as I said,0395

cells can communicate with each other through both electrical signals as with the nervous system and through chemical systems.0400

And hormones are a chemical signal, so substances released from one cell such as a cell in the thyroid gland.0408

Thyroxin is released, travel and go through a several step pathway to illicit a response in the target cell.0417

Now, there is two major mechanisms of action. I am going to talk about the first one here where it is a more complex pathway.0426

And this pathway, there are several steps. The first one is the reception of the signal.0435

The second is the transduction of the signal, and the third is the response phase.0447

Recall that in the first step, the ligand, which is a molecule that binds to a target cell, reaches the target cell, binds to the receptor.0456

So, that is the reception phase, and right now, I am just looking at extracellular receptors, receptors that in the cell membrane.0473

But, we will talk about intracellular receptor soon, so this first phase is binding.0481

This binding often causes, it binds, and the result is a conformational change in the receptor that may activate another molecule.0486

So, there might be a conformational change in this receptor that causes it to be able to bind this molecule.0497

This molecule may in turn activate via an enzyme that activates another molecule that, then, cleaves another molecule and on down.0504

And what this second phase is, is cascade of one molecule activating another molecule is transduction.0511

Recall, we talked about signal transduction pathways.0518

And during these pathways, the message is passed from the exterior of the cell to the interior and at the same time, the signal is amplified.0522

And signal transduction pathways often involve, so this step two often involves second messengers such as cyclic AMP or IP3 or diacylglycerol.0533

So, first phase, the signal is received.0552

The second phase, the signal is transduced or sent from the place where it has been received to the interior of the cell.0555

Finally, that message is passed all the way to the interior.0564

And here, we have the nucleus, and what this message may do is result in increasing the level of transcription of a gene.0568

It may decrease the level of transcription of a gene, or it may act on the protein level, so gene or protein level.0577

So, the response maybe to activate an existing protein or inactivate it.0587

Now, this is the situation for the extracellular receptor. However, some hormones can cross the cell membrane.0595

There are several major classes of hormones. Two, in particular, that we will focus on, the first two, the first one are steroid hormones.0606

Now, steroid hormones are derived from cholesterol. Therefore, they are fat soluble.0618

They can cross the cell membrane. An example is the hormone cortisol.0624

The second class of hormones are the peptide hormones.0631

The peptide hormones are generally insoluble to the cell membrane, so cannot cross the cell membrane. An example is insulin.0635

There is a third class of hormones that are amine hormones derived from the amino acid tyrosine, and sometimes these are fat soluble.0653

Some can cross the cell membrane. Others cannot, so it depends on this one.0662

A peptide hormone is going to have to bind to an extracellular receptor and use signal transduction to get its message into the interior of the cell.0670

Now, the situation is different for a steroid hormone.0679

For a steroid hormone, the receptor might actually be here inside the cell, and then, this steroid hormone can enter the cell,0682

bind to the receptor, travel to the nucleus and act directly as an initiator of transcription or to shut off transcription, sometimes, even permanently.0693

It does not need to use this mediator of a signal cascade. In fact, some steroid hormones even have receptors that are in the nucleus.0707

So, what will happen is, the hormone will travel in, bind, and it is already ready to go to activate or turn off transcription.0718

These are two mechanisms of hormone action.0731

Another topic involving mechanism of hormone actions is the idea of negative feedback inhibition.0739

We have touched upon negative feedback a few times in the course.0749

But, just to remind you how this would work and put it in the context of hormones, is that the production of a hormone will cause a response.0753

That response in negative feedback or some aspect of that response will turn around and0764

decrease the production of the hormone so that you will not end up with too much hormone.0772

You will not end up with too little. It is a loop of regulation.0779

Let me explain this, giving in an example of testosterone. Testosterone is produced by the testis, and the signals starts up here in the hypothalamus.0783

And the hypothalamus we will talk about regulates the anterior pituitary.0799

In this case, it releases a hormone called gonadotropin-releasing hormone that acts on the anterior pituitary.0804

You do not need to remember all these details right now just talking about negative feedback.0814

The anterior pituitary, then, releases luteinizing hormone, which acts on the testes, and the testes in response produce testosterone.0818

Testosterone exerts a variety of effects that we will talk about, so it exerts its effects on target organs.0833

However, it also suppresses further production of testosterone. It suppresses the hypothalamus from initiating this flow of testosterone production.0842

So, it inhibits the hypothalamus from releasing the GnRH, which will in turn cause the anterior pituitary to not release LH and on down.0861

The feedback is "alright, there is testosterone produced, turn around and stop production of more testosterone".0873

Otherwise, there would just be more and more testosterone, and there would not be anything regulating it.0883

The hypothalamus releases GnRH, and one of the results of that is testosterone. That is a response.0890

And that response goes and shuts off the initial signal.0897

We are going to now go on and talk about specific endocrine organs, and we are going to start with the pancreas, which I already mentioned briefly.0907

As I said, the pancreas is both an exocrine gland.0917

This is its function in digestion. It produces and secretes digestive enzymes via a duct and an endocrine organ.0920

In its endocrine function, the pancreas secretes two hormones.0929

And these hormones are actually produced by special groups of cells called islets of Langerhans, and there are two types.0934

There are beta cells, and the beta cells produce insulin, and the alpha cells produce glucagon.0943

These two hormones have opposing actions. They are what is called antagonistic hormones.0952

Beginning with insulin, let's say that blood glucose is high. You eat a bunch of sugary foods, and it is broken down; and blood glucose is high.0961

What this is going to do is it is going to trigger the pancreas to release insulin.0978

The effect of insulin is to bind to target cells and cause them to take up glucose, so it stimulates cells to take up glucose.0992

If the cells take the glucose up, it takes it out of the blood stream, and the result will be blood glucose is decreased.1004

So, we started out with high blood glucose. The result, the final effect of insulin is to decrease blood glucose levels.1018

And just to note that various cells can take up glucose, but in the liver, excess glucose, remember, is stored as glycogen.1026

So, that is another function of the liver. In the liver and some other cells, as well, the glucose is stored as glycogen.1034

Now, glucagon has the opposite effect. Let's say that you do not eat for a while.1050

You are getting pretty hungry. Your blood sugar gets low.1056

Now, you have blood glucose is low. That is going to stimulate the pancreas to release glucagon.1059

Glucagon tells the liver to break down the glycogen, so it stimulates the liver to break down glycogen, and the result is going to be a release of glucose.1077

The glycogen is broken down into glucose. Therefore, blood glucose is increased.1096

The effect of glucagon is to increase blood glucose. The effect of insulin is to decrease blood glucose.1104

You have probably heard of diabetes. Diabetes mellitus is a disorder in which there is either a deficiency of insulin or insulin is being produced.1114

So, there is enough insulin, but the cells are unresponsive to insulin.1127

The result of either form of diabetes is that blood glucose levels end up being too high.1131

And over time, a persistently high blood glucose level can cause damage to various organs to the kidneys, to the eyes, and to the cardiovascular system.1138

This is treated either by giving an individual insulin if they do not produce insulin or medications that allow them1147

to utilize the insulin more effectively if they are producing it, but their body is not responding to the insulin.1156

The second organ we are going to cover is the anterior pituitary, which produces many hormones.1166

The pituitary gland is located in the brain below the hypothalamus.1173

The hypothalamus regulates the anterior pituitary, and there are two parts of the pituitary: the anterior pituitary and the posterior pituitary.1179

And they have very different functions, so we are going to focus on these separately.1193

First, the anterior pituitary, and as I said, it secretes multiple hormones. Some of these hormones are what is called tropic hormones.1197

Tropic hormones act on other endocrine glands.1207

So, instead of just acting directly on a tissue like say thyroxin goes and acts on a tissues,1211

what a tropic hormone does is it acts on another endocrine organ, and then, it might stimulate the release of a hormone from that organ.1217

So, we will start out with some of the tropic hormones that are produced by the anterior pituitary for example thyroid-stimulating hormone.1225

And these are often known by their initials, so TSH. This is just often called TSH.1237

This is released from the anterior pituitary at a signal from the hypothalamus.1243

And it travels to the thyroid and stimulates the release of thyroid hormone, so stimulates the thyroid gland.1250

So, it is not having a direct effect on the physiology of the body.1262

What it is doing is stimulating another endocrine organ that will release a hormone that has physiological effects.1267

The second hormone that I am going to discuss is adrenocorticotropic hormone, and right here in the name, it tells you it is a tropic hormone.1277

ACTH stimulates the adrenal cortex, and as we are going to discuss, the adrenal gland even though it is one organ, it is almost like two glands in one.1291

The adrenal cortex and the adrenal medulla have different function, so it stimulates the adrenal cortex.1303

And the adrenal cortex in turn releases mineralocorticoids like aldosterone and glucocorticoids like cortisol.1314

The anterior pituitary also secretes hormones that are important for reproduction for example follicle-stimulating hormone, usually known as FSH.1324

It stimulates the production of gametes, so ova in females, sperm in males.1348

Luteinizing hormone, LH: this causes the rupture of the follicle in the ovary so that the ovum is released during ovulation.1366

I will just put "release of ovum", and it also stimulates the production of testosterone in males.1389

And we are actually going to talk in more detail about FSH, LH and testosterone, progesterone, estrogen in detail in a separate lecture on reproduction.1400

But for right now, you just need to know that these are released by the anterior pituitary and affect reproduction.1417

Additional hormones produced by the anterior pituitary are growth hormones, so growth hormone is not a tropic hormone. It has a direct action.1426

It stimulates the growth of bones and muscles.1440

A disorder called gigantism where people grow extremely tall can be a result of too much growth hormone.1447

There is a form of dwarfism that results from too little growth hormone and can sometimes be treated with growth hormone.1456

Prolactin: the name tells you lactin like milk, and it stimulates the development of the mammary glands1465

and also, the production of milk by the mammary glands, so stimulates development of mammary glands and milk production.1475

The last hormone we are going to talk about with the anterior pituitary is melanocyte-stimulating hormone.1497

In humans, this hormone stimulates the production of melanin, which is a pigment that is responsible for skin color, eye color, so pigment.1514

It is a pigment, so it stimulates melanin production.1525

And I do want to note that different hormones can elicit a different response in a different species.1530

So, in a human, melanocyte-stimulating hormone might have one function, and it elicits a slightly different response in another species.1537

Next, we are going to talk about the hypothalamus and the posterior pituitary glands.1548

As I mentioned, the hypothalamus regulates the anterior pituitary.1555

So, it secretes a bunch of hormones that act on the anterior pituitary, and it also secretes a couple of hormones with direct actions.1562

First, talking about regulation of the anterior pituitary, what happens is the hypothalamus releases hormones that are either stimulating or inhibiting.1570

They either stimulate or inhibit the anterior pituitary to release its hormones.1589

And this occurs by the release of the hormone from the hypothalamus into a group of capillaries.1595

So, it releases the hormone into a group of capillaries, so hypothalamus that surround it. There is a capillary network surrounding it.1603

Then, the hormones travel via the capillaries to the portal veins. It is called the portal veins.1615

And these portal veins connect to a second network of capillaries, and these capillaries surround the anterior pituitary.1621

And in that way, the hypothalamus can regulate the anterior pituitary.1632

Just to give you some examples of these hormones, one is thyrotropin-releasing hormone- TRH.1637

Another hormone that I already mentioned is...1653

So, this will act on the anterior pituitary and stimulate thyroid-stimulating hormone1657

to be released from the anterior pituitary, which will go on to stimulate the thyroid.1664

Gonadotropin-releasing hormone: GnRH stimulates the release of luteinizing hormone and follicle-stimulating hormone by the anterior pituitary.1670

There are many others. There are growth-hormone, releasing hormone, and it indicates that it stimulates the anterior pituitary to release growth hormone.1692

There is corticotropin-releasing hormone. It stimulates the release of ACTH by the anterior pituitary.1706

This is the function of the hypothalamus to regulate the anterior pituitary.1724

It is located in the brain near the pituitary, and what the hypothalamus does is it secretes...1735

The hypothalamus and the pituitary are right about in this region where the purple is.1743

So, the second function of the hypothalamus is to produce two hormones that are stored in the posterior pituitary.1748

It regulates the anterior pituitary, one function, and second function is to produce oxytocin and antidiuretic hormone.1755

These are stored in the posterior pituitary, and then, at a particular signal, they will be released or secreted by the posterior pituitary.1774

The function of oxytocin is to stimulate uterine contractions, so during child birth, the uterus will contract to cause the baby to be delivered.1791

So, it stimulates uterine contractions.1804

A second hormone, ADH, antidiuretic hormone is discussed in detail in the lecture on the excretory system.1814

But, recall briefly that what ADH does is it cause increased reabsorption of water by the kidney.1821

So, If there is an increase in osmolarity, the blood becomes more concentrated in terms of solutes that signals that the body needs to conserve water.1840

It needs to hold on to water, and that will trigger the release of ADH by the posterior pituitary,1850

which the ADH will, then, act on the kidney to cause the reabsorption of water.1858

The result is that more water will stay in the blood stream to dilute out the blood.1864

And the urine will become more concentrated thus, decreasing the loss of water into the urine.1869

OK, this was the hypothalamus and the posterior pituitary. The next set of glands we are going to talk about are the adrenal glands.1876

And the adrenal glands are located right on top of the kidneys, so on top of the kidneys, that is where they are located.1884

And the two parts of the adrenal glands, the adrenal cortex and the adrenal medulla,1895

even though they are together, they are functionally really separate endocrine glands.1902

They have separate cell types and separate hormones that they release.1905

So, these two sections are the adrenal cortex and the adrenal medulla.1911

So, the adrenal cortex, what happens is the anterior pituitary secretes adrenocorticotropic hormone, ACTH.1916

ACTH, then, acts on the adrenal cortex, and the adrenal cortex releases mineralocorticoids and glucocorticoids.1930

Mineralocorticoids are involved in maintaining sodium and water balance in the body. An example of a mineralocorticoid is aldosterone.1950

Recall that aldosterone, again, we talked about this in the lecture on the excretory system and the kidney,1964

but that aldosterone increases sodium - let's just talk about aldosterone for a minute - reabsorption in the kidney.1971

And when more sodium is reabsorbed, water will follow. So, this in turn triggers increased water reabsorption.1986

So, If an individual has low blood pressure or decreased volume, their body needs to hold on to more water.1996

So, low blood pressure, low volume, would stimulate the production and secretion of aldosterone,2005

which would signal the kidney to hold on to water to thereby, increase blood volume and blood pressure.2015

So, those are mineralocorticoids.2023

The second type of hormone produced by the adrenal cortex are the glucocorticoids.2025

An example is cortisol and the gluco right here, tells you that these are also involved in glucose regulation.2036

So, glucose levels are increased by glucocorticoids like cortisol.2047

They also have various other functions such as suppressing the immune system and the inflammatory response.2053

The second area of the adrenal glands is the adrenal medulla.2062

The adrenal medulla releases epinephrine, which is also known as adrenaline and norepinephrine or also known as noradrenaline.2070

These are responsible for the fight or flight response.2091

If a person or an animal is...there is a threat, somebody is chasing you, your body will release epinephrine and norepinephrine.2099

And what that is going to do is give you what you need to either fight the threat or run away from it.2111

And what you are going to need to do that? What you are going to need more oxygen, more sugar, more energy.2116

So, what is going to happen is an increase in heart rate, increase in blood pressure. Bronchodilation open up the airways, get more oxygen in.2124

Blood will be shunted to the skeletal muscles so that you can run or fight. Glucose levels will increase.2138

The pupils will dilate, so that is the fight or flight response.2147

Next, we are going to discuss the thyroid gland, which I mentioned as an example early in the lecture.2156

The thyroid gland is located in the neck, and it produces thyroxine and calcitonin.2162

So, recall that the anterior pituitary releases thyroid-stimulating hormone, which acts on the thyroid to promote the release of thyroxine.2170

Now, I am going to talk a little bit about some terminology that you will see.2185

We are focusing on thyroxine, but there is another hormone released by the thyroid gland.2187

The other name for thyroxine is T4, and the reason it is called T4 is there are 4 iodine atoms in it.2192

There is a second hormone produced by the thyroid gland called T3. As you can imagine, it has three iodine atoms.2201

Mostly, what the thyroid produces is T4, thyroxine, but in its target cells, that T4 is usually converted to T3.2211

For simplicity, I am just going to talk about thyroxine, T4, but this converts to T3; and then, that has an action on target cells.2219

The action of thyroxine ends up being to increase the metabolism.2231

Because iodine is required to make thyroid hormones, T3 and T4, a lack of iodine results in a condition called a goiter.2241

If there is iodine deficiency, lack of iodine, the result can be goiter, and in goiter, the individual with the condition will have a very enlarged thyroid gland.2251

In the U.S. for example, iodine is added to salt to help prevent iodine deficiency.2264

To understand the effects of thyroid hormone, let's look at the extreme cases.2273

Let's say somebody has too much thyroid hormone, and the result is a condition called hyper - so high thyroid - hyperthyroidism.2277

Their metabolism is going to be really increased, and the symptoms that they will get are they are going to be very hot.2292

They are going to lose weight. They might feel anxious.2304

Their heart is going faster. They might even have palpitations, problems sleeping2308

They are really hyped up, and thyroid increases the metabolism; but if that goes too far, too much, then, we can have some negative symptoms result.2313

Graves' disease is a cause of hyperthyroidism. It is actually an autoimmune disease.2324

What happens is there is an antibody that mimics TSH. Remember that TSH stimulates the thyroid to produce thyroxine.2332

Now, normally, the pituitary will not release TSH unless there is a stimulus like a person's cold.2348

And their body says "OK, we need to increase the metabolism here".2354

But this antibody is not under the normal controls of the body like TSH release would be.2359

It is an imposter but it is still can act on the thyroid and cause it to release thyroxine the way normal TSH would.2364

As a result, the person's metabolism is increased.2373

The other possibility is hypothyroidism, and in this case, we have decreased levels of thyroid hormone.2379

There is also an autoimmune disease that can cause this.2394

It has different mechanism in the opposite situation with Graves' where the thyroxine is not being produced in high enough levels.2397

These individuals, instead of losing weight, they will gain weight. They will feel cold.2406

They might have hair loss, lethargic. They will sleep too much.2413

They might become depressed. So, everything is decreased instead of increased like with hyperthyroidism.2420

The second hormone that you should be familiar with regarding the thyroid gland is calcitonin, and calcitonin decreases blood calcium level.2430

It does this by stimulating, so it stimulates the uptake of calcium by the bones.2448

So, the bones are reservoir to store calcium. Therefore, if blood calcium levels are high - so this is in the blood - then, calcitonin is released,2461

uptake of calcium by the bones, also, calcitonin stimulates the loss of calcium via the kidney and loss of calcium via the kidney, so it is excreted out.2477

Then, the result is blood calcium levels will be decreased, so it helps to maintain homeostasis in that way.2492

There is a set of four glands located right behind the thyroid gland in the neck, and these are the parathyroid glands.2505

The hormone they secrete is called parathyroid hormone or PTH.2513

PTH has the opposite effect of calcitonin, so we have the parathyroids. They release PTH, and PTH is going to increase calcium levels in the blood.2520

How does it do that? Well, what PTH actually does is stimulates the breakdown of bone to release calcium.2541

A second action is to stimulate the reabsorption of calcium in the kidney.2561

Calcitonin was causing calcium to be stored in the bone and was causing calcium to be lost by the kidneys,2577

so getting rid of calcium or storing it to decrease blood calcium levels.2591

Here is the opposite: increasing blood calcium levels by breaking down bone and having the kidney hold on to or reabsorb calcium.2597

It is a physiological antagonist, PTHs to calcitonin.2606

The last set of endocrine glands that we are going to talk about are the ovaries and the testis.2613

And these release the sex hormones: estrogen, progesterone and testosterone.2617

Now, we are going to again talk about this in more detail under the separate set of lectures on reproduction.2622

But, for completeness to just introduce this topic as part of the endocrine system, starting with the ovaries,2630

the ovaries produce estrogen and progesterone.2638

As it says right here, estrogen is responsible for the development of the secondary sex2647

characteristics in females during puberty and maintains those characteristics later on.2654

Secondary sex characteristics are not directly involved in reproduction, so it is not the actual production of sperm or egg.2659

But, secondary sex characteristics have to do with attracting a mate or their characteristics that are different in males and females.2670

So, that is part of estrogen's job is to cause the development of secondary sex characteristics during puberty2677

in females such as stimulating breast development also, a change in shape of the pelvis in females.2685

Estrogen plays a role on the menstrual cycle.2693

Progesterone, I just remember pro-gestation because it is a hormone that maintains pregnancy. It prevents the uterine lining from shedding.2697

It promotes an environment that will allow for the embryo and fetus to survive.2705

The testis secrete testosterone.2712

Testosterone stimulates the development of secondary sex characteristics in males such as facial hair, deepening of the voice and increase muscle mass.2715

And again, we are going to talk about more functions and how the progesterone, estrogen, testosterone work in this section on reproduction.2726

Now, though, we are going to go on and do some review on the endocrine system.2738

Example one: Match the following hormones with their descriptions.2742

Parathyroid hormone is the first one, and let's look at the choices:2748

Parathyroid hormone increases the metabolic rate, increases calcium level in the blood, stimulates the fight or flight response, stimulates uterine contraction.2753

Remember parathyroid gland is located near the thyroid in the neck, and PTH or parathyroid hormone causes an increase in calcium level.2766

It causes bone to breakdown and release calcium into the blood, so the answer here is B.2778

Two: oxytocin. We have choice of increase the metabolic rate, stimulate the fight or flight response or stimulate uterine contractions.2785

And in fact, oxytocin, which is produced by the hypothalamus and stored in2795

the posterior pituitary does stimulate uterine contractions allowing for child birth.2800

Thyroxine: as we talked about, thyroxine increases the metabolic rate, which leaves us with epinephrine and norepinephrine,2807

which are secreted by the adrenal medulla and stimulate the fight or flight response.2817

Example two: why is the pancreas considered to be both an endocrine organ and an exocrine organ?2827

Well, let's start with exocrine. Remember that an exocrine organ secretes substances via ducts.2834

In its function as part of the digestive system, the pancreas secretes digestive enzymes such as lipases and amylases through the pancreatic duct.2847

Therefore, it is an exocrine organ.2868

In its role as an endocrine organ, the pancreas also secretes hormones directly into the blood stream.2879

And these hormones involve the regulation of glucose. In the islets of Langerhans are produced insulin and glucagon.2892

Therefore, the pancreas is both an exocrine gland and an endocrine gland.2904

Example three: describe how the opposing actions of insulin and glucagon regulate the level of glucose in the blood.2910

Remember that these are antagonistic hormones. They cause opposite responses in the body.2918

Let's start out with when the blood glucose is high, so blood glucose increases.2929

If it is too high, the pancreas will release insulin. Insulin will cause uptake of glucose by cells, so it takes that glucose out of circulation.2937

The result by picking up the glucose and some of it is stored in the liver as glycogen2955

is going to be that the blood glucose level drops, so decreased blood glucose.2962

Let's say that the blood glucose level becomes too low, so low blood glucose. The result is going to be that the pancreas will release glucagon.2970

Glucagon is going to stimulate the breakdown or the hydrolysis of glycogen in the liver.2988

Glucose will be released into the blood stream. Therefore, blood glucose will go up.2999

So, between the two, glucose can be very tightly regulated.3008

Glucose gets a little too high, insulin is released. Glucose gets a little too low, glucagon is released.3012

Example four: symptoms of Cushing's syndrome include weight gain, thinning of the skin and easy bruising.3026

The cause is an increased level of cortisol in the blood.3034

Over secretion of hormones from which gland could result in Cushing's syndrome?3038

Well, recall that cortisol and other corticosteroids are released by the adrenal cortex.3042

Therefore, if an individual had over-secretion of hormones by the adrenal cortex,3051

they would have increased cortisol level and could exhibit these symptoms.3060

That concludes this discussion on the endocrine system here at

Thank you for visiting.3071