Introduction to the Endocrine System

hi everyoneon Dr Mike here in this video we're taking a look at an introduction to the endocrine system we're going to talk about what hormones are what triggers their release how do they get transported through the bloodstream all the various different types The receptors and so much more let's take a [Music] look all right to begin we need to Define what the endocrine system actually is a lot of us have an idea but it has a very specific definition and that definition is it's a collection of cells tissues and glands that release chemical Messengers into

the blood string so a couple of points here they can be cells tissues and glands meaning that while we might have explicit what we call endocrine tissues meaning their job is to release these chemicals into the bloodstream we've got other cells and tissues of the body which might perform one function but also an endocrine function so for example the stomach you don't think about it as being endocrine tissue you go well that's part of the digestive system not the endocrine system it tears Foods apart and chemically digests it so that we can absorb the various

nutrients but the stomach also produces chemical Messengers that it drops into the bloodstream which means that the stomach can be defined as endocrine tissue now these chemical Messengers we call hormones if they are dropped into the bloodstream that's important because there are many chemical messengers of the body and we know that the nervous system has chemical Messengers that it releases from neurons that Target tissues directly and they aren't called hormones they are called neurotransmitters so it's important to understand this because the endocrine system is a communication Network and so is the nervous system so if

this is an endocrine cell tissue or gland it's going to be releasing a chemical messenger like I said which we call hormone which will jump into the bloodstream and travel to areas that will have or other tissues that will have receptors specific for it so it can bind to it right so that is endocrine now let's compare this to the nervous system because like I said it's a communication system as well so if for example this was a neuron so there's the cell body there's the axon and there's the terminal so very simplistic neuron and

there is its Target cell with its receptor that neuron is going to be releasing its chemicals which are neurotransmitters that will bind now again they're chemical Messengers what are the differences between the endocrine and the nervous system as modes of communication well firstly if we look at the nervous system it is fast right it travels extremely quickly compare that to the endocrine system which is a lot slower so those hormones are only going to travel at the speed in which the blood is moving so that's dependent upon heart rate for example and blood pressure so

endocrine lot slower the nervous system is direct it is sending a signal from A to B and there's nothing in between but have a look at the endocrine system the endocrine system because it's going through the bloodstream we know that there are all these little stop offs on the way to the station that this hormone can get released so it is not direct it is definitively indirect and that's really really important a couple of other things is that when it comes to thinking about the um acting time of the hormone or in this case the

neurotransmitter it is short acting so you release the neurot transmitter it has its effect it then gets recycled or gobbled back up by the cell or the neuron and it's short acting but hormones are longer acting I don't want to say long because then I'll have to give you a time frame but it's longer acting so they have a longer halflife in the system compared to neurotransmitters so as you can see there are some important differences between the endocrine system and the nervous system as communication networks now I want to talk to you about what

these hormones actually are so I said they're chemical Messengers but what are the different types of chemical Messengers well firstly let's have a look at the categories so first thing is there's three major types of hormone categories you can have protein SL peptide hormones you can have steroid hormones and you can have amino acid derived hormones amino acid and to be more specific here these are from one amino acid called tyrosine so these are the three categories so we've got protein based lipid based but no carbohydrate based there are no solely carbohydrate based hormones that's

important some of the steroid hormones can have sugars attached to them sugar moities and we can call them uh um and proteins as well and so that would be called glycoproteins for example but generally speaking no carbohydrate hormones so let's have a look at these differences firstly protein peptide let's distinguish between the two broadly if you've got more than 100 amino acids it's recognized as a protein if it's got less than 100 amino acids it's recognized as a peptide so that's an important first point the protein or peptide based hormones they are the most abundant

so if you ever get a question in an exam asking you to classify the type of hormone and you have no clue just guess protein or peptide based so what are some examples all right the examples of the protein or peptide based hormones are all of the pituitary hormones so all the hormones released by the pituitary gland they are protein or peptide based what are the these hormones all right so for example if we have a look at the anterior L of the pituitary gland that's going to include the gonadotropins and those gonadotropins are follicle

stimulating hormone and lutenizing hormone we've got the adrenocorticotropic hormone we've got growth hormone we've got prolactin and we've got if we have a think about the last one thyroid stimulating hormone so these hormones from the anterior pituitary are protein or peptide based they also include those of the posterior pituitary and these include antidiuretic hormone and oxytocin so these are protein and peptide based but that's not all importantly other protein peptide based includes those of the pancreas so importantly what are these well these are going to be insulin and glucagon there we go there is not

an exhaustive list but a good list of the protein peptide based hormones that you should be aware of when it comes to steroid based hormones these are made from cholesterol so not amino acids but cholesterol so they are lipid soluble not water soluble lipid soluble and if we ever think about where they are produced most of these steroid hormones come from the adrenal cortex come from the adrenal cortex now let's have a look at this the adrenal cortex if I have a look at and I'm going to incorporate this here as well so I'm going

to just draw up uh I'll do it in blue if we have a look at the I'll draw it over here so I've got the the kidney and on top of the kidney is the adrenal gland and the adrenal gland has two major sections it's got this outer section here which we call the cortex so that's the adrenal cortex and then you've got the inside side here which is what we call the adrenal Medela so the adrenal cortex it produces steroid-based hormones made from cholesterol now I'm going to draw this up like this so imagine

we take this whole adrenal gland and do a cross-section and have a look into it what you're going to find is this you're going to H have the medala here so there's the medala and and then you're going to have the cortex here so I'll write it like this there's the cortex now the cortex has three major layers associated with it we've got the glosa and it's called the Zona glosa the Zona facul and the Zona reticularis and they produce different hormones so the glosa produces a hormone called aldosterone the facul produces a hormone called

cortisol and the reticularis produces androgens which are male sex hormones now all three of these cuz they're in the cortex they are steroid hormones now importantly Oster is also known as a Minella corticoid so this is just for completion sake right minell corticoid cortisol is known as a gluco corticoid what's the difference minello mineral because what aldosterone does is it tells sodium which is a mineral to be reabsorbed corticoid it's at the cortex so that's the cordi and oid tells you it's a steroid so cortisol gluco it plays around with blood glucose levels increases blood

glucose levels again corticoid because it's in the cortex and it's a steroid all right so these are the steroid hormones of the adrenal cortex but that's not all of them what we also have is those of the gonads which is obviously going to be the ovaries and the testes so they also produce steroid hormones the steroid hormones that they produce include estrogen progesterone and testosterone so we've got our steroid hormones here now when it comes to the amino acid derived from tyrosine it's important because I've drawn the medala here now I know it's a bit

of an angle cuz that's how unfortunately when I'm looking looking at this whiteboard it's a bit of an angle but just so you know the amino acid derived actually come from the adrenal medala the adrenal medala and the adrenal medala produces the catacol amines catacol a means now what are the cacol the camines now specifically the medala produces one important camine which is or you could say two noradrenaline SL adrenaline similar but different mostly it's adrenaline that gets released from the adrenal medala but camines also include dopamine now dopamine isn't produced here at the adrenal

Medela but it is a catac colomine and it is amino acid based tyrosine based but there's another hormone that is amino acid based and this is from the thyroid gland and the hormone from the thyroid gland is unsurprisingly thyroid hormone or you could argue thyroid hormones which is T3 and T4 just remember T3 is the active form right so what we've got here are the different types we got proteins peptides steroids and amino acids you can see the most abundant are the protein peptides steroid based in the adrenal cortex and gonads and the amino acid

based include the catac colomines noradrenaline adrenaline and dopamine and also the thyroid hormones what I want to do now is have a look at what stimulates these hormones to get released right so we've spoken about all the different types let's talk about what stimulates these hormones get released so there are three main stimuli that trigger hormones to get released so let's draw these up first and then we'll write them down we've got now you might go hey I remember you drawing this thing before so let's have a look let's see what it is okay hopefully

you're looking at this gun I think I know what this is okay there's the first one let's drw the second one what's this one doing hm this might look a little bit similar to what I dropped before okay there's the second one let's drop the third one all right looks very similar to the second one doesn't it all right these are the three way so all of these down here are Endocrine cells tissues or glands that need to release hormones how are we triggering them to be released first one what we've got is a neuron

is triggering the adrenal medal to release a hormone do you remember what I just said before right what does the adrenal medala release the adrenal medala releases adrenaline the adrenal medala releases adrenaline that's a hormone and it releases that adrenaline into the bloodstream so here because there's a neuron triggering this hormone to be released this is called neural a neural stimulus so you can have neuron stim stimulating hormones to be released I'm going to come back to this one in one second but let's first go through all of them here we've got an endocrine cell

or tissue or gland releasing a hormone that's going to jump into the bloodstream and in the bloodstream it's going to travel to another uh uh endocrine cell tissue or gland and tell it to release its hormone into the bloodstream okay okay because it's a hormone telling a gland to release another hormone this is a hormonal stimulus so that's the second one a hormonal stimulus and the third one looks very similar this here can be a nutrient right or a mineral for example some substance so let's just say it's a nutrient and let's say this nutrient

is glucose and this glucose nutrient is jumping into the bloodstream and binding to what would glucose bind to the pancreas right beta cells within the pancreas and it tells it to release insulin and then that insulin is going to jump into the bloodstream so that's not a hormone that's a nutrient and so we call this a humoral stimulus a humoral stimulus so these are the three different types of stimuli hormones can be released from neurons triggering them other hormones triggering them an important Point here when a hormone is getting released to tell another hormone to

be released it generally has in its name the suffix Tropic it's Tropic like um uh you could say uh an example of this is going to be corticotropic hormone it goes it's released by the hypothalamus and tells the anterior pituitary to release adrenocorticotropic hormone so corticotropic hormone tells another hormone to be released act adrenocorticotropic hormone you might say that's also got Tropic in it that's right because its job is to travel to the cortex to tell it to release more hormones so when you see Tropic just know it's not the last hormone to be released

and then finally you've got the humeral when it's a nutrient or it could be calcium right if it's low blood calcium in this instance it would be traveling to the parathyroid gland to release parathyroid hormone to try and increase blood calcium levels so this is the stimulus I said I was going to tell you more about this yes I do want to tell you more about this because it's an important thing to understand because none of these systems work in isolation we've got here a neural stimulus to a hormone being released this neuron because it

ultimately is releasing triggering the release of adrenaline this has got to be part of the sympathetic nervous system so I'm going to just take a little bit of a detour to talk about the sympathetic nervous system because it is important so here's the brain from a side view and we're going to have a look and there's the brain stem and spinal cord and for good measure there's the cerebellum what you probably already know is that the autonomic nervous system has two divisions your sympathetic which is fight ORF flight and your parasympathetic which is rest and

digest now your sympathetic the final flight it's neurons exit the spinal cord from either the thoracic or the lumbar re uh region that's why the sympathetic nervous system I'll write the whole thing for you that's why the sympathetic nervous system is sometimes also referred to as the thoro lumbar system now I said that there's neurons that come out from either the thoracic or Lumar let's draw up this particular neuron coming out of the thoracic and there's one neur and it speaks to a second neuron that will go to a Target tissue let's say it's the

heart now what generally happens here is that the first neuron called the preganglionic neuron releases a hormone called uh sorry a neurotransmitter again just another chemical messenger but in this instance released by neurons called acetal choline that binds to the postganglionic which releases another neurotransmitter which is called nor adrenaline or if you're from the US nor epinephrine all right all of them are tunon chains except in one instance there is one instance where a neuron will exit and this neuron will not sign upse with another neuron it will sign ups in the medala of the

adrenal gland right so let's draw the kidney underneath here right and so this pre gang urine is just like that one so what does it release it releases acetyl choline so it's releasing acetyl Coline here and it's the aetl choline that triggers this adrenal medala and this adrenal medala is acting just like the postganglionic neuron triggering it so if it's acting like the postganglionic neuron what does that release noradrenaline so in this instance it is the hormone version of noradrenaline which is adrenaline and because adrenaline jumps into the bloodstream it goes to the whole body

and you have an entire body effect of fight or flight I thought that was an important point to highlight now what I want to talk about is how once these hormones are released they we know that they jump into the bloodstream how do they travel through the bloodstream so couple of points what were the different classes I said right so let's just say we have peptide protein we have steroid and we have amino acid specifically tyrosine based right how do these travel through the bloodstream so firstly peptides as you probably know proteins are mostly negatively

charged and as we know water has both a positive and negative charge so if you don't know this already water being hydrogen oxygen hydrogen H2O the hydrogen have a slight positive charge water has a slight negative charge now proteins and peptides which are these threedimensional folded structures are mostly negatively charged and the negative charge loves the positive charge of water so we say that proteins love water they're hydrophilic so they can jump straight into the bloodstream which is mostly water and mix beautifully so peptides are actually they travel from freely within the bloodstream freely within

the bloodstream so that means not bound not bound to something to carry it steroids on the other hand they're lipid soluble and we know fats and lipids don't like water so they need to be bound and they they are bound to plasma proteins and there's many different types of plasma proteins one important one is albumin but pretty much all of them are produced in the liver most of them are produced in the liver and why is this important it's important because if your liver doesn't function very well anymore because of hepatitis or therosis or whatever

liver disease that somebody might get it's going to affect the way these steroid proteins are bound and can travel and why is that important it's important because if you compare bound to free proteins the active form is the free protein so regardless of the two right free hormone I said free protein free hormone is the active hormone now you might think so that means none of the steroid hormones are active not when they're bound but they can be released and they become free so that also means that a good thing for the steroid hormones is

they have a longer halflife in the bloodstream they have a longer halflife meaning they don't get destroyed as quickly and that's because they're carried by these proteins right and they don't get targeted for basically metab metabolism and get to ex to be excreted longer halflife but it also means that it increases their pull available pool because they're bound to those proteins but if the liver doesn't work uh very well these proteins aren't produced and the steroid hormones have a shorter half life and they don't work and then the balance of free to bound is off

and there can be a problem there what about the amino acid ones well these the catac colam meines if we think of the catacol meines and I'll just tell you what they are again just in case you forgot that's adrenaline and nor adrenaline and dopamine they are free right they're transported freely however I said there's another amino acid based uh hormone what was it do you remember it was thyroid hormone T3 T4 so again I should say thyroid hormones now interestingly T3 T4 tells you about how many iodine molecules are attached to it right it's

important because once iodine is bound to this tyrosine based hormone it no longer is water soluble so it needs a carrier protein it needs a carrier protein CU it's no longer water soluble so keep that in mind the peptides free floating not bound to carry proteins steroids they are a good thing is that gives them a longer half life and keeps a pool of them available however the active form is the free form only if the liver doesn't work steroid based hormones can go off in the bloodstream and the amino acid based ones the cacola

means they're free floating but the thyroid need to carry protein so now they're transporting the bloodstream they get to their target tissues what happens at the Target tissue this is really important because if you have a think about hormones in the bloodstream their concentration is really really low so hormone concentration hormone concentration generally sits at around about 10 -7 to 10 -12 molar and you might think what the hell is that all right let's just put this into context 107 if you've got 1.0 molar and I want to figure out what 107 is I'm going

to move it seven decimal places into the negative 1 2 3 4 5 6 7 so there's the new decimal point and I'm going to put zeros under here so that's the concentration of 10 the7 it's 0.000000001 molar that is the pretty much the strongest concentration of hormones that you'll get in the blood 10 -2 well you're going to have to add what 7 8 9 10 11 12 5 more zeros in that direction right so my point is that they're in very low concentration what does this mean it means two things that hormone receptors

hormones and their receptors must have strong affinity and specificity then spell that right specificity okay so first Affinity Affinity is how strongly will it bind to the receptor so that means the hormone and the receptor must have a perfect key lock mechanism so it must bind really really tightly so that's the first thing because it's such low concentration second thing is its specificity it needs to be very specific the hormone must be very specific to the receptor so that you don't get any cross hormone binding or you don't get other chemicals binding to it so

the uh specificity must be high as well two important things now once the hormone is bound to the receptor well firstly where are these receptors located that's a really really important point so where are these receptors located let's draw up a cell there's a nice big cell and there's the nucleus inside of that cell and we know that inside the nucleus we've got our DNA all right I said you've got different types of hormones right we've got our peptides we've got our steroids we've got our and I'm going to break up the amino acids into

the two right so camines and the thyroid hormones so we're going to go through these so firstly peptide because peptides are water soluble they were transported freely right but when they get to a cell remember the cell is surrounded by a phospholipid Bayer so a fatty layer the peptides cannot pass through the fatty layer so they must bind to what we call membrane receptors membrane receptors so peptides peptides will bind to membrane receptors steroids steroids are lipid soluble they are bound to proteins in the blood stream then they are released and they can because they're

lipid soluble they can be transported through the layer the phospholipid B layer and they will bind to receptors in the cytoplasm and those receptors that they bind to generally are associated with what we call transcription factors and they then will once they're bound once they're bound will translocate into the nucleus and then have its effect on the DNA transcribing then translating the DNA this is going to be steroid and like I said this is a cytoplasmic receptor cytop plasmic receptors and generally they're transcription factors all right so we've done that what about the catacol meines

the catacol meines look like the peptides so they are membrane receptors because amino acids and peptides are pretty much the same thing however thyroid hormone is different thyroid hormone needs to get because it's got to carry a protein right it's got to carry a protein because of the iodine that's attached to it then when it gets to the cell it's released but it's not lipid soluble so thyroid hormone even though it acted like it was lipid soluble in the bloodstream can't just transport through the membrane so it needs specific membrane Transporters so membrane Transporters and

this allows for thyroid hormone to enter the cell and then the thyroid hormone can freely diffuse into the nucleus and in the nucleus it binds to receptors that are on the DNA so these are what we call nuclear receptors nuclear receptors and again they're going to help transcribe DNA they're going to say turn things on turn things off so we can tick that one off so again the peptides and the catac colomines they both work together through the membrane receptors the steroids they work through the cytoplasmic receptors and the thyroid hormone works via the nuclear

receptors brilliant now they can have their particular effect all right that is my introduction to the endocrine system I hope that makes sense I'm Dr Mike