Lecture 10 - Endocrine System

Added: 2026-05-15

[00:00:01]hi everyone welcome back this week we finish off the course looking at chapter 17 where we focus on the endocrine system so for this week's material i've created a summary chart that you can fill in yourself there's also a completed version with all of the answers included i recommend you go through the lecture and try to fill this out as much as you can by yourself first use this to be able to consolidate all of the information that we've covered this week on this one chart and so the main outcomes for this week are to know the names of the various hormones know where they're produced know the stimulus for their production as well as the overall function of that hormone now similarly to the nervous system the endocrine system is also involved in communication where the nervous system is involved in rapid communication using neurotransmitters to be able to propagate those nerve signals around the body the endocrine system is involved in both the production and the secretion of hormones and those hormones are chemical messengers that once secreted are going to pass into our bloodstream and then circulate throughout the body to their final destination and similarly to the nervous system the endocrine system is also involved in positive and negative feedback mechanisms so today's main focus is going to be to look at the endocrine glands and discuss their main primary functions as well as the hormones that they secrete now next semester in biology we're going to be looking further into not only these glands but others in the body involved in the production and secretion of various hormones and looking at a variety of disorders so for this week i've tried to pare down the material as much as possible so the main focus is looking at the gland looking at the hormone that releases and thereby its primary function now as you can see from this diagram unlike other systems in the body the endocrine system is made up of structures that are relatively spread throughout the body and from this diagram it's difficult to be able to appreciate what it is that's connecting all of these various endocrine structures to one another and as i mentioned before endocrine glands are involved in the secretion of hormones into our bloodstream and so the structures connecting all of these different endocrine glands throughout our body those are the blood vessels that circulate our blood to reach all the various different tissues so that when a hormone is secreted and released into the bloodstream it can then circulate to reach its next destination now the main control center we've talked about this in other capacities right but now we're going to look at the hypothalamus in terms of how it controls the entire endocrine system the hypothalamus as you may recall is located within the diencephalon located inferior to the thalamus and anterior to the pineal gland now off of the hypothalamus we have another key gland the pituitary gland that is involved in the secretion of specific pituitary hormones that are then going to act on various glands and tissues as well the pineal gland is considered part of the endocrine system because it's involved in the production and secretion of melatonin the melatonin is that hormone that we talked about in the nervous system which is involved in regulating our sleep wake cycles now as we move inferior throughout the body we then come to the thyroid gland located in the anterior portion of the throat here just in front of the larynx the thyroid gland is involved in the production of your thyroid hormones as well as calcitonin which is involved in blood calcium regulation and if we look at the posterior aspect of the thyroid gland there are four small glands two located on the left and two located on the right known as the parathyroid glands and the parathyroid glands work in conjunction with the thyroid gland to help to regulate calcium levels and the parathyroid glands work in tandem with the thyroid to be able to help to regulate our blood calcium levels the thymus which is an endocrine structure located just ventral to the heart is involved in the production and maturation of our lymphocytes which are key cells involved in our immune response moving down to the abdominal cavity we have the adrenal glands the adrenal glands are located on top of each kidneys we have two adrenal glands and they are responsible for the production not only of norepinephrine and epinephrine as we saw with the sympathetic nervous system but are also involved in the secretion of our stress hormone cortisol as well as aldosterone the pancreas which we've talked about briefly involved in that negative feedback mechanism regulating blood glucose levels the pancreas secretes two different hormones we have insulin and we have glucagon and finally the gonads the ovaries and the female as well as the testes in the mail are both endocrine glands that are involved in the production of our sex hormones so if you recall in the body we have both endocrine glands as well as exocrine glands and we've spent a lot of our time thus far looking at exocrine glands those that secrete their substances to the outside of the body now with endocrine glands they are ductless they do not contain ducts as they secrete their hormones into the blood vessels that surround them and then those hormones circulate throughout the body to wherever they're destined now as i mentioned before the endocrine system is involved in slower communication than the nervous system and also a key component in regulating both positive as well as negative feedback mechanisms now in contrast the exocrine glands which we spent much of our time looking at so far they have ducts and those ducts are connected directly to the external surface of the body in some capacity so whether we're looking at our pseudoreference glands remember sweat glands both apron in the axillary region and genitalia or eccrine which are located everywhere else through the body both of them contain ducts similarly the sebaceous glands as well as the mammary glands salivary glands and cerumenous glands that are responsible for producing earwax and our external acoustic canals any exocrine gland that we've talked about so far they all have ducts that are responsible for secreting their substances to the external aspect of the body when we get looking today at the pancreas we're going to see how the pancreas has both an endocrine as well as an exocrine function now as i mentioned the hypothalamus is that main control center this is the main computer that's going to be calling the shots and the hypothalamus is responsible for producing and secreting hormones that are then going to communicate with the pituitary gland telling it what it should be releasing now you'll notice with the pituitary gland it extends directly from the hypothalamus this stock-like extension from the hypothalamus that extends to the pituitary is known as the infundibulum and the infundibulum contains two main structures we have axons of a very specialized group of neurons as well as blood vessels and as we move inferior looking at the pituitary gland you can see the pituitary gland is made up of two separate lobes we have the anterior pituitary gland and the posterior pituitary gland and the pituitary gland sits right inside this sphenoid bone in a recessed structure that's known as the cella turcica now the posterior pituitary gland is responsible for storing and releasing two specific hormones there are eight hormones in total we're going to cover six from the anterior pituitary and two from the posterior pituitary it's important for you to remember that the two hormones oxytocin and antidiuretic hormone that are released from the posterior pituitary are not produced here they are produced by the hypothalamus and the hypothalamus once it produces oxytocin and antidiuretic hormone and it transports those two hormones along these specialized neural secretory cells for storage in the posterior pituitary then when we need oxytocin or adh it's the posterior pituitary that's going to secrete those hormones into the bloodstream releasing them so that they can have their impact on the body now in contrast the way that the hypothalamus controls the anterior pituitary is that the hypothalamus secretes specific releasing hormones that signal the anterior pituitary to release the hormones that it wants it to release now for our purposes here at this point we do not have to worry about any of those releasing or inhibiting hormones the hypothalamus is going to secrete to control the anterior pituitary so in terms of knowing what's going to be causing the release of those six anterior pituitary hormones we're going to talk about at this point it's just going to be the hypothalamus once we move into biology next semester and we start to look at this system in more detail then we'll get into the specific hormones that then cause the release of those six anterior pituitary hormones so now we're going to take a look at those two hormones that are stored in the posterior pituitary and then move on to looking at the six anterior pituitary hormones now as i mentioned the posterior pituitary does not make the hormones that it releases right it only is involved in storage and ultimately secretion so the hypothalamus produces oxytocin an antidiuretic hormone also known as adh and then the posterior pituitary stores those two hormones until they are required at which time the posterior pituitary will release them into the bloodstream now with oxytocin oxytocin is involved in producing uterine contraction and we've talked about this feedback mechanism we just haven't referred to the hormone itself ultimately during labor and delivery the fetus's head is causing stretching of the uterine walls that sends signals back to the brain will wear specifically to the posterior pituitary to cause it to secrete oxytocin oxytocin then is released from the posterior pituitary travels throughout the body and ultimately causes the uterus to contract okay so that whole positive feedback mechanism of labor and delivery is involving oxytocin which is produced by the hypothalamus released by the posterior pituitary now oxytocin is also involved in producing contractions of other smooth muscle tissue and that is in the mammary glands specifically oxytocin is involved in causing milk ejection in mothers that are nursing there's two hormones that are involved in nursing one anterior pituitary hormone is involved in the production of milk and oxytocin is involved in actually causing those glands to eject the milk that's stored in them now with antidiuretic hormone it is released by the posterior pituitary as well but it is involved in regulating water balance and if you think about what a diuretic does something like coffee diuretics cause us to lose more water so an antidiuretic is going to do the opposite and so the antidiuretic hormone is going to be involved in causing us to reabsorb more water so we're keeping more of it in our blood and when we keep more water in our blood we increase blood pressure all right so oxytocin involved in both uterine contractions and mammary gland contractions an antidiuretic hormone is involved in increasing blood pressure by causing us to reabsorb water as i mentioned before with the anterior pituitary hormones the hypothalamus is communicating with it calling the shots by releasing specific hormones that are telling the anterior pituitary what to do we're going to get into more details of this next semester so for now as long as you understand that these six anterior pituitary hormones are all regulated and controlled by the hypothalamus then you're all set now the six anterior pituitary hormones are thyroid stimulating hormone adrenocorticotropic hormone follicle stimulating hormone luteinizing hormone prolactin and growth hormone now with the top four thyroid through luteinizing hormone these four hormones are all going to be stimulating other glands to secrete further hormones and so there's sort of an intermediate in a whole cascade where the hypothalamus signals the anterior pituitary and then the anterior pituitary releases one of these four hormones to then stimulate another endocrine gland to release another hormone with prolactin this is the hormone involved in milk production and growth hormone its main target structures are going to be our skeletal system and muscular system because it's involved in growth so with prolactin and growth hormone these two hormones have their final destination at a specific tissue or organ so first we're going to start off with a thyroid stimulating hormone so ultimately the thyroid stimulating hormone is secreted as a result of the anterior pituitary gland being signaled by the hypothalamus the thyroid stimulating hormone is then going to be secreted by the anterior pituitary and is going to pass through our bloodstream until it reaches the thyroid and this is one of them that's really easy to remember what its function is going to be because thyroid stimulating it's going to stimulate the thyroid to produce thyroid hormones and so it's involved in negative feedback regulation whereby if our thyroid levels are low the anterior pituitary is going to release more thyroid stimulating hormone to stimulate the thyroid to produce more thyroid hormone now our two thyroid hormones are t3 and t4 t3 is known as triidothyronine and t4 thyroxin now the difference between these two will get into structures more next semester but the t3 contains three iodine atoms and the t4 contains four iodine atoms and so these two are our thyroid hormones secondly our anterior pituitary can also secrete adrenocorticotropic hormone an adrenocorticotropic hormone is going to act specifically on the cortex of the adrenal gland so some of these anterior pituitary hormones you can use the name to be able to help you think what it's actually going to be doing so the adrenocorticotropic hormone is another anterior pituitary gland hormone which is released when the hypothalamus tells the anterior pituitary gland to release it as i mentioned the adrenocorticotropic hormone acth is going to be acting on the outer region of the adrenal gland which is known as the adrenal cortex if you recall when we looked at sympathetic activation we saw that there were specific sympathetic neurons that synapsed directly with the adrenal medulla and that was then responsible for secreting norepinephrine and epinephrine into our bloodstream so the cortex is the superficial region that surrounds the adrenal medulla and the adrenal cortex is responsible for the secretion of aldosterone which is involved in regulating water balance as well as cortisol which is our stress hormone then we have the gonadotropic hormones follicle stimulating hormone as well as luteinizing hormone fsh and lh they are both released by the anterior pituitary and are ultimately going to act on the gonads so the testes in the male and the ovaries in the female fsh is going to stimulate the follicle in either structures and the follicles are the structures where the sex cells are produced those gametes so with the follicle stimulating hormone it's going to be stimulating production of sperm and males and ovum in the females in contrast lh the luteinizing hormone is going to be stimulating those gonadal structures so stimulating the testes and the ovaries but it's to produce other hormones now it's important to mention that males and females secrete all three hormones on your chart we have estrogen as well as testosterone in females there's much more estrogen production and in males much more testosterone production but those levels vary from individual to individual now prolactin and growth hormone i've placed at the end here because these are the two anterior pituitary hormones that are going to be acting on specific structures prolactin acts directly on the mammary glands so prolactin is again produced by the anterior pituitary in response and is regulated by the hypothalamus so the hypothalamus tells the anterior pituitary produce prolactin so following delivery the infant that is feeding is going to be stimulating a positive feedback loop that will be signaling the hypothalamus to tell the anterior pituitary to release prolactin prolactin then will act on the mammary glands themselves stimulating the gland to produce milk where oxytocin comes into play is that oxytocin which is produced by the hypothalamus and released by the posterior pituitary gland it is going to transport through strain to ultimately cause each individual mammary gland to contract and secrete the milk so remember oxytocin think muscle contraction so it's involved in milk ejection and prolactin is involved in milk production and then lastly the growth hormone now i have a number of different functions here ultimately the growth hormone is released by the anterior pituitary it's regulated again by the hypothalamus and its main function is growth and so we can see it targets our skeletal system causing uh an increase in causing an increase in bone growth also within our muscular system increasing protein synthesis to increase skeletal muscle bulk it's also involved in the breakdown of glycogen and fat to be able to increase blood sugar so think about this if you're wanting to build up more cells you're going to need more atp how do we make atp through glycolysis krebs cycle electron transport and chemiosmosis so we need blood sugar and so growth hormone is going to be stimulating the liver to break down that stored up glucose that glycogen we can break down fat through beta oxidation to also increase atp synthesis so growth hormone is involved in stimulating the immune response and when we're stressed out we release a lot of cortisol from the adrenal gland and that cortisol is going to inhibit the action of the growth hormone thereby decreasing our immune response so now we're going to move from the pituitary gland to looking at the other peripheral glands located throughout the body starting off here we're going to look at both the thyroid gland as you can see here from an anterior perspective and the parathyroid glands which you can see located at the posterior aspect of the thyroid gland now the thyroid gland is responsible for the production of not only our thyroid hormones but also another hormone known as calcitonin the thyroid hormones are released in response to thyroid stimulating hormone which is released from the anterior pituitary and the way to think of your thyroid hormones both triado thyronine and thyroxine is that they regulate metabolism so if you have elevated levels of thyroid in your bloodstream you're going to have a higher metabolic rate and if you have lower levels of thyroid such as in hypothyroidism you are going to have a lower metabolic rate and as a result you're going to see either an increase or decrease in a lot of those different functions all linked to your metabolism now the para now there are other cells within the thyroid gland that are also involved in producing calcitonin and calcitonin is the other thyroid hormone that's involved in the regulation of blood calcium as i mentioned the thyroid and the parathyroid gland are both involved in helping to balance the calcium levels in our blood and i want you to think for a second what is calcium so important for and that would be in muscle contractions where calcium needs to bind to troponin to allow for the crossbridge to take place and in stimulating the transmission of an action potential from the synaptic end bulb of a neuron to some other either neuron or muscle cell so for both production so for both propagation of a nerve so for both propagation of an action potential as well as allowing a muscle contraction to occur calcium levels need to be regulated in our bloodstream to make sure that they are at optimal levels so we have four parathyroid glands two located on the left and two located on the right lobe of the thyroid the parathyroid glands are responsible for secreting the parathyroid hormone also known as pth now the parathyroid hormone is really important for helping to regulate calcium levels and it helps to ultimately increase blood calcium levels it's important for you to know between parathyroid hormone and calcitonin which is involved in increasing blood calcium and which is involved in decreasing so for the parathyroid hormone it is involved in increasing blood calcium levels in contrast calcitonin which is released from the parafollicular cells of the thyroid gland it's responsible for decreasing blood calcium now the way in which we can increase and decrease blood calcium is by either taking it from the major reservoir in our body or putting it back into that major reservoir and where do we store up 99 of all of the calcium in our body that's within the skeletal system right within our bones and so if our calcium levels are too high the thyroid gland is going to release calcitonin and as a result calcitonin is going to inhibit osteoclast activity and when it does that you're going to be inhibiting the rate at which your bone can break down and so we end up storing up more calcium in our skeletal system when you store up more calcium in your skeletal system your blood calcium is going to start to go down ultimately bringing us back to homeostasis so you can see how this is a negative feedback loop so if you have rising blood calcium levels that stimulates the parafollicular cells of the thyroid gland to secrete calcitonin which inhibits osteoclast activity causing your blood calcium to come back down now in contrast if you have decreased blood calcium levels that's going to stimulate the parathyroid glands and as a result they're going to secrete parathyroid hormone and they're going to increase osteoclast activity which is going to cause the breakdown of some bone tissue ultimately leaching calcium back into our bloodstream which will cause our blood calcium levels to rise so if you have rising blood calcium thyroid the thyroid gland releases calcitonin to help to bring it back down so if our blood calcium levels are declining the parathyroid glands secrete parathyroid hormone which are ultimately going to cause our blood calcium to rise now as you can see here there are other ways in which calcitonin and parathyroid affect other organs and those two organ systems you can see the kidney which is part of the urinary system and the intestines and in both cases here this is a way in which we can either increase how much calcium we're going to be excreting in our urine or we can take it out of our urine and out of our digestive system back into our blood to ultimately reabsorb any calcium before it's excreted from the body as either urine or solid waste and that will allow us to keep those blood calcium levels high as well so it's not only action on the skeletal system there's also action going on with the kidney as well as with the intestines to either cause us to secrete more or to help us to keep more calcium into our blood by secreting less now moving on we're going to be taking a look at the adrenal glands the adrenal glands are a pair of glands located on the surface of our kidneys as we've already learned the inner region of the adrenal gland is known as the adrenal medulla and the adrenal medulla is responsible for the secretion of epinephrine and norepinephrine in response to our stress response right the sympathetic nervous system innervates the medulla and causes it as part of our sympathetic response to secrete and flutter bloodstream with epinephrine and norepinephrine this is ultimately what causes you to feel that total body response during an acute stress response now the cortex of the adrenal gland is responsible for the secretion of a number of different hormones and there are different zones which we're going to talk about different regions to the adrenal cortex but we'll look at those next semester for now what you need to know is that the adrenal cortex is involved in the secretion of different hormones so we have aldosterone and cortisol are the two most important the androgens those are similar to your sex hormones your steroid hormones such as testosterone and estrogen the two main hormones i want you to focus on for the adrenal cortex are aldosterone and cortisol now cortisol is our primary stress hormone and during stressful events cortisol will kick into play especially when you're dealing with more chronic type stress remember the endocrine system is capable of producing a much more sustained longer term response although it's slower in communication as compared to the nervous system the endocrine system can sustain a response after the acute period so if you're dealing with an acute stressful period it's likely the sympathetic is innervating the medulla and that is what is producing a lot of that initial stress response but when you're dealing with more chronic periods of stress it's cortisol levels which will be elevated within your blood and ultimately that's also going to have impacts on your metabolic rate so cortisol has a direct impact on our blood glucose levels whereby when we're stressed our metabolic rate's increasing and so we are going to be increasing our blood glucose levels by breaking down glycogen as well as the conversion from fat now in contrast aldosterone is responsible for regulating water balance and it does this by regulating blood ion levels specifically sodium when we reabsorb sodium due to aldosterone secretion water is going to follow through right when we bring more sodium back into our bloodstream to maintain that osmotic gradient and so water is going to be reabsorbed as well so aldosterone works similarly to adh antidiuretic hormone to increase blood pressure but aldosterone actively reabsorbs sodium to then draw water in passively whereas adh causes the reabsorption of water directly now the pancreas has a dual function it is classified as both an exocrine as well as an endocrine gland we're going to get to look at more of the exocrine function of the pancreas next semester but to give you some perspective the way that the pancreas is set up is that there are exocrine glands located in the pancreas that produce digestive enzymes those digestive enzymes are secreted into this main duct that travels along and ultimately empties into our small intestine that's where a lot of that digestion is going to be taking place and remember the gi tract the gastrointestinal tract it is connected to the external environment at both the superior and inferior portions and so any gland secreting substances into the gi tract is considered exocrine because it's connected to the outside of your body now the endocrine portion of the pancreas is made up of these specific cells that together are known as the islets of langerhan and the islets of langerhan are made up of both alpha cells and beta cells now the alpha cells are responsible for producing glucagon and the beta cells are the cells that are responsible for producing insulin and insulin and glucagon work together to be able to help us to regulate our blood sugar and remember regulation of blood sugar is a negative feedback mechanism because the blood sugar goes up we are going to try to bring it back down okay similarly if our blood sugar is low we're gonna act to try to bring it back up so let's take a look at the two scenarios first we're looking at a situation where we have high blood glucose so you've just eaten a meal your body is breaking down all of the carbohydrates into the simple carbs you have lots of glucose in your blood so what do we need to do we need to get the glucose from our bloodstream into our cells because once the glucose is in our cells we can then break it down through glycolysis krebs cycle electron transport chain chemiosmosis to be able to produce atp so when we have just eaten a meal our blood glucose will be high that is going to stimulate beta cells to release insulin insulin is then going to bind to the receptors on the surfaces of our cells which allow them to take glucose into the cell and as a result your blood glucose levels will decrease back to normal levels now if we have been fasting for a long period of time our blood glucose may be low remember when you need glucose where do you go you go to the liver to break down glycogen and ultimately low blood glucose is going to be stimulating the alpha cells to secrete glucagon which is then going to act on the liver to break down glycogen and when we break down glycogen we are going to increase the amount of glucose that will be in our bloodstream so these two hormones glucagon and insulin are both active to help us to regulate our blood sugar so if we've just eaten a meal it's insulin that's going to be allowing us to take that glucose into our cells and if we're fasting and we need glucose it's glucagon that's going to stimulate the breakdown of glycogen in the liver to ultimately increase blood glucose levels now we're going to finish off the lecture briefly looking at the gonadal hormone and these are steroid hormones that are released in response to luteinizing hormone that has been secreted by the anterior pituitary ultimately in males the luteinizing hormone is going to act on specific cells known as leydig cells and these cells are located within the seminiferous tubules which are the regions that produce spermatozoa and the latex cells are ultimately going to produce testosterone during the ovarian cycle there will typically be one primary follicle that will grow and mature that will release an egg cell through ovulation now during the development of this follicle in the first half of the ovarian cycle we see an increase in estrogen levels and that's because the follicular cells that make up the follicle that contains that ovum they're going to be secreting estrogen and as it grows there will be more estrogen produced and then ultimately once the egg is released this follicle now empty starts to degenerate in a structure known as the corpus luteum and as it degenerates it releases progesterone so remember all three hormones are produced in both males and females it's just estrogen and progesterone are more abundant in females and testosterone more abundant in males and these sex hormones are responsible ultimately for the development of those secondary sexual characteristics typical of both males and females so with testosterone you have the increased production of body hair as well as deeper voice and with estrogen and progesterone levels higher in females we see the secondary sexual characteristics such as the development of breast tissue okay and that brings us to the end of our lecture for this week's so remember use these lecture notes to help you to focus your attention and guide you during this chapter the main outcomes from this week are to know the endocrine glands know the hormones that they release and ultimately their primary function in the next semester we're going to dig deeper into this system where we're going to look at the structure of hormones look at various disorders and map out the various feedback mechanisms that exist so if you have any questions please feel free to reach out otherwise i look forward to seeing you all in tutorial take care and have a great week