Hello everyone, here is Dr. George Law. In this lecture, I would like to introduce the endocrine system, which is a very important system that controls a lot of things inside our body. In Chinese, it is called 內分泌系統. We have to be very careful that it's not the same as an exocrine system. Okay, this time this system we call it an endocrine system, another important system is exocrine system. Don't mix them up. And, I would like to introduce two important components of an endocrine system. First of all, it's a gland, and secondly, it's the hormones. What is a gland? It is an organ or structure inside our body which produces and secretes hormones. Hormone, it is an important substance which helps to modulate the activities of cells or organs inside our body. The glands and the hormones work together to form the endocrine system and then carry out the following functions: it regulates and coordinates the growth, metabolism, sexual development and a lot of important functions in our body. In the following part of my talk, I will explain to you. Here's a diagram about how an endocrine system is working. First of all, this greenish structure is the gland. It has some endocrine cells to produce these small greenish substances, which are hormones. After the production of the hormones, the glands release the hormones, and then the hormones will go into the bloodstream. Through the bloodstream, these hormones can go through our body. The hormones act on certain target cells or target organs and these hormones bind to certain kinds of receptors. When these receptors are activated, it causes a sequence of changes inside the cell and then brings along the functions of these hormones. Here is a list of the important endocrine organs in our body. I would like to highlight some of them in my talk, which are the most important glands inside our body including the hypothalamus, pituitary gland, thyroid gland, parathyroid gland, adrenal gland, pancreas, and the sexual gland which includes the testes in male and the ovaries in the female. So first of all, it's the hypothalamus. Hypothalamus, in Chinese, is 下丘腦. We call the highest center of our body, it is the “big boss” of the endocrine system. Why do we say so? It's because the hypothalamus basically controls nearly every endocrine gland in our body. Here is a picture showing the brain of our body. The hypothalamus is located at the base of the brain. This blue structure is the hypothalamus. Hypothalamus produces a list of hormones to control the functions of other glands, which include the corticotropin-releasing hormone that controls the release of corticotropin; thyrotropin-releasing hormone that controls the release of thyroid hormone; gonadotropin-releasing hormone which controls the release of sexual hormones. Other than that, the hypothalamus also releases oxytocin, antidiuretic hormone, and somatostatin hormone, which have various important functions throughout our body. In addition, the hypothalamus produces growth hormone-releasing hormone which directly controls the pituitary gland in secreting the growth hormone. The next one is the thyroid gland. In Chinese, it is called 甲狀腺. Why is it called 甲狀腺? Because when you look at the shape of the thyroid, it is somewhat like the Chinese word 甲. Thyroid is situated in the anterior part of our neck. It secretes two important hormones, thyroxine (T4) and triiodothyronine (T3), which are the activated hormone of thyroxine. The main function of the thyroid gland is to control the metabolism of our body. With more thyroxine, our body will metabolize faster; with not enough thyroxine, our body will metabolize too slowly that it cannot maintain the normal functions of our body. The functions that are controlled by thyroid glands include heart rate, gut motility and body weight. With more thyroxine, our heart beats faster; when thyroxine is not enough, our heart rate drops. For gut motility, with more thyroxine, our gut moves faster. When it goes to the extreme, it can cause diarrhea. On the other hand, if our thyroid function is not enough, it causes the gut to move too slowly, and it can cause constipation. For the body weight, when there's too much thyroxine in our body, such as in the case of hyperthyroidism, our body metabolism will go too fast. It burns too much glucose and fat, so our body weight drops. On the other hand, if our thyroid function is not enough, then our metabolic rate will be too slow. In that condition, we will gain weight. Next one, the parathyroid gland. In Chinese, it is called 副甲狀腺. It is called 副甲狀腺 because the parathyroid gland is actually just located at the back of the thyroid gland. These four small dots are the parathyroid glands, two on the left and two on the right. The main function of the parathyroid gland is to produce parathyroid hormone, it controls the blood calcium level, which is very important for the normal functions of our body. How could parathyroid hormone control the calcium level in our body? It is by acting on three major organs, which include the bone, the kidney, and the gut. When there is more parathyroid hormone in our body, it helps the bone to break down, to release the calcium into the blood vessel. It helps to increase the calcium level in our bloodstream. For the kidney, the parathyroid hormone will increase the reabsorption from the urine and decrease the excretion of calcium from our kidneys. So, in turn, it helps to preserve calcium levels in our bloodstream. For the gut, parathyroid hormone helps to increase the calcium uptake from our gut, it helps to increase the bloodstream calcium level. The next one, is the pancreas. In Chinese, it is called 胰臟. It is in close proximity to our stomach, our liver, and our gallbladder. The pancreas is actually just located at the back of the stomach. As an endocrine organ, the main function of it is to secrete insulin and glucagon. It can help to maintain our blood glucose level, which is a very important field of our body for metabolism. Insulin and glucose have two opposite effects. Insulin helps to decrease our blood glucose level; on the other hand, glucagon helps to increase our blood glucose level. In the case of Type-1 Diabetes Mellitus, our pancreas cannot secrete enough insulin, the blood glucose level goes up which causes Type-1 Diabetes Mellitus. The end-acting organs of insulin and glucagon are the liver, fat, and muscle. Liver is very important storage of glucose in our body. Insulin helps to stimulate the liver to release glucose into the bloodstream. However, glucagon stimulates the liver to take up more glucose from the blood vessels to increase the liver glucose stored. For fat and muscle, insulin will stimulate the fats and muscles to take up glucose. On the other hand, glucagon will stimulate fats and muscles to release blood glucose. Another important endocrine gland in our body is the adrenal gland. In Chinese, is called 腎上腺. Why it is called 腎上腺? Because the adrenal gland is at the top of our kidney. It is at the top of our kidney, so it is called 腎上腺. This triangular structure is the adrenal gland. It has two major parts, the medulla, and the capsule. Both of them have different functions and secrete different hormones. But in short, the adrenal gland will secrete two stress hormones which include the glucocorticoid hormone and also the adrenaline. These two are the very important stress hormones which respond when we face stress. They help to increase our heart rate, increase the pumping force of our heart and let us have the fight or flight response. On the other hand, the adrenal gland also secretes another important hormone, which is called the mineralocorticoid hormone. Its main function is to control our blood pressure and also to maintain our electrolyte levels in equilibrium, which include sodium and potassium. Finally, our sexual organs, which include the ovaries for females and testes for males. Ovaries, in Chinese, is 卵巢; Testes, in Chinese, is 睪丸. This is the picture of the testes, which itself has a lot of glands, and a lot of cells inside which produce sexual hormones. For the male is the testosterone. The two pinky structures are the ovaries, which are in close proximity to our uterus, and also the fallopian tube. The ovaries have two main functions. As an endocrine gland, it produces female sexual hormones which include estrogen and progesterone. These sexual hormones are released into the bloodstream to go through our bodies. On the other hand, they also produce eggs. When the egg is produced, it goes through the fallopian tube and then goes into the uterus. It fertilizes with the sperm for reproduction purposes. We have already gone through those important endocrine glands in our body. Let me introduce an important concept of our endocrine gland, which is the feedback system, including positive feedback and negative feedback. First of all, negative feedback. It is a negative regulatory system in which it feeds back to the input a part of a system's output so as to reverse the direction of change of the output. Take a room temperature control of a heater as an example. Let's say, we have set the room temperature at 20°C. When the room becomes too hot, that means the room temperature is above 20°C, it ignites negative feedback. In turn, it will turn off the heater. And so, the temperature will drop back to 20°C. On the other hand, when the room is too cold, that means the room temperature is too low. The system will turn on the heater, so as to increase the room temperature back to the equilibrium. That is what we talk about to reverse the changes. When the room is too cold, the heater will try to increase the room temperature to go back to normal; when the room is too hot, the heater will be turned off so as to let the temperature drops back to the equilibrium. The main function of a negative system is to maintain an equilibrium in a system. In this condition, the equilibrium is the set temperature of 20°C. How about the endocrine system in our body? One common example is the blood glucose control. Blood glucose control is just like room temperature, and the insulin is the heater. What does it mean? When our blood glucose level is too high, our pancreas will secrete more insulin. The insulin acts on the liver, muscles, and fats. These organs take up more glucose and help to decrease the blood glucose level, that means to reverse the changes, reverse the high blood glucose level. So that the blood glucose could go back to normal. This is one of the common examples of negative feedback in our endocrine system. On the other hand, what is positive feedback? Positive feedback means the process in which the end organs of an action cause more of that action to occur in a feedback loop. That means the positive feedback is not to reverse the stimuli, but it causes more stimuli in the system. One common example is the feedback when we try to put a microphone very close to the loudspeaker, there will be a very loud feedback you heard. Because when the sound comes out from the loudspeaker, it will be taken in by the microphone and then through mixer and amplifier, finally the sound will be going out through the loudspeaker again. After that, the loud sound will go back to the microphone again and form a positive feedback. The sound becomes louder and louder, and this causes a very loud feedback. What will be the condition of our endocrine system? One classical example is the suckling of the baby on the breast and causing milk release. When the fetus/baby sucks on the nipple of the mother, this stimulus causes the release of the hormone cause of oxytocin. This oxytocin stimulates the milk coming out from the breast of the mother. When the milk is released, the baby could drink; when the baby feels more milk is coming out, they will suck more vigorously. These stimuli cause more oxytocin to release more milk to come out. It causes a positive feedback. That means when you suck more, there will be more milk; and more milk, then the baby will suck more. This is a positive feedback in our endocrine system. So, near the end of this lecture, let me take the growth hormone axis to illustrate these all common important features of our endocrine system. In our endocrine system, there are actually a lot of axes. One of the axes is the growth hormone axis, that means the axis controls the release of the growth hormone. In this system, it involves two glands, which are the hypothalamus and also the anterior pituitary, which is the anterior part of the pituitary gland. The hormone involved in this growth hormone axis, for sure, is the growth hormone (GH). The regulation of the growth hormone axis is controlled by two negative feedback systems. One is short negative feedback and the second one is the long negative feedback. I will go through it later. Finally, the end organs are our body tissues, the adipose tissues, muscles, and liver which the growth hormone will act on all these end organs to carry out its function. Another important feature of the control of endocrine system is multi-level control. That means the hypothalamus controls the actions of the pituitary. Pituitary produces growth hormone to control several end organs. In different levels, there are some promoting factors and inhibiting factors. For example, in the hypothalamus, it produces growth hormone-releasing hormone, which stimulates the release of growth hormone from the pituitary gland. And in turn, the hypothalamus produces another inhibitory factor, which is the growth hormone inhibitory hormone, that decreases the production of growth hormone from the pituitary gland. These are the promoting and inhibitory factors. Finally, it is the feedback loop. In the growth hormone axis, there're two layers of feedback loop. First of all, the anterior pituitary gland feedbacks to the hypothalamus, which means when there is excessive secretion of the growth hormone from the pituitary gland, it will suppress the production of the growth hormone-releasing hormone and cause more growth hormone-inhibiting hormone to be producing out so as to reverse the excessive growth hormone, to let the growth hormone level goes back to the equilibrium. Another level of control is from the end organs. When there are too many actions from these end organs, which include the liver, the muscle, the adipose tissue, etc., it feedbacks to the hypothalamus. It stimulates the hypothalamus to secrete more growth hormone-inhibiting hormone and also decrease the production of the growth hormone-releasing hormone, and to let the actions go back to the equilibrium. This is the summary of the important features and the feedback control system in a growth hormone axis. In the next talk, I will introduce how this endocrine system is important in our practice of sports medicine. Thank you.
