Welcome to the first lecture in introductory human physiology, and today, we want to talk about homeostasis. This is the basic theme for physiology. All of the organ systems are going to integrate in order to maintain homeostasis of the body. And the homeostasis of the body is to maintain conditions within the body that are compatible with the life of the cells. So the things that we want to look at today, the learning objectives are first, to explain the basic organization of the body. Secondly, we want to define the fluid compartments of the body. Third, explain how solutes such as sodium, chloride, glucose and so forth distribute within the body. And fourth, we want to explain what homeostasis is and the homeostatic mechanisms that regulate this, we're going to deal with in the very next lecture, which is coming right up next. And then last we're going to very quickly talk about mass balance, and how the body maintains mass balance. All right, so the first thing that we want to consider then, is the body components. So as you all know, the human body starts with a single fertilized egg, and this egg then undergoes division to make multiple copies as well as differentiation. The differentiation allows the specific cells to acquire specialized functions. These functions, then, these groups of cells that have the same specialized function, will work together to form what are called tissues. We have four tissue types within the body. There are muscle, nervous tissue, connective tissue, and epithelium. These four tissue types will form the organs. And the organs will work together to perform a specific function for the body. And then at that point, if we have more than one more organ functioning, that is we have several organs functioning together, then they're called an organ system. So for instance, an organ system would be called, the organ would be the kidney and the organ system, the renal system or urinary system would be the kidney with the two ureters that are taking the urine that's generated from the kidneys down to the bladder. Where the urine can be stored in the bladder, and then eventually expelled to the outside of the body through what's called the urethra, so that's our urinary system. So the organ systems that we're going to consider, there are ten organ systems of the body, we're going to consider nine of them. And they are going to perform very specific functions. So for instance the skin. The skin is the largest organ of your body. It has its specific function. It is protective, so it forms a barrier to the outside world and keeps all of the inside materials sort of organized. The skin is a very important barrier for the loss of water. So it's a hydrophobic barrier, so it allows the body to retain water even though we have conditions where we would normally become dehydrated. The second of the organs that we need to deal with then are the organs that are going to overcome the barrier of the skin, and that is organs that allow us to have entry into the body. For instance the respiratory system, which allows the entry of oxygen and the expulsion of CO2. So we have gases then that can come in and out of the body, and we have the GI tract or the gastrointestinal tract, which allows food or nutrients to enter into the body, and then solid waste to be removed from the body. We also have transport systems, and the transport system is predominantly the cardiovascular system. The cardiovascular system takes the nutrients that are entering from the GI track and delivers it to all of the cells. It takes the gases which are coming in from the lung, the respiratory system, and delivers that to all of the tissues and organs of the body. This is done by bulk flow, and we'll talk about this when we get to the cardiovascular system, but this is moving materials through a series of vessels which are the vasculature. Once they get to the tissues, then we have to move the gasses and the nutrients and the solutes out of the vasculature, and actually into the tissues themselves. And they have to cross a very small space. And this space then is between the tissues of the cells and the vasculature. And we're going to talk about that in just a few minutes. And that is going to occur by diffusion. So that's going to be a very slow process that's only a local delivery system. And then we have to be able to remove materials from the body, and this is done by the renal system as I said. So liquid waste are removed, excess ions are removed, excess water is removed from the urinary system and of course the GI tract, the gastrointestinal tract will remove solid waste products. But the physiologist looks at the bodies in a slightly different manner, and that is they divided into what are called fluid compartments. We have effectively two major fluid compartments. One is where we take all of the cytoplasm, that is the liquid components that are within cells. The cells are bounded by a plasma membrane, and this liquid component, this cytoplasm, we take all of that from all of the cells and put it into one fluid compartment. And that would be called the intracellular fluid compartment, or the ICF. And it is bounded by the plasma membrane. And that's what's shown here. And then outside of the cells, we have this extra cellular space, and this extra fluid compartment or the ECF, is what is immediately outside all of the cells. The ICF, the intracellular fluid compartment is the largest of these two fluid compartments. And it is effectively two-thirds of the total body water or the total fluid of the body, and the extracellular fluid compartment is one-third. Now these two compartments are dissimilar in content, that is is that inside cells we have very high levels of potassium and very small concentrations of sodium. We also have present within the cells, proteins, which are negatively charged. In the extracellular fluid compartment, we have very high concentrations of sodium and small concentrations of potassium. So we have completely different types of an environment. The other thing about these two environment is that extracellular fluid compartment, can be divided further into two compartments. One is the intravascular compartment, and that's within the blood vessels, and the other is this interstitial fluid space. And this interstitial fluid spaces is that little space that's between the vascular and the cells themselves. This is usually filled with connective tissues. So these two compartments actually have the same content of ions and solutes, so that the amount of sodium that is present within the vasculature is equal to the amount of sodium that's present within the interstitial space. And the amount of potassium and so forth is equal between these two compartments. So there is an equilibrium of equal distribution of these solutes between the two spaces, and this is because the barrier, that is the epithelial cells that are lining the blood vessels, are a bit leaky, and so they allow this material to move from one compartment to the other, and to form an equilibrium. The two compartments do differ in that the intravascular's fluid compartment also has proteins, which are not present within the interstitial space. Now one other thing about these two compartments then is that we have a equilibrium between the intravascular space and the interstitial space, but we have a disequilibrium between the ECF and the ICF. But that is maintained at a constant or a steady state, and this is done so by the presence of an enzyme, which is an ATPas, which cleaves ATPase. So the enzyme uses energy to move the sodium out of the cells, so 3NA is pumped out of the cells for every 2K that enter the cells. This is needed because there are little leaks between these two compartments, which allow potassium then to slowly leak out of the cells and into the extocellurar fluid space. And this pump then reorganizes the distribution of the ions and keeps the ions at a disequilibrium. So that we have a steady state that is input is equal to output, but that the amount of sodium on the outside of the cells is different from the amount of sodium that's inside the cells. And the amount of potassium inside the cells is different from the amount of potassium that's on the outside of the cells, So the fluid compartments then or the total body water is about 60% of your total body weight. So if we have an individual who is a 70 kg male, then 42 liters of that individual is fluid, is water. That means that the intracellular fluid space or the cytoplasm which is two thirds of the total body water, would be equal to 28 liters. And that the extracellular fluid space, which surrounds the cells and is this interface between the cells and the external environment, this will be equal to 14 liters. Then within the ECF or the extracellular fluid space, we have this intravascular fluid. And the intravascular fluid is actually only one twelfth of the total body water. That is, it's one fourth of the ECF. So we have one fourth of the ECF is equal to the intravascular space times one third, which is the ECF, that is of the total body water. And that gives us then one twelfth of the total body water is equal to the fluid phase of the blood, of the vasculature, that is equal to the plasma. So that's pretty amazing, if you think about it, because, when you think about the body, you think of the fluid phase of the body is the blood, that is the plasma, which is the fluid portion, the liquid portion of the blood, and not all of the other fluids that are within the body. But it's actually the smallest amount of fluid that's within the body. So we have self-regulating mechanisms then, which are active between these different fluid phases, these different fluid compartments. We have an equilibrium, which is allowing equal amounts of substance to be distributed between the intravascular space and the interstitial space. So sodium, potassium, chloride, the calcium, they equally distribute between these two phases, these two compartments. There's no net transfer of substance or of energy between these two compartments, and there's no barrier to movement. As I said, the epithelial cells that are dividing these two compartments are fairly leaky and there's no energy expenditure to maintain this equilibrium. In contrast, we have a steady state which is present between our exocellular fluid space and the intracellular fluid space. And here, we have a constant amount of substance within the compartments and that the input is going to be equal to the output, but that the concentrations within these two compartments can be dissimilar. And that this requires energy to maintain. We need to use ATP, the energy of the cells, in order to maintain this gradient between the two compartments. So why are we so interested in these fluid compartments? Why is it the physiologists are asking about the fluid compartments of the body? And the reason for that is that as that the cells themselves require specific factors to be within a very tight range. These factors are the amount of oxygen, the amount of CO2, the amount of hydrogen ions, the temperature, the amount of glucose which is presented to the cells. So the cells then are requiring this very tightly regulated environment and yet as you go through your daily life, you are bringing into your body a very diverse amount of material. So you're constantly changing, your environment is constantly changing. And it is the ECF that is the buffer zone. What do I mean about that, well just think about it, if you eat a large hamburger for lunch, you're bringing in glucose, fat, proteins, amino acids, into the body and that material will go from the gastrointestinal track directly into the blood. And then from the blood it will then be distributed to the cells. But the organs of the body are trying to maintain that ECF, that buffer zone, which is where all this material is being delivered within a normal range or within a very set range. And it's the maintenance of this ECF, the constituents of the ECF is relatively constant, which is the main theme of physiology. And this is what homeostasis is about. So that's our central theme, and what we're going to see is that all of the organs of the body are going to act on that ECF to try to keep the contents of the ECF under this very narrow range which is compatible with the life of the cells. So what happens if we do not maintain the ECF in this very tight range of needed factors? When we have input is equal to the output we'll have wellness. So under those conditions then, as long as the materials that are within the ECF or within the range that's compatible with life of the cells, everything is fine. But when we have input, say for instance, that says effectively is increased over output, then we can get illness or pathophysiology. And the converse can occur if we have output that is greater than input. Then again, we can have illness or pathophysiology. And so it is this balance, this very tight balance that has to be maintained at all times in order to keep the body at a constant activity. If the organ system does not perform its function then we can end up with input or output which is not equal to the opposite. Under those conditions then, we will have pathophysiology. So one of the major ways that the body is going to regulate this ECF is by using homeostatic control systems or reflex loops. And that's what's diagramed here, and that reflex loops have essentially three components. They have a Sensor which is going to detect a specific signal or stimulus, and that Sensor then sensing information to what is called the Integration Center. And this Integration Center is usually the brain. The Integration Center has within it the set points that are compatible with the life of the cells. And so it will then evaluate the incoming signal to see whether or not the incoming signal matches the set point that the body needs or whether it exceeds it or is below it. It then will decide whether or not it needs to make a response and will send out an effector path wave to the effectors. So this is an efferent path wave going out to the effectors, which will generate a response that will then bring the body back to its normal condition. This is exactly analogous to the temperature control system that you have in your house for heating. So, the integration center would be a but we set a specific temperature that we want within the room. And then the stimulus is the incoming reading that is, what is the temperature of the room. And the output would be whether we have to turn on the heat or we have to turn on the air conditioning to bring the temperature of the room back to normal. So, this is a simple reflex loop. And it's essentially the types of reflex loops that the body is going to use. So let's consider one of these systems where we have a case where we've decided that in a given week that you want to eat nothing but a high salt diet. So on Monday, the amount of sodium that's coming into your diet is equal to the amount that's being released from the body in urine. And so we have then, what is called, a neutral balance. So the mass balance then is equal. What's coming into the body is equal to what's leaving the body. But by Wednesday, with this high salt diet, you're eating a lot of sodium. You're taking in Chinese food with a lot of soy sauce on it, and so it's really salty. And so that on this diet is very high on salt we have now a positive balance where the amount is coming in from the diet exceeds that which is lost in the urine. And so this now is a positive balance for sodium. But by Friday now the amount of sodium that's coming in from the diet is equal to the amount that's lost in the urine, and so were again under neutral balance. We're under neutral balance, but look what's happened to the body. We've actually increased the amount of sodium, the content of sodium within the body, and I just finished telling you that the body wants to maintain a very tight regulated amounts of sodium within the ECF at all times. That's one of the regulated factors that the body is interested in keeping constant. And yet we have with this diet, we have increased the total amount of sodium within the body. So how could we do that? With increase the total amount of sodium within the body, but what happens when you are eating in a high salt diet. What happens when you taking a lot of salted food like a potato chips, you eat a bag of potato chips, what happens? You get thirsty, and as you get thirsty then you drink water, and as you drink water that fluid will come in to the body and dilute the content of the sodium, so that it now has a concentration which is the same as the concentration of sodium that we had on Monday. So the concentration of the sodium in the body is going to stay equal, but the content, the amount of sodium that's added to the body has increased. And where did it go? It went to the ECF. All of the sodium went into the ECF. It's not able to cross that plasma membrane that hydrophobic barrier, and instead it's staying in the ECF. So where the volume of water go that you drunk? It also goes into the ECF, so that we could dilute them the sodium concentration within the ECF. So all of the volume, all of the fluid volume is into the ECF. So we have increased the sodium content that was in the ECF, and we've increased the water content in the ECF, but we've maintained the concentration of sodium in ECF as constant. At what cost? So let's think about it. So what is within the ECF? We said that there's a vasculature, and the interstitial fluid space. The vasculature, within the vasculature, we have increased the volume of blood, and by increasing the volume of blood, we have increased the pressure within the vasculature, so by holding this extra sodium and holding this extra fluid within the body, we increase the volume of the blood. And by doing so, we then increase pressure within the cardiovascular system. So there is a cost, then, to maintaining the ECF at normal range. So what are general concepts? So the first is that the human body, then, is this interdependent set of self regulating systems whose primary function is to maintain an internal environment compatible with living cells and tissues, and this is homeostasis. And this is the primary theme of physiology, and it is what all of the organ systems of the body are trying to maintain. The second is is that we have stability of these internal variables, and it can be achieved by balancing our inputs and outputs to the body and among the organ systems. But what we need to remember is that there's a hierarchy among the organ systems, and that the two organ systems that always win out is the brain and the heart. And often, they will take dominance and allow the body to maintain the brain and the heart, say, for instance, perfusion of the brain and the heart, but then lose the perfusion to other organ systems. So there is now going to be a tradeoff where the body is going to make some decisions which may not, under difficult conditions, or pathological conditions, which may not maintain everything as a constant. Okay, so the next time we come in then let's look at all the different mechanisms that we can use to maintain this homeostasis. Okay, see you then.
