Welcome to the first lecture in introductory human physiology. And today we want to talk about homeostasis, this is the, 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. Alright, so the first thing that we want to consider then, is the body components. So, as you all know the body starts with this, with, a human body starts with a single fertilized egg and this egg then undergoes divisions to, to make multiple copies as well as differentiation. The differentiation allows the specific cells to acquire specialized functions. These functions then, this 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, they are muscle, nervous tissue, connective tissue and epithelium. These four tissue types will form the organs and, and the organs will work together as, to, to perform a specific function for the body and then, at that point if we have more than [INAUDIBLE] 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, an organ would be the kidney and the organ system, the renal system or the 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 is called the urethra. So, that's our, our urinary system. So, the the organ systems that we're going to consider, there are ten organ systems in 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 is protective, so it forms a barrier of tissue to the outside world, it keeps all of the inside materials, sort of, organized. The skin is a, is a very important barrier for the loss of water. So, it's a hydrophobic barrier, so it allows the body to maintain water even though we have conditions, where we would normally become dehydrated. The second of these, of the of the organs that we need to deal with and are 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 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 a transport system is predominantly the cardiovascular system. The cardiovascular system, it takes the nutrients that are entering from the GI tract and delivers it to all the cells. It takes the gases which are coming in from the, 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 [UNKNOWN] series of vessels, which are the vasculature. Once we get to the tissues, then we have to move the gases and the nutrients and the solutes across, out of the vasculature and actually, into these tissues themselves. And they have to [INAUDIBLE] they have to cross a very small space and this space then is between the tissues of the cells and, 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 wastes are removed excess ions are removed. Excess water is removed through, through from the urinary system. And of course the GI tract, the gastrointestinal tract will remove waste products, solid waste products. Alright but the, the physiologist looks at the bodies in a slightly different manner and that is they divide it 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 bound by a plasma membrane and this liquid component, this cytoplasm we take all of that from all of the cells and, and put it in to 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 extracellular space and this extracellular fluid compartment or the ECF, is what is immediately outside of all of the cells. The ICF, the intracellular fluid compartment is the largest off these two fluid compartments and it is effectively two thirds of the total body water or the total fluid of the body. And the extra side of food compartment is one third now, these two compartments are dissimilar in content. That 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 a completely different types of an environment. The other thing about these two environments is that the 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 space is that little space that's between the vasculature and the cells themselves. This is usually filled with connective tissue. So, these two compartments actually have the same content of ions and solutes. So, that the amount of sodium that's 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 fourth is equal between these two compartments. So, there is an equilibrium distribute distribution, an equal distribution of the, of these solutes between the two spaces. And this is because the, the barrier, that is the epithelial cells that are lining the blood vessels are [INAUDIBLE] a bit leaky. And so, they allow this material to move from one compartment to the other, and to form an equilibrium. They, the two compartments do differ in that the intravascular fluid compartment also has proteins, which are not present within the interstitial space. Now, the, one other thing about these two compartments then, is that we have an equilibrium between the intravascular space and the interstitial space. But we have a disequilibrium between the ECF and the ICF. And that in, 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 ATPase, which cleaves ATPs. So, the enzyme uses energy to move the sodium out of the cells, so 3 Na is pumped out of the cells for every 2 K that enter the cells. This is needed because there are little leaks between these two compartments would allow potassium then to slowly leak out of the cells and into the extracellular fluid space. And this pump then reorganizes the distribution of the ions and keeps the ions at an, 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 are, the total body water is about 60% of your total body weight. So, if we had an individual who was a 70 kg male, then 42 L of that individual are as fluid, as water. That means that the intracellular fluid space or the cytoplasm, which is two thirds of the total body water, will be equal to 28 liters. And that the extracellular fluid space, which surrounds the cells and is, is interface between the cells and the external environment, this would be equal to 14 liters. Then within the, the ECF of the extra cellular fluid space we have this intravascular fluid and the intravascular fluid is actually only one twelfth of the total body water. And this is one fourths of the ECF, so we have one fourths of the ECF is equal to the intervascular space times one third, which is that 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, or 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 in between 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 lined, 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, which is present our extracellular 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, 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 a specific factors to be within a very tight range. These factors are the amount of oxygen, the amount of CO2, the amount of the hydrogen ions, the temperature, the amount of glucose which is presented to the cells. So, the, 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 tract directly into the blood and then from the blood, it will then be distributed to the cells. So, this, but the, but the organs of the body are trying to maintain that ECF, that buffer zone, which is where all of this material is being delivered within a normal range, or within a very set range. And it's the maintenance of this ECF, the compon, this constituents of the ECF as 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 a reg under this very narrow range. Which is compatible with the life of the cells. So, what happens if we do not maintain E, the ECF in this very tight range of of needed factors? When we have input is equal to the output we'll have wellness. So, under those conditions then, as long as they, the materials that are within the ECF are within the range of that's compatible with life of the cells, everything is fine. But when we have inputs, say for instance this is effectively out, 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, 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, to 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, this ECF, is by using homeostatic control systems or reflex loops. And that's what's diagrammed here, and that reflex loops have essentially three components. They have a sensor, which is going to detect a specific signal or a stimulus. And that sensor then sends the 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 the bod, 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 response, and will send out in an affector pathway to [UNKNOWN] , to the affectors. So, this is an efferent pathway going out to the affectors, which will generate a response that will then bring the body back to a normal, 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 our rheostat where we set a specific temperature that we want within the room. And then the stimulus is the, 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's 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 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 to 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 on this diet that's very high in salt, we have now a positive balance where the amount that's 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, the amount of sodium that's coming in from the diet is equal to the amount that's lost in the urine and so we're again under neutral balance. We're under neutral balance, look at what's happened to the body, we've actually increased the number 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 amount of sodium within the ECF at all times. That's one of the regulated factors that the body is interested in reg, in, 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? Well, we've increased the total amount of sodium within the body, but what happens when you are, are eating in a high salt diet? What happens when you take in 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 get drink water, and as you drink water that fluid will come into the body and dilute the content of the sodium. So, that it now has the concentration, which is the same as the concentration that, 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? Well, 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 is staying in the ECF. So, where did the, the volume of water go that you drank? It also goes into the ECF so, that we could dilute then, 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 the ECF, as constant. But at what cost? So, lets 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, of, of the, of the blood, and by doing so, we then increase pressure within the cardiovascular system. So, there was a cost then, to maintaining the ECF at a, at a normal, normal range. So, what are our general concepts. So, the first is the bo, 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 the 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 that we have stability these internal variables, and it can be achieved by balancing our inputs and our 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, they will, take dominance and allow the body to maintain the brain and the heart, say for instance profusion of the brain and the heart. But then lose the profusion to other organ systems. So, there is then going to be a trade off, where the body is going to make some decisions which may not under, under difficult conditions or, or pathological conditions. Which may not maintain everything as a constant 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.
