 homeostasis is the maintenance of near constancy of internal environment by means of physiological processes. So, let us see what this internal environment means and how is the homeostasis maintained by various physiological processes. So, a unicellular organism actually had lot of needs. It has to ingest nutrients from the environment. It has to take up oxygen so that metabolism can take place and there would be production of a lot of waste products which need to be excreted out into the environment. So, there is a relationship between the unicellular organism and the environment. Basically, it has to synthesize the stuff to maintain the living processes, utilize the resources from the environment and whatever is the waste products it has to excrete and obviously for the propagation of the survival it has to reproduce and you see unicellular organism is basically self-sufficient in every aspect. There is a relationship between the environment and the unicellular organism and we know that this unicellular organism in our body is basically a cell which is the structure and functional unit of life. But when we talk about a single cell there are certain limitations that it functions only within a limited range of temperature, pH, osmolarity and electrolyte composition because all the metabolic processes require a particular temperature, the enzymes which are there, they are active in a particular environment and that includes a pH, osmolarity, electrolyte composition. That means these components have to be maintained within a limited range. If they shift to this side or that side of that range then the functioning of the organism or the cell will be compromised. So basically immediate environment of the cell needs to be relatively uniform that is very important because it is in a relationship with its environment and it is not a problem for unicellular organism because the environment for the unicellular organism is huge so it can take up a lot of products and when it excretes into this environment the excretory products will be lost in the sink of the environment it is so huge but for a multicellular organism that becomes a problem because multicellular organisms are complex machines and they perform complex actions and obviously they require complex maintenance. So each human body consists of about 100 billion cells and each cell we saw that each cell is functioning at its own level right. So each cell has a life of its own and these cells are arranged in a particular manner. So there are certain chemical processes going on inside the cell and the cell has the entire machinery for keeping it alive. Then the cells, the many cells are arranged in form of a tissue, a group of cells actually forms a tissue. Then various tissues combine together and form an organ for example when we consider say suppose liver there are hepatocytes are there which are cells and a lot of cells together are forming a lobule but there are other tissues as well there is blood flowing within the liver then there is connective tissue supportive tissue is there so lot of tissues combine together to form an organ and multiple organs function as a system right. So when we talk about say suppose cardiovascular system we have heart we have the vascular system and these systems are organized in a full organism level. So there is a relationship between the various cells the tissues and how these various cells and tissues are interacting with different organs at a system level. Now there is a problem in this because each cell is not directly in contact with the environment so it cannot take product directly from the environment and cannot excrete out directly into the environment rather each cell is surrounded by a fluid and this fluid is known as interstitial fluid. So the picking up of products and the excretion of the waste products occurs into this interstitial fluid but you see the amount of this interstitial fluid is very very small before we were talking about the whole environment where the unicellular organism is living. Now this fluid is very very small where all the cells are dumping their products and they have to take their resources from that interstitial fluid only. So we have to maintain the constancy of this interstitial fluid. Now how it is being done? See we have blood flow so basically there is another channel such that the blood flow will be able to supply whatever the cells are needing so all the nutrients and oxygen they enter into the interstitial fluid and the excretory products which are released into the interstitial fluid enter back into the blood and it is this blood which carries it to the various excretory organs or the resource providing organs for example kidney right. So they carry it to kidney from where the excretory products are excreted and maybe the oxygen which is taken up from the lungs and carbon dioxide is excreted via the lungs so this provides another channel. So you see how various systems are working together so as to provide resources to individual cells and remove the excretory products. So we have a respiratory system we have digestive system we have excretory system then we have cardiovascular system which is important for maintaining this blood flow then obviously we have endocrine system which is basically regulating the functions of all the cells together that we will see because that acts as a process by which the activity of the cells and of the various organ systems acts as a process to maintain the homeostasis. So various systems are acting as a link between the external and internal environment and ultimate aim being to achieve constancy in the thin layer of fluid surrounding the cells. So you see how various systems are keeping the cells alive isn't it and cells are basically forming the system so it is like I work for you and you work for me isn't it. Now it was Claude Bernad in 1857 who gave the term milieu interior that is the internal environment of the body. So the each cell which is surrounded by a small quantity of fluid that is the interstitial fluid that is the immediate environment of the cells. So before this interstitial fluid was the milieu interior internal environment of the body. Now later on this concept was projected to the entire extracellular fluid because you see the constancy of interstitial fluid can only be maintained if proper blood flow is maintained. So that is why now milieu interior is known as both interstitial fluid plus the plasma flow or blood flow which is happening. So whole ECF is considered as a milieu interior and what is the job of the entire body basically to maintain the relative constancy of internal environment for a free and independent life. Now I want you to take note of certain terms here first of all obviously internal environment which we have discussed what is the internal environment of the body plus there is something known as relative constancy it is not that it is fully constant it is one value no. It keeps on changing there is a particular range in which the functions of the cell is compatible with life so it is like approximately constant relatively constant and there might be certain conditions in which this constancy requires exaggerated actions of the various systems why say suppose how we exercise then what happens then there is increased requirement of the nutrients oxygen and increased waste products are being produced in the muscles isn't it so we require that blood flow to the tissue should increase so this is how that various organ system pump up their actions to meet the requirements of a particular state so that whatever is happening whatever is the disruption or change in the state in that condition also constancy of internal environment is maintained and remember that it was Walter B. Cannon who coined the term homeostasis to describe the constancy of milieu interior so two terms here very important milieu interior which was coined by Claude Bernard and homeostasis which was coined by Walter B. Cannon so how is this homeostasis maintained basically various systems participate directly or indirectly in maintaining homeostasis so what is system basically the group of organs that perform related functions so it is not that one system is acting independent of the other whenever the activity of one system changes it is required that the functioning of other system is also changed for example again we will take the example of the exercise when muscles are working too much then in that case we need that the cardiovascular system should change its functioning isn't it so say suppose fight or flight reaction is there in that time we need to run from a particular place so we need the activity of the muscular system but to keep up that activity we need proper functioning of the cardiovascular system along with that we need a decreased functioning of some other systems maybe say suppose excretory system so there is a constriction of the sphincters then there is a diversion of blood flow from the other organ systems so basically all the organ systems are working in tandem with each other and not in isolation from one another so regulation of these systems is brought about by endocrine and nervous system so they are like the communication channels which are sending the information to various system that what needs to be done so this concept is one of my favorite where basically all the body systems are acting to maintain the homeostasis that is the relative constancy of internal environment of the cell and which is essential for survival of the cells and you see various cells make up the body system so it is like the cells have teamed up to form the systems for their own survival and the maintenance of the body systems for the homeostasis that is the controlled processes which we are going to talk about now what is this suppose homeostasis there is imbalance then what will happen any imbalance in homeostasis will result in disease so for physiological processes of the cells to function properly homeostasis has to be maintained if homeostasis is not maintained then there will be disease okay so let us proceed with the processes so say suppose this is depicting a system and there is some input input is basically any relationship what we are having with the environment you see everything is constantly changing even when we are talking right this state is different from when I am sitting silent isn't it so there is some input some activity going on and this input will tend to change the homeostasis so this is known as disturbance and whenever there is a disturbance everything has to be kept within normal range and this normal range in homeostasis processes is defined as a set point and this set point basically is different for different processes so whenever there is a input that is tendency to change there is a disturbance there will be physiological systems which will recognize the disturbance or recognize the change in the variables of the homeostasis then they will try to compare it with the normal range and based on that okay this much has to be corrected they will give one output so this process is known as regulation so basically to maintain homeostasis either the functioning of the system has to be increased or decreased so that the homeostasis is maintained so what are these homeostatic control processes in order to maintain homeostasis control system must be able to detect deviations from normal there should be something which should tell the system yes there is a deviation and this is known as a sensor and we have lot of sensors for example there are sensors for detecting blood pressure there are sensors for detecting the chemical changes which are occurring for example the partial pressure of oxygen partial pressure of carbon dioxide then they should be able to integrate this information with other relevant information that means whenever we are talking about a disturbance see one disturbance alone is not happening there are lot of things going on right so it should receive the information from various processes and then it should be able to integrate this information kind of say suppose algebraic sum what is the end result of this how much it needs to be changed right so that is done by a controller and based on the output which is coming from the controller the system should make appropriate adjustments in order to restore the variable to its desired value and that is the effector so there should be a sensor there should be a controller and there should be an effector and obviously the information from the sensor should reach to the controller and the information from the controller should reach to the effector so then only everything will be connected so based on this concept there are five components of control system there is a sensor which will sense the disturbance there is an afferent which will send the information from sensor to the control center then there is an efferent efferent will send the information from the control center to the effector and what will sensor sense it will sense the stimulus that is the change in the variable and the effector will bring out the response so these are the five components of the control system there is a sensor there is an afferent control center efferent and effector for example in case of maintenance of body temperature what will be the stimulus stimulus will be the change in the temperature either in the core temperature or the outside temperature outside temperature is basically going to send the information advanced information to the body that okay outside information is different outside temperature is changed so if you do not do something then maybe the core temperature will also change so body makes appropriate changes so there is a stimulus then sensor will be the thermal receptors will be there efferent will be the neural signals which go from the receptors to the control center in this case it is hypothalamus from control center again neural signals come to the efferent so this will cause either vasoconstriction or vasodilation or they will be sweating or maybe shivering depending on whether the temperature change is cold or warm so that is the response of the system and what will be the effect this effectors will be the blood vessels there will be vasoconstriction or vasodilation or the sweat glands right so these are the components of the control systems coming to the mechanisms for homeostasis now there are feedback mechanisms in homeostasis then there is feed forward mechanism now in feedback mechanism we have negative feedback and positive feedback in feed forward mechanism when it the feed forward mechanism has the capability to learn that is known as adaptive control and then there are hierarchical control hierarchical control is something where if the disturbance is little bit only then only a particular system will be affected if the disturbance is too much then maybe some other system will also be recruited for example hierarchical control say suppose blood pressure control when blood pressure is maybe between 70 to 180 millimeter mercury mean artery repression then it is the barrel reflex which come into action that control system comes into action if the blood pressure falls from 70 millimeter mercury maybe between 40 to 70 millimeter mercury then chemo reflex also comes into action and if the blood pressure falls below 40 millimeter mercury then it is the cns ischemic response which also comes into action so this is a hierarchy of control so with this now let us discuss each of the control mechanisms for homeostasis first we will discuss the feedback mechanisms so what is feedback feedback is basically the property of a control system to use its output as its input so the output is fed back into the system as its input and this is for continuous monitoring so there are two types of feedback mechanisms one is negative feedback and the other is positive feedback in negative feedback the deviation in the controlled quantity is counterbalanced by the control system we will see what is it and in positive feedback deviation in the controlled quantity is further amplified by the control system what does this mean we will see there is an input okay there is sensor afferent there is a controller efferent effector and output and this output is fed back as input okay so this is what this is feedback whatever output is happening whatever is the end result it is again fed back as input so this is a continuous process which is happening there will be deviations also so even if there is certain change the system is telling okay i will again see what is the change and whether i have done a correct job or not but there are continuous disturbances isn't it something is changing in the environment also continuously so system should always know what is the current value of the output and what is this changing isn't it so this is feedback control mechanisms now these feedback control mechanisms can be either via the neural mechanisms or via the hormonal mechanisms you will not go into the details that which hormones which neurons but you see how the information is going that can be either neural or hormonal or it can even be neurohormonal that means the afferent can be neural but the efferent can be hormonal so those are known as neurohormonal feedback control systems so coming to the types of feedback systems which we said they are negative feedback system and a positive feedback system and we try to recall the definition of these what did negative feedback say negative feedback say the control system counter balances the original change and positive feedback amplifies the original change so this is a negative feedback example so whenever there is a decrease in the input the feedback system will increase the output and whenever there is increase in input the feedback system will decrease the output for example whenever blood pressure tends to decrease the whole system will try to increase the blood pressure for example changing your position from line to standing that will tend to decrease the BP but if BP actually decreases and you will not be able to stand only you will fall right so the system makes it possible to correct the BP and helps you maintain that state of increased BP on the other hand suppose say there is some condition in which BP increases then the system will try to bring back the blood pressure so negative feedback by counter balancing the original change in the variable maintains stability of the controlled variable so it is keeping it within the normal range in that within the set point example I told you that maintenance of BP or maintenance of temperature these are types of feedback mechanisms now how effective is negative feedback system to maintain that variable that is given by gain of the system and gain is defined as correction upon residual error when the gain of a system is high it is much more effective for example for the temperature control system gain of the system is very high 33 so it is a very effective control system okay now let us see example of positive feedback positive feedback the definition was that there is amplification of the output in the same direction as that of the input so if there is increase in input output is going to increase further if there is decrease in input output is going to decrease further so this happens in certain conditions for example blood clotting generation of action potential part duration ovulation lactation these are certain examples of positive feedback but you see in positive feedback what is the system doing system is taking the variable away from the set point so if the system detects that input has increased it is further increasing the input that is further taking it away from the set point and similarly in if the input is decreasing it is further decreasing the output so is it good or bad well if it is controlled then it is good that means positive feedback after a certain time has to stop okay and basically positive feedback is used in body to change the state of the system for example in case of blood clotting flowing blood is being changed to a clotted blood so the system state is changed right similarly resting membrane potential is changed to action potential we are not talking about resting membrane potential change and bringing it back to resting membrane potential okay resting membrane potential the state is changed to action potential okay in that cases we need positive feedback but positive feedback should stop after a certain point otherwise it becomes stileterious it can lead to death of the person one example is in shock so positive feedback if it continues uncontrolled is harmful that's why positive feedback always works under the loop of negative feedback please remember it always for works in body under the loop of negative feedback that means whenever a certain state is reached sensors detect that state and a negative feedback loop starts working or there is another thing that the stimulus itself ends so if the stimulus ends then the positive feedback cannot go on so that is positive feedback coming to next that is feed forward control what is feed forward control in feed forward control what is happening first we will see the flow chart this is the control system we have detected that there is input there is output and the output is fed back as input into the system but if a feed forward controller is there then feed forward controller detects the disturbance itself please remember here we are not talking about the variable it detects disturbance itself and anticipating that what change in the variable the disturbance will cause feed forward system tries to correct the variable value beforehand itself very important one example of feed forward control is temperature regulation yes temperature regulation is there in feedback also and feed forward also see when we are detecting the shell temperature that is the outside body temperature core body temperature has not yet changed okay shell body temperature your maybe outside body temperature is basically giving information about the environmental temperature and this information via the neural signal reaches to the hypothalamus okay now this hypothalamus what it does it beforehand starts the action so heat too much heat suppose outside it will beforehand start vasodilation so that the core body temperature does not rise otherwise if it waits for core body temperature to change then there will be a problem the activities of the system will be affected on the other hand when it is actually detecting the change in the core body temperature itself and correcting the core body temperature then that is negative feedback system so that is one example of feed forward control so it is detecting the disturbance itself second for example when you are drinking water when you drink water when you feel thirsty right so that means there is certain change in the body or similarity or the blood volume has decreased that's why you are feeling thirsty fine now when you take the water water is there in the digestive system but beforehand itself you know okay this much water is enough how do you know that okay this much water is enough to correct that body or similarity no actually what happens when we drink the water the sensors from our tongue itself kind of meter the amount of water which is taken so this is also kind of a feed forward control because the body is not waiting for the water to get absorbed and then deciding okay this much water is enough or not enough because then we will never be able to drink the adequate amount of water obviously the body or similarity and the plasma volume is being sensed that feedback is going on so you see feed forward again is under the negative feedback control so it is under the loop of negative feedback system so both positive feedback and feed forward control work under the loop of negative feedback so that is the main pillar of the control systems but because we are complex organisms we require positive feedback as well and feed forward control as well another example of feed forward is smell of food it is itself increasing the gastric secretion and motility so it is preparing the stomach beforehand then increase it heart rate before start of exercise that is also a feed forward control so as I told you positive feedback and feed forward mechanisms work under a loop of negative feedback next coming to adaptive control adaptive control is basically a learned control where whatever output the feed forward control is making right this is the output of the feed forward control it is compared with the desired output and then an error signal is produced I will give you one example say supposes when you are learning to do cycling what happens initially it is difficult to cycle because you don't know that what should be your body position what should be the pedaling position but still some output is made because of the feed forward control you are getting information that okay this is a cycle position and you are getting information from your eyes that okay this is the road then your body position you know by your vestibular system and by your proprioceptor so some feed forward control is going on making you to make certain movements but if the movement is incorrect then what will happen an error signal will be generated and again the information is fed back into the feed forward control so it is learning right and what happens that in the more you practice the more this error signal keeps on reducing and you become skilled in that particular aspect in our case cycling so basically our cerebellum is doing all this thing it is a very important feed forward controller so modification in the way system responds to future inputs that is adaptive control so in summary negative feedback or any control system has five components negative feedback monitors the end result of the activity it is the most common homeostatic system very important and it stabilizes the value of the variable then it has some time lag time lag means because it is detecting the end result of the variable and then after that only making the changes in the response right so it is kind of waiting the end result then the change in the response is occurring by that time maybe the output has changed again because due to some other disturbance isn't it so there is some time lag in the response and this is time lag is countered by means of feed forward mechanisms they are very fast and they are only neural then there is gain of the system which determines the efficiency of the system and it can be neural humoral or neuro humoral positive feedback and feed forward system positive feedback loop is used for switching states quickly it can be disastrous it goes uncontrolled feed forward control sensors disturbances and not the actual change it anticipates a change which could occur based on the disturbances makes corrections accordingly and when this feed forward control can be changed based on the experience that is adaptive control both positive feedback and feed forward systems work under negative feedback system so that was all about homeostatic control systems see you again in the next video