 regulation of body temperature is very important process because the various chemical reactions and enzymatic activities which are happening within a cell depend on the temperature. So, when we talk about temperature regulation we are basically talking about core body temperature. So, it is the core body temperature which is kept in a very narrow range in a constant manner. On the other hand there is shell temperature as well. So, what is the meaning of this core and shell? Core body temperature is the temperature surrounding the deep issues and it is this core body temperature that is kept within a constant range and shell temperature is the temperature of the surrounding areas basically the skin and this temperature varies as that with the environment. So, when the environment is cold shell temperature will be in a very lower value. When the environment is hot then shell temperature may be much higher that is closer to the environmental temperature. So, the normal value of this core body temperature is 98 to 98.6 degree Fahrenheit or if we talk in Celsius then it is 36.5 to 37 degree Celsius. But there are variations in this core body temperature as well. There are basically diurnal variations where it is lowest when we are sleeping. Then if we are awake then it is lowest in morning 6 a.m in the morning it is lowest and it is highest in evening. So, this diurnal variation also occurs. This is important because when we are recording the body temperature that is also important we should keep all these things in mind. Similarly in women what happens during ovulation this basal body temperature increases by 0.5 degree Celsius. Then there is also increase with the exercise and depending on the emotional state of the person with increase in emotion this core body temperature increases. So, these are the normal variations of the core body temperature but apart from that this core body temperature has to be kept within a very narrow range. So, what are the mechanisms by which it is regulated? See always in body what is happening that there are metabolic processes going on within the cells and due to these metabolic processes there is generation of heat. So, even if the environmental temperature is much closer to the body temperature this heat generation is always going on and needs to be lost. So, obviously these regulating mechanisms are always active. So, let us see what are the mechanisms by which there is temperature regulation. Fundamentally temperature is always a balance between the heat gain mechanisms and heat loss mechanisms because ultimately it is the heat which determines the temperature. So, what are these heat gain and heat loss mechanisms? So, heat is gained in the body by the basic metabolic processes which are going on inside the body. So, even if remember that when we are at complete physical and mental risk then also these metabolic processes go on inside the body because they are important for the survival of the cells. So, this metabolic process which goes on even when we are at complete mental and physical risk that is known as basal metabolic ray. Then obviously if there is any increase in activity so any kind of muscular activity will cause the increase in the heat production. So, that is second. Third very important process is something known as a specific dynamic action of food. What is this? When we take food energy is required for the digestion and absorption of the nutrients. So, that is why there is heat production as well and that is known as specific dynamic action of food. Before we proceed let us see that why actually heat is gained. See when any nutrient is being metabolized for the production of ATP it is not that the whole energy of the nutrient is converted into energy. Most of it it is lost as heat. So, heat is produced even when the ATP is being generated and also with the metabolic processes of the utilization of ATP again the entire energy which is present in the ATP it is not utilized for the activity that is for the work done or for the metabolic processes. Instead most of it again is lost as heat. So, heat is gained during the production of ATP as well as in the utilization of the ATP. So, these are basic mechanisms by which heat is being generated in the body. But when needed there are other mechanisms also which can become active and cause increase in the heat production. That is when the body needs to gain more heat it increases this muscular activity and that is known as shivering thermogenesis because heat is being generated. So, thermogenesis and it is increasing the muscular activity which is basically shivering that is known as shivering thermogenesis. There is another mechanism that is non-shivering thermogenesis and this is brought about by increasing the release of epinephrine. So, there is sympathetic activation and there is increase in the release of epinephrine and non-epinephrine which cause this non-shivering thermogenesis. So, we will see what is happening in this non-shivering thermogenesis a little bit later. Coming to heat loss mechanisms these are the fundamental heat loss mechanisms which we study in physics that is radiation, then there is conduction, convection and finally the one which is in which biological activity is involved that is sweating. So, these are very important heat loss mechanisms. So, what is this radiation? If this is the body surface this is the body surface and there are surroundings around the body surface. There are certain infrared rays which are emitted from both the surroundings as well as from the body surface. Now, depending on the temperature difference between this surrounding and the body surface heat is either lost or gained. So, if the skin temperature is more than that of the surrounding then these heat infrared rays will cause the loss of heat from the body. On the other hand suppose the surroundings are too hot they are hotter than that of the skin temperature then the heat will be gained from the body. These are basic physical process. So, this radiation mechanism is very important for the heat loss and it is responsible for 60 percent of heat loss from our body in normal conditions. However, if you understand here that if the surroundings are hotter than the body temperature or we are talking about the skin temperature here then the heat will be gained by the body by this radiation mechanism. So, that means that time this radiation will not be an effective mechanism for heat loss. Then there is conduction what is conduction that is the particles which are in contact with the body in our case say suppose air particles which are in contact with the body. Depending on the difference between the temperature of these contact particles and the skin heat will be lost from the body. So, conduction is basically the transfer of heat from the skin to an object which is in contact with the skin. But you see it is not much effective process because very soon the temperature of these two will equilibrate then what then it will not proceed further. Then the convection comes in convection is basically the movement of these particles away that is because when the air particles become warm they rise up again fundamentals of physics they rise up and cooler particles come down. So, this conduction heat loss will continue because of the convection. So, you might have felt that when it is windy outside then that time we feel more chill why because more and more heat loss can occur because the wind can take away these air molecules away from our body the warmer ones will move away and the colder ones will come closer to our body. So, that is the factor which is responsible for something known as wind chill why we feel chilly in case the environment is windy. But remember this conduction and convection are not very important as far as heat loss from the body is considered the contribution is very less. We can say that this radiation and conduction both contribute around 60 percent. Then finally very important process is this sweating and with sweating what happens that the water molecules come to the surface of the skin and with evaporation they carry the heat energy from the body. Actually with the 1 gram of loss of water there is loss of 0.58 kilocalories of heat. But remember here it is not only sweating which is important along with sweating the water particles should evaporate. If you wipe the sweat off then it will not be able to carry this heat along with that. So, in total the sweating contributes to 30 percent of heat loss from the body and finally with the normal body processes of urination and defecation also there is heat loss. So, this urination and defecation contributes to a loss of approximately 10 percent of body heat. So, fundamentally you see that the heat gain and heat loss mechanisms they depend on the skin temperature. First of all if a skin temperature changes then this radiation conduction convection will change. They depend on sweating and they depend on the metabolic processes and muscular activity. So, if this skin temperature is controlled then we will be able to control the heat gain and heat loss mechanisms in the body. So, that is how the temperature regulating system works. Let us go into temperature regulating systems that how this is brought about. So, whenever we talk about regulation we always think about certain components that is the receptor then what is the afferent center then there is efferent and finally what is the effector. So, receptor in this case there are thermal receptors and these receptors are both for hot and cold. So, they detect this change in the temperature and where are they located? Both they are located on the shell they are located on the skin and they are located surrounding the deep issues of the body surrounding the great veins as well. So, there are skin thermal receptors and there are core thermal receptors to detect the core temperature. Now these receptors via the nerve fibers that is the afferent will give information to the center and center in this case are two which are located in the hypothalamus it is an anterior hypothalamus and then there is posterior hypothalamus. So, anterior hypothalamus is responsible for responding to heat. So, it activates the heat loss mechanisms it responds to heat and posterior hypothalamus is responsible for responding to cold. So, it decreases the heat loss mechanism and increases the heat gain mechanisms. Then what will be the efferent? Efferent again are the nerves there is sympathetic nervous system which is there which gets activated or is inhibited and also the supply to the sweat glands the muscular activity activation. So, that will all be the efferents and nerve impulses involved in that and what will be the effector? Again same thing there will be sweat gland activation muscles and there is either vasodilation or vasoconstriction. So, vessels are very important effector because this vasodilation and vasoconstriction is going to affect the skin temperature and hence the amount of radiation conduction which can happen from the skin to the surroundings. So, let us just see the role of these skin vessels which are important for temperature regulation. Say suppose this is the skin ok this is the epidermis right and below there are vessels. So, there are these vessels capillaries the skin capillaries but here these vessels are connected to the veins as well. So, here there is connection directly first of all to the veins it may not go to the surface and also from here there will be vein connection right. So, let me just draw this. So, from here the returning blood it goes to the veins. So, like this ok. So, you can imagine that when there is vasodilation what is going to happen? There will be more blood flow to the skin right. So, more blood flow to the skin it is going to increase the delivery of heat to the skin. So, blood is carrying the heat along with it to the skin and hence there will be rise in the skin temperature causing the loss of heat by means of radiation conduction. On the other hand if there is vasoconstriction what will happen? If there is vasoconstriction then here these vessels will be constricted because the arterioles are basically constricted and there will be less blood flow to the skin. In fact, there will be direct shunting of blood from arteries to the veins here. So, skin temperature will be much less ok. But keeping the skin temperature less the core body temperature can be maintained because heat is not being lost from the skin to the environment now it is directly being shunted and heat is being conserved. So, even though the extremities are kept cold we are conserving the heat. So, by this simple process of vasodilation and vasoconstriction we can increase the heat loss or conserve heat. So, with this now let us move on to the fundamental mechanism by which temperature is regulated. We will take it in the form of a flow chart. So, as I told you that anterior hypothalamus is responsive to heat and posterior hypothalamus is responsive to cold and these are responsible for various effector mechanisms and the effector mechanisms can be autonomic somatic that is a muscular activity then there are endocrine changes and also there are behavioral changes. So, first let us see the response to the cold. So, posterior hypothalamus is the one which will be responsive to cold and this posterior hypothalamus will receive information from the receptors from the skin as well as the core body temperature. Now, this core body temperature what it is telling that what is the current core body temperature. So, here it is acting in a feedback mechanism that if core body temperature is lesser then posterior hypothalamus will initiate to the mechanisms for heat gain and decreasing the heat loss mechanisms. On the other hand skin body temperature you see what it tells is that even of the core body temperature is not changed but the environmental temperature has changed. So, environmental temperature will affect the skin temperature. So, that is going to be detected by the receptors and this posterior hypothalamus will initiate the responses even when the core body temperature has not yet changed. Try to understand this little bit difficult that core body temperature that what is the current state of core body temperature if it decreases posterior hypothalamus will try to increase the core body temperature. But skin temperature it is not a surrogate for core body temperature it is a surrogate for environmental temperature. So, this skin temperature is giving the information about the environment not the core body temperature. So, the skin receptors are basically important for generating an anticipatory response if somebody has suddenly moved to cold then this anticipatory information is going to the posterior hypothalamus that yes the temperature has changed. So, be prepared and you start the responses so that the core body temperature does not dip because of this disturbance in the environment. So, this is basically a feed forward mechanism it is anticipatory. So, very important that thermo regulation is an example of both feedback mechanism and feed forward mechanism. Now, coming to posterior hypothalamus that once these neurons activate what it does it initiates responses for decreasing the heat loss right and increasing the heat gain and what are these responses. So, decreasing the heat loss what it will do is it will cause vasoconstriction. So, less heat delivery to the screen and that is brought about by sympathetic activation okay so that is vasoconstriction. Then it also causes contraction of arcter pili muscles. These muscles are responsible for what is known as goose bumps the hairs they become erect and just besides the skin a layer of air is trapped and it cannot just move away. So, a layer of air becomes trapped and this mechanism is very important in case of animals not for human because we do not have that much thick layer of hairs. So, we are keeping a warm air just in near our skin as I told you what happens in conduction that soon that the body temperature and the contact object which is there the temperature will become constant and heat loss will not occur any further for that the air particles should move away. So, here we are just trapping the air and not allowing the air particles to move away. So, this is creating a kind of insulator area right very important for muscles. For us this mechanism is basically more important in behavioral change. So, like when we are wearing thick layers of clothing what we are doing basically this air is being trapped between different layers of clothing. So, that is another mechanism coming to increase in heat gain mechanisms. So, what are these that is first of all shivering that is shivering thermogenesis actually in posterior hypothalamus there is a center for shivering primary motor center for shivering in there in dorsomedial posterior hypothalamus and what it does is activation of the descending tracts in the brain stem which are there basically the extra pyramidal tracts because the shivering is involuntary isn't it and this causes activation of the alpha motor neuron and there is increase in tone of the skeletal muscles. So, this increase in tone we can feel when we are in too much cold we feel our body tightening and shivering occurs because that is stretch reflex which is getting activated at a local level. So, the increased tone by the descending tracts stretch reflex tries to relax it back, but again there is increase in tone. So, it is kind of oscillation which is occurring at the local level. Fundamentally from the shivering center it is increase in tone of the skeletal muscles. Then another heat gain mechanism is the sympathetic activation which is responsible for non shivering thermogenesis and non shivering thermogenesis is because of the release of the norepinephrine what it does that it causes activation of iodinase enzyme which is important for conversion of T4 to T3. So, there is increase in this T3 and this causes increase in the expression of uncoupler protein UCP1 also known as thermogenin. So, what is this uncoupler protein? See whenever ATP is being generated then the hydrogen gradient is being coupled to ATP generation. So, this uncoupler protein what happens that hydrogen gradient is uncoupled with ATP generation and all the energy which is in the gradient it is lost as heat. So, ATP is not generated everything is being lost as heat. So, this is basically happening at the level of the mitochondria and actually this non shivering thermogenesis mechanism is very important in a particular tissue known as brown adipose tissue. Brown adipose tissue is in high amount in case of infants. So, for them this non shivering thermogenesis is a very important mechanism because this brown adipose tissue has constitutional expression of this uncoupler protein and it is responsible for heat generation. But with age the amount of this brown adipose tissue decreases and it is not very important in case of adults. So, some facts about this brown adipose tissue it is present in high amount in newborns infants and the location of this brown adipose tissue is basically in the inter scapular area and this is richly supplied by the sympathetic nerves. So, this norepinephrine is very important for the activation of this uncoupling protein. Now apart from this there are behavioral responses as well. So, what are the behavioral responses? See we try to reduce the body surface area. Suppose you are exposed to cold how is your body position? You try to fold your hands isn't it? So, what you are doing? You are trying to decrease the body surface area so that the heat is not lost from the surface to the surroundings. Again this mechanism is very important in case of animals because we as humans have various other mechanisms. We have houses we can keep our houses warm then we can cover ourselves with blankets so all those mechanisms are activated. Then there is increase in the activity as well. So, one is shivering but increase in muscular activity also occurs and it may be subconscious like tapping of the feet movement of the hands all this increases. Then there are endocrine mechanisms also activated that is there is increase in release of TRH and as I told you that thyroid hormone is important for increased expression of uncoupled protein. So, this increase in TRH will cause increase in the release of the thyroid hormones. However again this response is not an acute response in fact it is activated when there is exposure to cold for several weeks. So, especially for the residents of high altitudes and if there are people who are posted there for some job. So, in those people we see this increase release of the thyroid hormones and increase in T3 levels. So, that was about responses to the cold but before we go to the heat responses just one thing I want to mention here something known as Lewis hunting phenomena. What is Lewis hunting phenomena? Basically as I told you that there is vasoconstriction to conserve the heat but with this vasoconstriction the peripheries will be kept cold right. So, blood flow is not happening to the peripheries but every cell requires oxygen. So, whatever is little amount of blood is going there they will extract oxygen and then there will be deoxygenated blood but decrease in oxygen in the local tissues will lead to vasodilation. So, vasoconstriction is happening because of the sympathetic activation but because of the local accumulation of the metabolites and decrease in the oxygen this will lead to vasodilation. So, there is vasoconstriction and ultimately with accumulation of metabolites it leads to vasodilation. Once these requirements of the tissues are fulfilled and the metabolites are washed away again the sympathetic activation will take over and again it will lead to vasoconstriction. So, there is alternating vasodilation and vasoconstriction in cold. Now coming to anterior hypothalamus responses to heat. So, what it does anterior hypothalamus first of all remember that as I told you that there are obviously the thermal sensors that is the receptors yes they are there but these receptors on the skin and core body are more for the cold they detect cold much better heat receptors are much less compared to the cold actually cold receptors are 10 times more than that of warm receptors. On the other hand anterior hypothalamus itself has heat sensitive neurons so the warm blood which reaches to the brain this is detected by the heat sensitive neurons in the anterior hypothalamus and what do they do they basically inhibit the responses of the posterior hypothalamus. So, all these will reverse there will be decrease in sympathetic activation leading to vasodilation this response of electropelai will be gone then there is a decrease in the heat gain mechanisms plus there will be activation of the sweating mechanism which will cause the evaporation of the water and loss of heat from the body. So, that is very important and finally there will be some behavioral changes again so what are the behavioral changes like switching on the fan then as I told you before that the specific dynamic action of food also generates heat. So, in a hot environment what happened we feel enorexia we don't want to eat anything right then there is decrease in muscular activity we don't want to move around much because that will also increase the heat gain right. Just before we end one very important point I want to bring out that as I told you that radiation and conduction are important for 60% of the heat loss and sweating is important around 30% of the heat loss but in case of very hot environments where the surroundings are at a higher temperature than the skin temperature then radiation and conduction will lead to heat gain and that case sweating will be the only mechanism which is responsible for the heat loss. On the other hand if it is hot and humid environment so humidity is also there then in that case the water will not be able to evaporate from the skin so sweating mechanism will also be not able to take away heat from the body so then it becomes very dangerous so that is why we say that hot and humid environment don't exercise especially in hot and humid environment because exercise is leading to heat gain in our body and that is very important that sweating mechanism should lead to loss of the heat but if it is humid environment then that is not happening. So that was all about the temperature regulation mechanisms what will be the problems which can occur if temperature regulation mechanisms are not effective that we will see in another video. Thanks for watching the video if you liked it do press the like button share the video with others and don't forget to subscribe to the channel Physiology Open. Thank you.