 Heat illnesses range in a spectrum from mild to severe most which includes the heat syncope, heat cramps, heat exhaustion and heat stroke. So basically this heat illnesses result when the body is not able to balance the heat gained versus the heat lost. So in especially hot and humid environments these illnesses can occur because in these environment what happens that the body is trying to lose the heat but in hot environment the body is restricted to lose heat only by sweating. Why? See when the environment is greater than 37 degrees Celsius say suppose 40 degrees Celsius what will happen that instead of losing heat by the physical mechanisms of radiation and conduction the body will start gaining heat by means of radiation. So the only mechanism which is left for the body to lose heat is sweating and because of this the various heat illnesses result. So let us see each of them one by one. First dealing with the mildest one that is the heat syncope. What is this? Heat syncope is basically collapse after exercise and it can also occur in elderly people because the elderly people are prone to volume depletion. So what happens that when we exercise in hot environment as I told you before that sweating remains the only mechanism for losing the heat. So body will cause excessive sweating and obviously there will be vasodilation will be there yes because skin blood flow is increasing and when we are exercising muscle blood flow is also increasing and because of these two mechanisms there will be volume depletion. Excessive sweating is going to cause volume depletion that is there obviously and due to vasodilation there will be decrease in peripheral resistance. So when peripheral resistance is less what happens there is decrease in diastolic blood pressure as peripheral resistance is the main determinant of diastolic blood pressure and due to volume depletion there will be decrease in venous return and hence decrease in cardiac output this will in turn cause decrease in systolic blood pressure also. So the person is going to collapse after exercise mainly due to excessive sweating. So that is heat syncope. Moving on to the next one that is heat cramps. What is heat cramps? Again it occurs due to excessive sweating. So heat syncope was exercise induced collapse and heat cramps is exercise induced muscle cramps and remember this heat cramps occur after the exercise not during the exercise these are not the cramps which occur in athletes during the exercise but these heat cramps manifest after the exercise at rest. So when we exercise in hot environment again there is excessive sweating as we have seen before there is volume depletion yes but along with the water loss there is sodium loss as well because when we sweat the sweat includes not only water but the electrolytes as well and the major electrolytes which are lost are sodium and chloride. So there is sodium and chloride depletion and if now the person who has exercised because of the dehydration drinks excessive water and that to that water is not having any electrolytes. So what is happening we are only replacing the water and sodium deficiency remains there. So it leads to hyponatremia and hypochloremia and because of this hyponatremia and hypochloremia muscle cramps occur. These muscle cramps especially manifest in unaclimatized individual. Why? Unaclimatized individual the sweat glands actually lose both the sodium and water profusely but after acclimatization the sodium loss and sweat decreases. So that is why these muscle cramps are more common in an unaclimatized individual and chances of development decrease as the person becomes acclimatized. Coming to the next heat illness that is heat exhaustion. Heat exhaustion again is because of the compensatory mechanisms which the body is taking to lose the heat. So you see in all three heats syncope heat cramps heat exhaustion the compensatory mechanisms are intact. The body is trying to lose the heat and the manifestations which occur is because of that compensatory mechanism. So in heat exhaustion what occurs that there is dehydration again because of excessive sweating but this heat exhaustion is of two types. There is water depletion heat exhaustion so it is basically isotonic heat exhaustion and the other is salt depletion heat exhaustion where again what happens that the person is drinking lot of water while working in a hot and humid environment. So only the water is replaced and not the salt. So this leads to salt depletion heat exhaustion and which kind of individuals the salt depletion heat exhaustion will be more common again in unaclimatized individual because in acclimatized individual the sodium loss in sweat is less. So if adequate water intake is not there not balanced by the amount of the water loss then it is going to lead to dehydration and the manifestations of heat exhaustion are mainly because of that. So whenever there is dehydration basically there is decrease in the extracellular fluid. So that is going to decrease the venous return decrease the cardiac output and once cardiac output decreases that means there is decrease in the blood pressure and also it will manifest as orthostatic hypotension that is the person will feel dizzy when he is trying to change the position from lying down to standing. Then again body will try to compensate because cardiac output is less. So baroreplex will activate and it will lead to increase in the heart rate so there will be tachycardia. Plus because of dehydration there are some general symptoms that is malice is there nausea vomiting. So these symptoms are present in both kind of heat exhaustion. On the other hand in salt depletion heat exhaustion there is other features also present. First of all as I told you before what happens in heat cramps it is due to the hyponatremia which develops. So similar to that in salt depletion heat exhaustion we have muscle cramps. Then in salt depletion heat exhaustion there are high chances of development of cerebral edema why so. See if this is extracellular fluid and we are telling that this extracellular fluid is hypotonic okay and here is intracellular fluid. So if extracellular fluid becomes hypotonic which side will the water move whenever there is change in tonicity the water starts moving by osmosis. So water always moves from hypotonic to hypotonic solution. So water will move from ECF to ICF and that is going to happen in the neurons as well so that leads to cerebral edema and this in turn leads to neuronal signs and symptoms that is vertigo impaired judgment. So remember that in salt depletion heat exhaustion we will see CNS involvement signs and symptoms as well. So that was all about the heat illnesses which occur when the compensatory mechanisms of the body are intact and there are consequences of excessive compensatory mechanisms taking place. But the last one heat stroke is the one where these compensatory mechanisms have stopped working. So this heat stroke basically consists of a triad of exposure to heat stress as I have been telling the hot and humid environment basically. The rise in core body temperature greater than 40.5 degree Celsius. So you see the body is trying to compensate but if the heat gain is not balanced by heat loss then the core body temperature is going to increase and once it increases above 40.5 degree Celsius then the person lands up in heat stroke because then there is CNS dysfunction and there will be loss of thermoregulatory mechanisms. And also you see there is a kind of a positive feedback going on here that when body temperature starts increasing what happens that because of continuous sweating there is dehydration that is there is a decrease in extracellular fluid volume or decrease in plasma volume we can say. So obviously this is going to decrease the skin blood flow and skin blood flow is very important for losing heat, isn't it? Plus also it is going to decrease sweating that is also very important for losing heat. So once these decrease what happens the heat loss further decreases. So these are going to lead to increase in body temperature. Anyways let us move on to the features of heat stroke. So features of a heat stroke first of all obviously it is the features of the dehydration because the person will be severely dehydrated so there will be increase in heart rate decrease in blood pressure as we have seen before also there will be increase in respiratory rate. Then since now the sweating has stopped severe dehydration ultimately will lead to decrease in sweating plus also because the CNS is affected so the thermoregulatory mechanisms will be affected. So sweating has stopped so if they touch the skin of such a person it will be hot and dry. If the person is sweating profusely then how will be the skin? Skin will be cold and wet right? But this stoppage of sweating appears as hot and dry skin. Then at such a high temperature of 40.5 degrees Celsius you see the body proteins start to de-nature. So we even start seeing organ failure and especially susceptible organ is liver. So hepatic dysfunction occurs since hepatocytes are very heat sensitive. Also there will be de-naturing of the proteins of the muscle. Muscle will start breaking down that is raptomiolysis will occur and because of raptomiolysis there will be renal failure. So we will see certain signs like if we go for lab investigation there will be increase in creatine kinase then due to renal failure we will see oligouria then microscopic hematuria may be present. Also because of muscle breakdown and the release of myoglobin from there there will be presence of myoglobinuria. So we will start seeing signs of organ failure. Then whenever the body is exposed to extreme conditions in this case extreme heat there is activation of heat shock proteins. Heat shock proteins are present in the cell and these are protective in nature. However when anything becomes too much excessive then it becomes destructive in nature in bodies. So there will be initiation of inflammatory response and that is going to lead to vasodilation plus increase in permeability of the blood vessels. Once permeability of blood vessels increase then the fluid will start moving out of the vessels more see already there is dehydration. So if the fluid moves out of the vessels it will further exacerbate the dehydration and there will be further decrease in the blood pressure leading to cardiovascular collapse. So you see everywhere we are seeing the signs and symptoms like all the organs of the body are getting affected. Then there is damage to vascular endothelium directly also because of this inflammatory response a lot of interleukins are being released. Also the heat itself directly damages the vascular endothelium and once the endothelium is damaged there is activation of the clotting factors because you see smooth endothelium is one of the mechanisms by which the clotting factors are kept inhibited it doesn't allow the coagulation cascade to activate. So the damage to the vascular endothelium will start activating the coagulation pathway leading to disseminated intravascular coagulation. So there will be decrease in platelets decrease in clotting factors in DIC what happens actually in a lot of places in the body they see unnecessary coagulation where it is not needed. Then also because of the temperature itself there will be damage to the neurons. Neurons are very sensitive to heat. So we will see CNS symptoms as well there will be loss of coordination in the patient. In fact there will be loss of consciousness itself due to neuron damage. So that was all about the various heat illnesses that is heat syncope heat cramps heat exhaustion and heat stroke. 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.