 cutaneous circulation has many special features. First of all remember that unlike any other circulation the amount of cutaneous circulation is not determined by its metabolic requirements. In fact the metabolic requirement of the skin is quite less and if solely based on metabolic requirement its circulation will be very very less. However the amount of cutaneous circulation is governed by the requirement of thermo regulation depending on that whether heat has to be conserved or heat has to be lost. So in case of hot environment where heat has to be lost from the body in that case cutaneous circulation can increase to as much as 150 ml per 100 gram per minute and in case where vasoconstriction occurs that means in cold it can be very low to as low as 1 ml per 100 gram tissue per minute. So this concept on thermo regulation I have dealt in the video on temperature regulation that you can have a look. But here apart from this we will deal with certain special characteristics of cutaneous circulation. So first comes the presence of arteriovenous amastomosses. So this diagram here shows that this is the presence of arteriole and these are the venules and these are running deep in the dermis and you see this connection runs in the superficial dermis as well and then at the level of the epidermis what we are seeing that there is a capillary loop arising. So there is arterios then there is a formation of the capillaries then veins which are taking away the blood back to these venous plexus. Now two things to note here one you see that the blood is flowing in parallel and opposite direction. So like suppose this is the incoming blood right and this is the taking away of blood in the venules. So this is parallel and opposite flow of blood and this is what is known as counter current flow. We will talk about it a little bit later. But here you see that there is presence of arteriovenous anastomosses that is direct connections between the arterioles and the veins and these arteriovenous anastomosses are present in apical regions in the body that is they are present in palms, soles, then the fingertips right and toes also, then ears, nose and some other face areas like lips. So presence of these AV anastomosses is important for thermoregulation. You see what happens suppose there is cold. So in cold there will be vasoconstriction and these AV anastomosses are supplied by a sympathetic vasoconstrictor fibers very important sympathetic vasoconstrictor fibers and when there is a activation of this sympathetic system it causes vasoconstriction of these anastomosses. So blood will be shunted from the arterios to the veins directly. It will not go via these AV loops so this AV anastomosses is basically closing this channel and there will be decrease in blood supply to the superficial surface that is the epidermis. So due to vasoconstriction there is decreased blood supply to the surface of the skin. So what? So if there is decreased blood supply there will be decrease in the heat loss from the blood vessels to the environment right. So that is very important the function of the arteriovenous anastomosses and that is also the reason that in these areas you see when there is cold what happens your hands and feet fingertips your nose tip they are cold right they are much closer to the environmental temperature why? Because the blood supply to these areas decreased and the heat is not coming to the surface so these areas are much closer to the environmental temperature. On the other hand when it is warm it is hot what happens that there is loss of this activation of the sympathetic system it leads to vasodilation and hence the blood flow occurs to the skin surface. So that is the importance of these arteriovenous anastomosses. So remember they are very important for thermoregulation and they are supplied by sympathetic vasoconstrictor fibres and unlike the other arterioles of the body they do not respond to metabolites no response to the metabolites please remember this because you know that locally what happens in other tissue beds and even in other regions of the skin where there are only arterios and not arteriovenous anastomosses the arterios respond to the accumulation of the metabolites as well so that whatever is the local metabolism in the skin the arterios will be responsive to that as well but these av anastomosses solely respond to sympathetic supply plus also to the local temperature local skin temperature whether it is hot so both to heat and cold they respond but not to the metabolites. Coming to the next important concept that is the presence of countercurrent flow in circulation. Now we will take one example suppose the environment is cold then what will happen that warm blood whatever warm blood is reaching to the skin even though there is vasoconstriction some blood flow will happen to the skin isn't it so once that warm blood reaches to the skin there will be heat loss from the skin to the environment right but due to this countercurrent flow what happens that heat moves from the arterial side to the venous side and hence whatever blood reaches to the skin that will become less warm understanding this is going to become less warm because already the heat transfer has occurred from the arterial side to the venous side and hence this warm blood is going back to the core areas otherwise what will happen there will be more heat loss and cold blood will go into the core regions of the body but because of this heat movement two things happen one warm blood reaches to the core regions second there is less loss of heat from the surface because here the blood is less warm and you know that energy loss depends on the energy gradient right so here because less warm so the heat gradient is less so less heat will be lost from the skin to the environment what will happen in case of hot environment hot environment opposite will happen so let us try to understand it in opposite manner what will happen in hot environment let me just delete it so in case of hot weather warm blood is coming right and here much much more warmer blood will reach to the skin surface so again the heat gain will be less because the heat loss and heat gain depend on the temperature gradient so if here it is much more warm so heat gain from the environment will be less since the temperature gradient is less between the environment and the blood so that is how counter current flow helps in heat conservation in cold environment and less heat gain in hot environment moving on to some local reactions of the skin out of which first one is white reaction what is white reaction see when the skin is stroked firmly by an object what happens that there appears a white line so if I stroke firmly there will appear a white line you can try it on yourself and why this white line occurs because once that stroking occurs there occurs due to this stroking constriction of the pre capillary sphincters okay pre capillary sphincters that is before the capillary there are certain sphincters they constrict because of the mechanical effect of the stroking and this prevents the entry of blood into the capillaries and whatever blood is there it drains out so the region which has been stroked becomes white so this is white reaction and why does it occur it occurs because of the constriction of the pre capillary sphincters next local reaction which we will see in a skin is triple response and what is this triple response triple response consists of three responses as its name suggests that is there is redness then there is flare flare means flaring of the redness increase in the redness in the area surrounding and there is formation of the wheel that is a local swelling so what happens when the skin is struck more firmly than that which is happening in the white reaction when it is struck more firmly we see this triple response and what is happening that redness occurs due to local capillary dilation here what we saw in white reaction there is constriction of these sphincters in red reaction these capillaries which are there they dilate and that is just by the mechanical effect of the injury right so when there is local dilation there will be increase in the blood flow and this blood flow will be seen as redness then flare flare is that suppose this is the area which is injured spreading redness to the surrounding area is flare and why this happens this happens because behind the capillaries there will be arterioles right so these arterioles also dilate and when these arterioles also dilate then whatever are the capillaries which are supplied by these arterioles they will also dilate so the surrounding area will also become red so redness first one due to capillary dilation flare due to arterial dilation okay and wheel wheel why it occurs that occurs due to increase in the capillary permeability increase in the capillary permeability and what happens when there is increase in the capillary permeability the water will flow out of the capillaries into the surrounding tissues that there will be extravacation of the fluid and this is going to lead to local swelling so this triple response we see in a skin and the cause is basically redness occurs due to capillary dilation due to local mechanical effect and flare and wheel occur because of arterial dilation and increase capillary permeability respectively but this response involves nerves and very interesting phenomena is seen here which is not seen anywhere that is the only example of orthodromic conduction so we will see what is this so what is happening that there is a phenomena known as axon reflex axon reflex and this is responsible for flare and wheel so suppose here this is showing the skin and this is the site of injury now the nerves there are stimulated and what happens that these nerves will carry the signal to the spinal cord that is the orthodromic conduction right but via one branch there is conduction back to the skin and this is anti-dromic conduction axon reflex is the only example in the body of anti-dromic conduction and these nerve fibers actually release two substances one is substance P and second is CGRP okay and these both of them are responsible for arterial dilation so they are responsible for flare and the wheel that is increase in the capillary permeability is caused by substance P substance P is responsible for increasing capillary permeability and hence responsible for local swelling of the skin so that was about triple response coming to the next part that is the nerve supply as I told you that there is very interesting nerve supply one is sympathetic vasoconstrictor which is supplying the arterioles everywhere and also the AV anastomosis right which I told you how it is important in causing shunting of blood during cold and the other one is sympathetic cholinergic and sympathetic cholinergic actually supplies the skin near the sweat glands right and when these sympathetic cholinergic fibers are activated there is release of acetylcholine and what happens you do this there is actually conversion of quinineogen to brady quinine see basically this sympathetic cholinergic causes secretion of the sweat right and sweat contains an enzyme which is responsible for conversion of quinineogen to brady quinine and brady quinine is a vasodilator that is how in case of hot environment both there is activation of sweat secretion and also there is local vasodilation in the skin so this is one example of sympathetic cholinergic supply in the body which basically supply the sweat glands now with this we have dealt with the major characteristics of the cutaneous circulation but I will answer some why questions here that one why we have blushing in case of embarrassment why our skin becomes redder when we are embarrassed and there is blanching in case of fear or anxiety see skin in the head neck and shoulder region it is supplied by sympathetic vasoconstrictor fibers which are directly under the central control so basically in case of embarrassment the emotional reaction which occurs in amygdala from there there is inhibition of the sympathetic activity and this inhibition of the sympathetic activity will lead to vasodilation and then hence it will lead to blushing on the other hand sympathetic activation occurs in case of fear anxiety so again amygdala reaction it leads to activation of the sympathetic activity and it leads to vasoconstriction and this blanching occurs in the head neck and shoulder region so that is first second is that why in case of cold the skin appears red rudiness occurs in cold see we are telling that in cold there will be vasoconstriction then why there is reddish appearance of the skin in case of cold that is because the muscles in the blood vessels respond locally directly to the temperature so initially when there is cold there is this spasm of the muscles so that leads to the vasoconstriction correct but as the temperature decreases more and more towards the colder side this muscles stop responding and hence there is loss of the muscle spasm thus leading to vasodilation understanding so it is this local vasodilation because the muscles have a stop responding because of too much cold that we see local redness of the skin in case of too much cold third is we may see an alternating phenomena of vasoconstriction and vasodilation because cold will lead to local vasoconstriction local arterial or constriction will occur right so that is going to lead to vasoconstriction this in turn is going to lead to ischemia okay decrease in blood supply to the skin and hence there will be accumulation of the metabolites okay accumulation of metabolites will again act on the arterioles and that will lead to vasodilation because these arterioles will dilate and there will be increase in the blood supply to the skin so in severe cold we may get alternating periods of constriction and dilation and here we are talking about the normal and here we are talking about the normal arterioles not the avianastomosis arterioles understanding because they are I said that they do not respond to the metabolites so this phenomena of local vasoconstriction and vasodilation is known as louis hunting phenomena louis hunting phenomena and when pathological it leads to renaught phenomena what is that that local vasoconstriction will be so much that ischemia will occur and ultimately it will lead to sinuses because more of oxygen will be extracted from the hemoglobin leading to accumulation of deoxygenated hemoglobin and there will be sinuses then accumulation of metabolites vasodilation and this in turn will lead to redness and this phenomena is known as renaught phenomena so two terms you remember louis hunting phenomena and renaught phenomena and finally there is a question asked in the exams also it is asked that why patient of shock should not be warmed beyond the point which can cause a rise in body temperature so warming of a patient of shock has to be limited why because if the person is warmed too much there will be arterial dilation isn't it so body temperature has risen that will lead to arterial dilation for the heat loss to occur so now the body wants to regulate the temperature so for the heat loss there will be arterial dilation however this arterial dilation will lead to decrease in the peripheral resistance okay decrease in peripheral resistance and this decrease peripheral resistance will lead to decrease in blood pressure so we are treating the person for shock and we want the blood pressure to be maintained but because of the rise in the body temperature for the heat loss to occur there will be arterial dilation but that is in turn going to decrease the peripheral resistance remember that the skin is a very important site for peripheral resistance because there are two types of arterioles one is avianastomosis is there which is also important resistance vessels and then there is a normal arterioles also and both contribute to peripheral resistance and both will dilate thus causing decrease in the peripheral resistance and that is also one reason that why it is so much difficult to exercise in hot environment because in hot environment both of these will dilate and there will be decrease in the peripheral resistance and for exercise we want blood pressure to be higher so blood pressure cannot be maintained if the person is exercising in hot environment due to the fall in the peripheral resistance and the person may collapse when exercising in hot environment so that was all about the special characteristics of the cutaneous 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