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The picture to the left depicts the various elements that are responsible for thermoregulation in human skin. The illustrations shows the various layers of skin along with the veins, arteries and capillaries of the circulatory system that assist in insuring that the thermoregulatory system works properly. The sweat glands are responsible for selectively removing materials from the blood the sweat glands then concentrates or alters these toxins, and secretes them for elimination from the body. The perspiration or sweat is then removed through the sweat pore. This has a twofold purpose: to remove toxins and thermoregulation (in this case cooling the body).
Thermoregulation involving perspiration is brought about by both internal and environmental heat and exercise. As it relates to the latter, there have been many studies related to exercise and thermoregulation. According to Marino (2004)
"thermoregulatory effector responses of humans and concluded that temperature regulation during exercise is dissimilar to temperature regulation during fever.
Essentially, Nielsen described the human thermoregulatory system during exercise as analogous to a thermostat, whereby the increase in core temperature is proportional to the metabolic rate and almost independent of the environmental conditions over a wide range. Since this early understanding of human thermoregulation during exercise, there have been a plethora of studies dealing with the human thermoregulatory system and its limitations during exercise
The author also explains that this subject matter has lead to the study of thermoregulation during exercise under different types of environmental conditions (Marino, 2004). The author explains that it has been established that people have the capacity to thermoregulate proficiently during exercise when the ambient air is cool to moderate, however such thermoregulation has been more difficult during exercise in warm conditions (Marino, 2004). The author further asserts that, "an analysis of the average ambient temperatures of the cities in which the fastest times for distance events ranging from 5000 m to 100 km road race are set (are rarely above an average of ~14 8C (Marino, 2004)."
Indeed Skin blood flow responds to heat exposure and exercise. In most human beings both skin blood flow and vasodilation increase to allow for the dissipation of heat. When people are in cold environments vasoconstriction occurs (Charkoudian, 2003). The skin reduces the amount of heat that is lost and protects the body against hypothermia (Charkoudian, 2003). Therefore, altered control of skin blood flow has important clinical implications and can substantially impair the ability to maintain normal body temperatures (Charkoudian, 2003).
When environments are thermoneutral the resting skin blood flow is around
250 mL/min (Charkoudian, 2003). This results in heat dissipation of 80 to 90 kcal/h, which is identical to resting metabolic heat production (Charkoudian, 2003). When people are exposed to heat or exercise a rise in body temperature causes cutaneous vasodilation and sweating. Cutaneous vasodilation is responsible for significantly increasing blood flow to the skin (Charkoudian, 2003). This process is also responsible for convective transfer of heat from the interior of the body to the exterior of the body (Charkoudian, 2003). When Cutaneous vasodilation occurs the large increases in skin blood flow often necessitates greater cardiac output and redistribution of blood flow from other areas of the body, including the splanchnic area in which vasoconstriction occurs (Charkoudian, 2003).
The adjustments that are made are normally enough to reflect the demand for greater skin blood flow, so that the amount of oxygen getting to vital organs is not compromised. The author also explains that cutaneous vasodilation and the drying up of sweat decreases skin temperature, which is responsible for cooling the blood in the expanded skin vessels before it returns to the body's core. The author also explains that
"In general, skin blood flow and sweating continue to increase in proportion to internal temperature until a steady state is reached at which heat dissipation and heat generation are equal, and therefore body temperature is constant, or until maximal responsiveness is reached. When internal temperature decreases toward normal, sweating stops, and skin blood flow returns to normal. In this sense, thermoregulation represents a classic negative feedback loop (Charkoudian,
When the entire body is heating there are certain temperature thresholds that are reach so that cutaneous vasodilation and sweating can take place (Charkoudian, 2003). These thresholds are referred to as the internal temperatures at which cutaneous vasodilation or sweating starts (Charkoudian, 2003). Additionally the occurrence of sweating or a vasodilator reaction as it relates to internal temperature is described as the slope of the skin blood flow or the internal temperature relationship following the reaching of the threshold (Charkoudian, 2003). This is why the body may have altered reactions in some situations. For instance, "lower skin blood flow at a given internal temperature during heat stress could be due to an increased threshold for vasodilation (such that vasodilation does not begin until a higher internal temperature is reached), a decrease in the sensitivity of the response, or some combination of both (Charkoudian, 2003)."
The picture above is a depiction of 2 cutaneous vasodilator responses as functions of internal temperature during body heating. The author explains that the responses shown have differing thresholds but their sensitivities are alike (Charkoudian, 2003). This leads to noticeable differences between responses as it pertains to the level of skin blood flow and as such heat dissipation for a certain level of internal temperature (Charkoudian, 2003). The author explains that issues that can change the threshold and/or sensitivity of cutaneous vasodilation include the status of reproductive hormones, circadian rhythm, heat acclimation, and exercise training (Charkoudian, 2003). When people are exposed to cold environments, skin blood flow is reduced as a result of cutaneous vasoconstriction (Charkoudian, 2003). The author explains "This results in a decrease in heat dissipation from the skin surface and less convective heat transfer from the core to the surface. With further body cooling, shivering begins (Charkoudian, 2003). The muscular contractions involved result in increased heat generation, which in combination with decreased heat dissipation helps to maintain core temperature in the face of cold exposure (Charkoudian, 2003)."
Reflex control of Skin Blood Flow
There are different types of controls as it pertains to the human body and the thermoregulatory system. The first type of control that will be discussed is reflex control. According to (Charkoudian, 2003) cutaneous circulation in people is controlled by 2 populations of sympathetic nerves this type of control is different from other animals. The sympathetic adrenergic vasoconstrictor nerves and the sympathetic vasodilator nerves (Charkoudian, 2003). The sympathetic adrenergic vasoconstrictor nerves are generally well-known and understood (Charkoudian, 2003). On the other hand the sympathetic vasodilator nerves is not as well studied and it is only activated during hypothermia.
The author further explains that both the sympathetic vasoconstrictor and vasodilator nerves supply nerves to all areas of hairy (non-glabrous) skin. On the other hand areas of, hairless (glabrous) skin are supplied with nerves only by sympathetic vasoconstrictor nerves (Charkoudian, 2003). The author also explains that there are other factors that distinguishes glabrous and nonglabrous skin which is the presence of arteriovenous anastomoses (AVA). AVA are thick-walled, low-resistance channels for liquid that permit high flow rates directly from arterioles to venules. The authors explains that when skin is glabrous, AVA are plentiful and richly innervated by sympathetic vasoconstrictor nerves (Charkoudian, 2003). Because this is the case, the opening or closing of glabrous skin AVA can result in significant changes in skin blood flow (Charkoudian, 2003). On the other hand nonglabrous skin has limited AVA and is innervated by both sympathetic vasoconstrictor and vasodilator nerves (Charkoudian, 2003).
The author also explains that Sympathetic vasoconstrictor nerves are responsible for giving off norepinephrine. Norepinephrine combines with postsynaptic ?1- and ?2-receptors on cutaneous arterioles and AVA (Charkoudian, 2003). The article also explains that noradrenergic vasoconstrictor nerves discharges at least one co transmitter that is responsible for vasoconstriction (Charkoudian, 2003). Even studies involving animal models have found
"roles for neuropeptide Y (NPY) and/or adenosine triphosphate as adrenergic cotransmitters mediating cutaneous vasoconstriction,32,33 the identity of those that participate in human reflex vasoconstriction remains to be elucidated. The vasoconstrictor system in human skin is tonically active in thermoneutral environments. Therefore, subtle changes in the activity of this system during most daily activities are responsible for maintenance of normal body temperature with slight changes in activity or ambient temperature (Charkoudian, 2003)."
According to the author the aforementioned scenario occurs because minute fluctuations in skin blood flow can lead to significant changes as it pertains to heat dissipation. For instance, the author points out that when skin blood flow changes from resting neutral levels of 8 mL per 100 mL/min over the whole surface of the human body a doubling of heat transfer to the environment occurs. The transfer is significant because it allows an area of thermoregulation known as the "neutral" or "vasomotor" area. This is only made possible through changes in cutaneous vasomotor tone (Charkoudian, 2003). The author also explains that the vasoconstrictor…[continue]
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