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Thermoregulatory Mechanisms
The temperature of the body is regulated by neural feedback mechanisms which operate primarily through the hypothalmus. The hypothalmus contains not only the control mechanisms, but also the key temperature sensors. There are 2 types of thermoreceptors, the core thermoreceptors and peripheral thermoreceptors. Core thermoreceptors are located in the spinal cord, gut, great veins and the hypothalamus itself whereas peripheral thermoreceptors are found mainly in the skin. The normal body core temperature is determined by the hypothalamic temperature set point and the body will use thermoregulatory effectors to maintain a constant core temperature around this set point. This set point is affected by skin temperature as well as fever.
In a cool air-conditioned room, the initial temperature drops below 37°C when the room temperature was regulated at 24°C. After resting for about 10 minutes, the temperature of the body was recorded to be about 37.1°C. This change was brought about due to an increase in the hypothalamic set point to about 37.1°C. Cold fibers in the thermoreceptors of the skin detected the decrease in temperature in the surroundings and this information is conveyed to the hypothalamus. This resulted in an increase in the set point to prepare the body to conserve heat in preparation for the cold. Hence, there is an increase in temperature even under resting conditions.
The loss of heat led to an initial decrease in the core temperature. When this information was relayed back to the hypothalamus, several effector responses was triggered to return core temperature to normal. Temperature was increased by a variety of responses initiated by the posterior hypothalamus. Vasoconstriction of the blood vessels occurs to decrease the flow of heat to the skin so as to conserve heat in the body. Furthermore, norepinephrine, epinephrine, and thyroxine are secreted to increase heat production. Cellular metabolism which is the chemical breakdown process that occurs within cells also increases. This process releases heat and warms the body. The basal metabolic rate also acts as a constant internal furnace. Its rate increases slightly when exposed to the lower temperatures at extended time frame. The hairs on the body also 'stand on its end', and trap a layer of air between the hair and the skin. This provides an insulation of warmer air next to the skin which reduces heat lost. Shivering may occur to increase heat production in the muscles. Shivering generates heat as muscles quickly contract and shake.
Muscles, which make up 50% of our body weight, produce 73% of our heat during work. However, after walking for 10 minutes in the cold air-conditioned room, the body temperature actually decreased to 35.8 °C. When exercise was done, it took a very long time to sweat as sweating is being suppressed to prevent heat loss. This could be due to a corresponding decrease in the body set point as the activity caused the body to expand energy thereby producing heat in the process and raising the core body temperature. This increase in temperature was again detected by the thermoreceptors that led to physiological events initiated by the hypothalamus to bring the core body temperature down to normal. Heat is lost through convection, conduction and radiation and is conserved is the ways mentioned earlier, thus temperature does not increase much as compared to exercise in a warm environment.
Between individuals in the cool environment, there are slight differences in the temperatures recorded. Generally, females have a higher percentage of fats than males, thus providing them with more insulation. Fats conduct heat only 1/3 as fast as other tissues insulator. Thus lesser heat is lost to the environment by conduction. So, females have a higher body temperature than males.
In a warm outdoor environment of 29.5°C, the control of thermosensitive mechanisms is by the anterior hypothalamus as the body reaches there a temperature of about 36.8°C. The temperatures taken are higher than the ones taken at the cool air-conditioned room. This initial increase was due to heat from the surroundings through convection and radiant heat gain from exposure of skin to direct sun. Not much heat is gained through conduction as no objects were in direct contact with the body. The resting body temperature after 10 minutes was recorded to be 37.5°C. The hypothalamic set point was then shifted to about 36.8°C as the body prepares itself for further increase in temperature. At exercise, sweating begins almost precisely at a skin temperature of 37°C and increases rapidly as the skin temperature rises above this value, which was recorded to be 37.9°C. Moisture from sweat glands or respiratory surfaces evaporates and cools the animal. With exercise, lesser time is taken to sweat in response to a faster increase in heat production due to the increased basal metabolic rate as muscles work harder. Sweating allows heat gain to be lost which is enhanced by the vasodilation of the blood vessels. The heat production of the body under these conditions remains almost constant as the skin temperature rises.
Here, there were rather wide fluctuations in the results obtained across individuals. This could be due to pace of walking, individuals that walked faster could experience a greater increase in temperature thus producing sweat, leading to heat loss via evaporation. It could also be due to the inconsistent environment as trees shaded certain areas of the location. Furthermore, the types of clothes worn will also affect heat gain and loss by radiation and convection, as it is a deciding factor in the amount of skin being exposed to the environment. Individual health is an important factor as well and activity of sweat glands also varies with each person. Furthermore, there exist variations in the core temperatures of each individual. These differences could be due to many factors such as diet, frequency of exercise and state of health.
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