The main organ in the human body that monitors and regulates body temperature is the hypothalamus (Boon, N. et al, 2006). In an effort to maintain the normal body temperature of 36-37.4 degrees Celsius, various mechanisms come into play when the body is exposed to increased or decreased environmental temperature (Sukker, M. et al, 2000). As warm-blooded mammals, humans maintain homeostasis by eliciting various responses of the receptors in the different parts of the body (Smil, V., 2008) and in employing various adaptive mechanisms that will result to acclimation either in a hot or cold climate (Parsons, K., 2003; Witzmann, F., 2008; Mclafferty, E., 2009).
Thermoregulation of Humans to Cold Climates
Humans exposed to cold weathers or are living in cold climates have the major challenge of conserving heat (Witzmann, F., 2008). How the body accomplishes this involves two main mechanisms such as changes in the metabolism of the body and changes in the circulatory system (Marino, F. 2008; Gropper, S., et al, 2008). The former is used by the body in order to increase the production of heat (Blatteis, C., 1998) while the latter is used to reduce the loss of heat and divert blood flow to major organs during acute exposure to cold weather or environment (Marino, F., 2008). Once exposed to cold temperatures, the primary response of the body is the mobilization of autonomic response leading to vasoconstriction of the arteriovenous shunt followed by shivering (Sessler, D., 2009).
A number of studies (see for example Iberall, A., 1986; Smolander, J., et al, 1992; Havenith, G., 2001; Sessler, D., 2009) explained that it would take approximately 10 days for the human body to adapt to a cold environment. The changes involved during this time consist of changes in the cardiovascular system (Iberall, A., 1986). For example, in order to conserve heat, the superficial and peripheral blood vessels will undergo constriction in order for the blood flow to be diverted to main organs of the body (Smolander, J. et al, 1992). This will ensure that heat is conserved and at the same time, blood volume will be concentrated to vital organs such as the heart, kidney, lungs, liver, etc. (Fox, J., 2007). Another illustration is the vasoconstriction that occurs on the superficial veins of the limbs. During exposure to cold, venous blood present in the venous limbs will be diverted to deeper limb veins (Rhoades, R. and Bell, D. 2008). In the process heat will be conserved in the deeper veins that lie in close proximity to major arteries of the limbs that do not undergo constriction during exposure to cold.
If the human body is continuously exposed to a cold climate for prolonged periods of time, the body will resort to metabolic adaptations. This means that the body will produce more heat. This is accomplished by increasing the concentrations of the hormones noradernalin and thyroxin (Havenith, G., 2001). These two hormones are important in increasing metabolic activities in the body (Sessler, D., 2009). There is also an increase in As Marino, F. (2008) remarked, there is also “…an increase in circulating free-fatty acids, the result of which is enhanced non-shivering thermogenesis which results in an increase in heat production.” (p. 108).
A good illustration of how humans adapt to cold environments can be seen among different populations in very cold areas such as the subarctic regions. The most common physiological response to persistent exposure to cold climate is the development of bodies that are more massive (Crawford, D. and Powers, D., 1992). As cited previously, metabolism increases and in addition there is an increased insulation of important organs in the body with fat (Marino, F, 2008) as well as changes in the pattern of blood flow (Fiala, D. et al, 1999). Populations living in very cold environment, in order to survive need to integrate one of the adaptive mechanisms. A prominent example is the Inuit population in the northern region of the West (Webb, J. et al, 2009). An investigation to their way of life would reveal that they have a diet that is highly rich in calorie and are considered to be fatty (Iberall, A., 1986; Behr, R. et al, 1991). This diet would allow the population to flourish since it would give them the needed high calorie content to elicit faster metabolism (Fox, J., 2007). This higher metabolic rate would result to increased production of heat. Aside from eating high calorie content food, these group also wear very thick clothing to protect them from the cold and are very active outdoors (Sukker, M. et al, 2000).
Thermoregulation of Humans to Hot Climates
Once humans are exposed to hot environment, the first adaptation is marked by an increase in plasma volume (Mclafferty, E., et al, 2009; Boon, N. et al, 2006; Gropper, S. et al, 2008; Nolano, M. et al, 2006). In the first week of exposure to a hot environment, the total body water also increases (Rhoades, R. and Bell, D. 2008). Autonomic defenses also results in to sweating followed by vasodilation of the precapillary vessels (Sessler, D., 2009). If the person has not acclimatized yet to the warm environment, sweating will be concentrated on the back or on the chest (Rhoades, R. and Bell, D., 2008). However, during acclimatization, sweating will be more apparent on the limbs to make use of the increased surface area of this section of the body. One problem arising from exposure to hot environment is not only the excessive loss of water in an unacclimatized person but also the loss of sodium.
Once the person has adjusted to the environment, his body will be able to conserve sodium “by secreting sweat with a sodium concentration as low as 5 mmol/L. This effect is mediated through aldosterone, which is secreted in response to sodium depletion and to exercise and heat exposure” (Witzmann, F., 2008, p. 559). Witzmann went on to explain that in comparison to the sweat glands, it will take several days for the kidneys to respond to the effect of aldosterone. In contrast, the sweat glands will respond immediately to the effect of aldosterone and conserve sodium and does so for several days even if the level of sodium has been restored. This process allows conservation of sodium during the acclimatization period of an individual. However, it should be noted that acclimatization to heat is not permanent especially if the person is not subjected to repeated exposure to a warm or hot environment (Rhoades, R. and Bell, D., 2008).
Persistent exposure to a hot climate will also result to evolutionary adaptation of the body over time. For example, people living near the equator would tend to have a slender or thin and tall body (Gropper, S. et al, 2008) as compared to population living in very cold areas. This adaptation is necessary for heat to be dissipated. A tall and thin body will allow heat to be radiated easily because of increased surface area (Boon, N. et al, 2006). People living in hot climate would also tend to harbor little fat (Blatteis, C., 1998) as compared to persons living in cold environments. Other adaptation includes a darker skin tone to act as protection against the sun’s radiation (Mclafeerty, E., et al, 2009).
Comparison of thermoregulation responses to hot and cold climate in humans
It should be noted that aside from the different thermoregulatory responses discussed in the previous sections, the human body utilizes six thermoregulatory effector pathways. According to Parsons, K (2003), there are six thermoregulatory pathways involved in the maintenance of core body temperature:
These thermoregulatory pathways present differences in how the body reacts to weather changes. During warm or hot weather, the body will utilize the sympathetic nerves and artine vasodilator nerves to to dilate cutaneous blood vessels and dissipate heat in the environment. The Sympathetic adreno-medullatory system and the non-medullated (cholinergic) sympathetic nerves are also utilized by the body to promote non-shivering and sweating. These three pathways enable the body to adapt to an abrupt exposure to a hot climate. On the other hand, the adrenergic non-medullated nerve fibres of the sympathetic system are mobilized during acute exposure to a cold climate. Blood vessels will constrict upon exposure to a cold environment whereas the ordinary skeletal supply will promote shivering to conserve heat. The release of catecholamines and adrenaline by the sympathetic supply to the adrenal medulla will allow increased production of heat and an increase in the cardiac output (CO) of the heart. This will result to conservation of heat and increased ability of the body to withstand a cold environment leading to acclimatization.
As detailed in the previous sections, thermoregulatory responses of humans to a hot or cold climate varies. Aside from vasoconstriction during exposure to cold and vasodilation during exposure to heat, the human body can adapt to cold climates by increasing metabolism in order to increase body heat; vital organs of the body are insulated with increased fat and changes in how blood flows through the body. In contrast, humans respond to a hot climate by either increasing the dissipation of heat, or conservation of sodium. The body also responds by maintaining little fat and retaining more water in the body.
Meanwhile, studies have also revealed that age is major determinant factor among humans on their ability to withstand cold environments (Havenith, G., 2001; Sessler, D., 2009). The elderly, more than gender, have lesser capabilities to withstand cold climates (Palca, W. et al, 1986). Meanwhile another study (Smolander, J. et al, 1992; Palca, W. et al, 1986) have shown that in comparison to older men and adolescent boys, the latter have significantly more capacity to withstand cold temperatures.
Thermoregulation of Animals to Cold Climates
Most vertebrates such as mammals, also release hormones that are responsible for increasing basal metabolic rate (Koga, A. et al, 2004). Some examples of these hormones are adrenaline and steroids (McArdle, et al, 2006). These hormones are responsible for facilitating the breakdown of glycogen responsible for increasing metabolism and heat production in the body. Other mammals also release thyroid hormones (Willmer, P et al, 2000) which have a similar effect as that of adrenaline and steroids. Thyroid hormones also allow increased activity of animals (Purves, W., 2001) leading to more production of heat. To preserve heat, animals living in very cold climates adapt by gaining more body fat for insulation (Wells, K., 2007). Some animals are also highly adapted to their cold environment and are called as freeze-intolerant (McGonigal, D., 2009). These animals, such as marine fishes, can survive in very cold environments.
Endotherms also have more adaptive capabilities as compared to ectotherms in conserving or producing heat. For example, endotherms have a high number of mitochondria in their cells allowing them to produce more heat (Willmer, P. et al, 2000). The mitochondria of these endotherms are also unique since it would allow them to produce more heat by as much as eight times compared to ectotherms (Willmer, P. et al, 2000). To increase the production of heat, most animals can keep their bodies warm through physical exertion (McArdle, W. et al, 2006). Some animals can also perform shivering or undergo non-shivering thermogenesis (Parsons, K., 2003).
Another animal that has learned to adapt to cold environments are the cetaceans. These animals are warm-blooded (Witzmann, F., 2008) and need to maintain constant body temperature. To counter the extremely cold waters in the polar region, they have blubbers (McGonigal, D. 2009) in their bodies. These blubbers, located beneath their skins, are a mixture of fats, oils and connective tissues (McGonigal, D. 2009). In some species of cetaceans, blubbers may consist a third to 40 percent of their body weight (Koga, A. et al, 2004). This huge amount of fat and connective tissue deposits are designed to provide these animals with enough energy during their long migration during the winter season. These animals also have massive bodies that allow them to conserve more heat easily once they are in very cold waters (Starr, C. and Taggart, R., 2001). Owing to their large sizes, they have comparatively lower skin surface to body mass ratio (Willmer, P., et al, 2000) as compared to smaller animals. Hence, they would loss heat more slowly (Willmer, P. et al, 2000). Aside from their massive size and blubber content, cetaceans also have a unique “heat-exchange mechanism in the circulatory system that helps to regulate their body temperature” (McGonigal, D. 2009, p. 184). Blood circulation of these animals is unique such that when it is extremely cold, peripheral circulation is diverted to vital organs of the animal.
Thermoregulation of Animals to Hot Climates
How desert animals adapt to continuous exposure to hot climates has been a hot topic for scientists and has garnered extensive literature. Literature and journal articles have revealed different physiological responses to these animals. First, animals living in the desert have temperature tolerance and exhibit heterothermy (Willmer, P., et al, 2000). Most terrestrial animals have temperatures that range from 40 to 50 degrees Celsius (Purves, W., 2001). According to Starr, C. and Taggart, R. (2001), arthropods are the most tolerant to desert, tolerating temperatures to as much as 55 degrees Celsius. During evenings, these animals exhibit a heterothermic behavior (Wells, K., 2007) which involves dropping their body temperature to accommodate the desert cold (Smil, V., 2008). To avoid dehydration, desert animals decrease their metabolism leading to reduction of hyperthermia and evaporative cooling (Wells, K., 2007).
Meanwhile, Smil, V. (2008) noted that even if some animals can tolerate extreme temperature such as the Saharan ant coming from the Cagtaglyphis genus which can tolerate temperatures as high as 55 degrees Celsius, this extreme adaptation is dangerous to the survival of the ant. Smil, V. (2008) explained that in order to for growth, metabolism and other metabolic processes of the body to proceed normally, an optimal body temperature must be present. He pointed out that animals living on land would mostly have a body temperature of 36 to 42 degrees Celsius only. Enzymes present in a mammalian body (Wells, K., 2007) would be inactivated once core body temperature nears 50 degrees Celsius (Wells, K., 2007; Starr, C. and Taggart, R., 2001; Willmer, P. et al, 2000).
In an effort to survive in hot climates, endothermic animals have learned to adapt to the environment. For example, panting (Starr, C. and Taggart, R., 2001; Willmer, P., et. al, 2000) and sweating are the most common form of adaptations (Koga, A. et al, 2004). Conserving water in their bodies is also another form of thermoregulatory response (Witzmann, F., 2008). Camels counter water loss by employing respiratory mechanisms that would allow them to cool exhaled air (Smil, V. 2008; Korb, J. and Linsenmair, K., 1999). Ectotherms, on the other hand, do not have good body insulation (Blatteis, C., 1998) and thermoregulation is mainly behavioral (Smil, V. 2008; Wikelski, M., 1999). For example, butterflies, lizards and other insects would position their bodies to increase their exposure to the breeze and decrease their exposure to the sun’s hot rays (Purves, W., 2001; Roberts, S. and Harrison, J., 1998). Others can also adapt to hot environments by changing the colors of their body in order to feel cooler (Purves, W., 2001; Smil, V., 2008; Willmer, P et al, 2000). Other adaptive mechanism or response to excessive loss of water in a hot climate is the adaptation of the renal function of vertebrates (Starr, C. and Taggart, R., 2001; Webb, J. et al, 2009). As an illustration, mammals and birds have highly adapted renal system that would allow them to excrete urine that would almost have the same osmolarity as that of blood plasma (Willmer, P. et al, 2000). Other animals have also learned to decrease the amount of urine especially when water availability is restricted (Wells, K., 2007).
Finally, just like humans, the main challenge for animals living in hot climate is the losing of heat. Animals, especially mammals, accomplish this by increasing the blood flow to the extremities or to the surfaces of their skin. This way, they will lose more heat and maintain internal optimal body temperature (MacArdle, et. al., 2006; Roberts, S. and Harrison, J., 1998). Other animals also resort to heat evaporation.
Animals have different thermoregulatory mechanisms in surviving in hot or cold climates. Others resort to behavioral changes in order to survive in arid deserts or in extremely cold places such as the Arctic region. However, most animals would depend on physiological processes in order to achieve their ultimate goal of survival. When found in places with hot climates, these animals learn to adapt to their environment through a number of ways. First, mammals can sweat and pant in order to release heat while some birds and animals resort to renal excretion of highly concentrated, small amounts of urine in order to conserve water. Others also either change their color or posture in order to shield themselves from extreme exposure to the sun. In very cold regions, animals learn to adapt by storing fats in their bodies. They also thermoregulate by drawing blood circulation to vital organs of their body. Others thermoregulate by keeping a constant temperature in their body through a number of neuronal pathways. Animals in these cold climates also have higher metabolic rates, enough to allow these animals to increase production of heat.