Adaptations To Heat Stress In Dry Environments Biology Essay

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Human beings have long inhabited hot environments, and have developed several physiological mechanisms in order to deal with heat stress. We are exposed to two kinds of heat stress, environmental and metabolic, on a regular basis. Heat can be gained from the environment through radiation, convection and conduction, as well as created during the course of metabolic activity, which is compounded by exertion. The heat gained from these sources must be balanced by convective heat loss to the environment and through the evaporative cooling mechanisms (Hanna and Brown). Failure of these cooling mechanisms can result in heatstroke, a serious condition which can cause severe damage to the central nervous system (Bouchama and Knochel 2002). Over reliance on evaporative cooling can lead to another debilitating condition, dehydration. Hot desert environments, such as those found in North Africa and the Arabian Peninsula, are a perfect storm of stresses on human heat tolerance. High ambient temperatures maximize heat gain from the environment, while severely reducing our ability to lose both environmental and metabolic heat by convective means. This causes an increase reliance on evaporative cooling in order to deal with rising body temperature. Increased sweating, combined with the lack of readily available water in desert climates severely increases the risk of dehydration. Populations native to desert areas exposed to the dual stresses of heat and lack of water employ a variety of cultural adaptations in order to buffer their natural thermoregulatory processes. Primary amongst these adaptations is the creation of a favorable microclimate through the use of clothing and architecture. (Hanna and Brown).

Human heat tolerance can be summed up through the equation;

Mb + Ma + S = E + R + K + C

In this equation Mb and Ma represent heat generated by the body through basal metabolic activity and physical activity respectively. S is representative of the amount of heat stored by the tissues of the body. The right side of the equation represents environmental heat, with E, R, K, and C standing for evaporation, radiation, convection and conduction, the primary methods by which heat is lost to, or gained from, the environment. Evaporative cooling takes place as water secreted by the endocrine glands onto the skin's surface undergoes a phase change and draws heat away from the body. Radiative heat gain is based on the transference of wide span length electromagnetic waves. Factors such as an objects temperature, reflectance and surface area, as well as those of objects around it, affect the net heat flow through this mechanism. While there is a positive gain of radiant heat by the human body from the sun under all circumstances, environmental conditions can greatly affect the magnitude of this gain. Heat flow by conduction functions in a manner similar to diffusion, between materials in contact with one another and from high to low heat areas. Environmentally, conductive heat exchange can occur between the body and the ground, while with-in the body it is the mechanism by which heat is transferred from the core to the surface of the skin. Convective heat flow occurs between an object and a moving fluid. The movements of the fluid can carry heat away from the body; this is the reason why a cool breeze can be so refreshing on a warm day. When ambient temperatures are at or above body temperature, however, convective currents can significantly increase heat load, sometimes beyond the body's ability to compensate through other means (Hanna and Brown).

Heat stroke is caused by an elevation of body temperature, usually due to failure of cooling mechanisms caused by exposure to high external temperatures, and is the most severe form of heat illness. Exertion can accelerate the onset of heat stroke by increasing metabolic heat production in active muscles. Symptoms of heat stroke include confusion and irrational behavior, weak pulse, elevation of breathing rate and unconsciousness. Severe cases can cause brain damage and organ failure, leading to death. An early symptom of the onset of heat illness is profuse sweating, caused by the body's attempts to return to homeostasis. Salt loss through this excess perspiration can lead to muscle cramps, while the increase in water loss itself further elevates risk of dehydration (Heat Emergencies). Dehydration is the result of an imbalance between fluid loss and intake. Illness is a common cause of this condition, especially in children and the elderly. Reliance on sweating as a cooling mechanism without adequately rehydrating is another frequent cause of this condition. In hot dry environments, where increased sweating is required in order to maintain body temperature, dehydration is particularly common. Cases of Dehydration can randge from mild to severe, depending on the percentage of overall body water lost. Mild dehydration is easily treatable, and has relatively few long term consequences. Severe dehydration, if not treated quickly can result in damage to the central nervous system or death. Symptoms of dehydration include low blood pressure due to reduced fluid volume, elevated heart rate, dry mouth, low urine production, and in severe cases, lethargy and coma. One sign of dehydration is a reduction in skin turgor. Reduced water levels in the tissues cause skin to lose its elasticity, causing it to slowly return to position after being pinched upwards (.

Cultural adaptations can serve as a buffer between the environment and the body, allowing survival in environments where physiological responses would be inadequate for or impede survival. Remarkable examples of this are seen in the populations of Northern Africa and the Arabian Peninsula. The populations native to these regions, such as the nomadic Bedouin and semi-nomadic Tuareg have developed specialized clothing and architecture that allow them to survive in some of the hottest, driest areas in the world, the Sahara and Arabian Deserts. The specialized material culture utilized by these groups facilitates the creation of a favorable microclimate around the body which allows them to maintain homeostasis without relying as strenuously on physiological mechanisms.

A desert environment is defined by the Köppen-Geiger climate classification system as a region where the mean annual participation in mm is less than or equal to 5 Pth, which is defined by the following equation,

In order to be designated as a hot desert, the region must also have a mean annual temperature greater than or equal to 18°C (Kottek et al). The Sahara Desert is the largest in the world, covering an area of 9,100,000 km2 across the breadth of North Africa. The central region of the desert receives less than 25 mm of rainfall annually, while regions on the eastern margin receiving as little as 5 mm annually. Rainfall is extremely sporadic when it does occur; a single storm sometimes accounts for multiple years' worth of precipitation. The region is also amongst the hottest on earth, with temperatures exceeding 50° C in the summer months. Diurnal variation in temperature can be considerable, the recorded maximum variation being 38° C. Extreme winds are common in the Sahara, which when laden with dust make the environment even less hospitable. Underground aquifers occasionally break the surface and create oases amongst the otherwise barren landscape of stone plateaus, salt flats and dunes (Berrahmouni and Burgess).

The Arabian Desert is the dominant eco-region on the Arabian Peninsula, covering 1,851,300 Km2. Contained within the larger desert region is the vast Rub'al-Khali, the Empty Quarter, which is the largest continuous area of sand in the world, stretching 500,000 km2. Salt flats and limestone cliffs are present in other regions of the desert. Average annual rainfall is less than 35mm, while temperatures average around 47° C in summer. Temperatures drop significantly at night, sometimes reaching low enough for frost to form (Llewellyn-Smith).

The Bedouin are a primarily nomadic pastoralist group located in the desert regions of the Arabian Peninsula and North Africa. Their subsistence is based around the raising of sheep, goats and camels, with limited dependence on seasonal agriculture in groups inhabiting regions with more predictable seasonal rains. The Bedouin's traditional form of shelter is the bayt, a large rectangular tent constructed from sheep's wool. The roof is supported by a series of tall poles, while the walls of the tent are pinned onto the edge of the roof. The bottoms of the wall pieces are buried in the sand or pegged down, depending on the nature of the terrain. The interior is divided into sections, one for cooking and storage, the other for the entertaining of visitors. (Chatty) The tent is typically open along one of its long sides. This open wall, called the face of the tent or wej al bait, is on the leeward side. In the event of a sudden change in the wind, the back wall of then tent is unpinned and moved to what was formerly the face of the tent. In particularly hot weather parts of the side walls are unpinned to increase air flow throughout the tent. Carpets are sometimes employed to thicken the roof of the tent when the sun is stronger than usual (Dickson). When the wind is not particularly strong the side walls can be lowered entirely in order to maximize ventilation and cooling (Musil). The Bedouin traditionally dress in long, light garments that cover a majority of the body's surface. For men, typical garments include a long tunic like shirt with wide sleeves that reaches to the ankles, loose trousers, a sleeveless cloak and a head cloth held in place by a woolen cord wound around the head. Women's clothing is similar, consisting of a long sleeved dress, black cotton over garment, loose trousers, cloak and various degrees of head covering, depending on the region the particular group hails from (Dickson).

The Tuareg are a predominantly nomadic group inhabiting the nations of Mali, Algeria Niger and Libya. Different Tuareg groups practice widely different modes of subsistence, ranging from nomadic stock-breeding and trading to oasis gardening, although the latter is viewed with somewhat of a social stigma (Rasmussen). The Tuareg utilize a variety of both portable and non-portable dwellings, depending on their particular mode of subsistence. Nomadic Tuareg generally utilize a combination of mat and skin tents depending on regional conditions and availability of materials. Both of these tent forms follow the same basic layout, with the roof and sides suspended from a series of poles anchored in the substrate, while a wooden bed forms the floor of the structure. The particular arrangement of the poles varies significantly by region and preference, ranging from a small number of arches to a series of upright posts. The more sedentary groups amongst the Tuareg construct a variety of mud and mud coated stone houses, some of which are subterranean or semi-subteranean. These houses are typically rectangular, with small windows and flat roofs, although conical and domed forms are not uncommon in some regions of the Tuaregs' territory.

The most conspicuous aspects of traditional Tuareg clothing amongst both men and women are elaborate head coverings, taking the form of a veiled turban-like garment amongst men and a less elaborate head cloth for women. The men's veil, called a teguelmoust, is typically worn so that only a small space for the eyes is exposed. The amount of the face exposed varies considerably, however, depending on social circumstances. Men's garb consists of loose trousers and multiple sleeveless gandurah (a loose fitting cloak-like garment). Although sleeveless, the gandurah is loose enough that it typically extends below the hands. It is folded up over the forearms or shoulders when necessary to free the hands for manual labor. Women's costume includes either baggy trousers or a cotton skirt reaching to the knees, a gandurah similar to those worn by men but stitched higher on the sides, and a combination of shawl and headscarf made from the indigo dyed cotton popular amongst the Tuareg. This is often folded across the face for protection from the elements when riding or in the sun (Briggs).

The primary benefit of clothing in a desert environment is its ability to minimize radiant heat gain by blocking solar radiation. Layers of clothing and air pockets caught between the layers insulate the body and prevent heat from being transferred to it. Baggy clothing allows for ventilation and air flow around the body, which encourage both convective and evaporative heat loss while still protecting the individual from environmental heat gain. Traditional populations in the regions of the Sahara and Arabian Deserts benefit from these concepts by dressing in layers of loose clothing that covers a majority of the body. A study performed by Henschel & Hanson demonstrates the effectiveness of protective clothing in maintaining body temperature. This study demonstrated that acclimatized individuals wearing desert fatigues sweat 30% less than unclothed individuals when at rest, due to protection from radiant heat gain. Further benefit is seen in a lower core temperature and reduced heart rate relative to unclothed resting individuals. Clothing becomes less effective protection as metabolic heat production increases through exertion. The same study demonstrated that the benefits experienced by the individuals in desert fatigues were reduced by half (15% less than unclothed individuals) while engaged in physical labor (Hanna & Brown). This reduction is due to an increased need to vent internally produced heat as compared to the benefit of preventing the uptake of environmental heat. The traditional clothing worn by the Bedouin and Tuareg is looser than typical fatigues, allowing for better circulating and larger pockets of insulating air, as well as providing superior protection to the head, neck and shoulders. This would likely increase the protective benefit gained by an individual at rest. The design of the clothing, allowing for more significant ventilation than desert fatigues, would likely allow an individual at work to still reap the maximum protective benefit while at the same time shedding metabolically generated heat. Another benefit of the head coverings worn by the Bedouin and Tuareg is their ability to protect the lungs and face from the desiccating effects of the hot dry winds. When pulled across the face, these veils can maintain a small pocket of air which is relatively humid compared to the environment, which greatly reduces the discomfort caused by exposure of the mucous membranes to the dry desert air (Briggs).

Amos Rapoport describes in his book House Form and Culture several architectural traditions employed in hot dry climates that create favorable microclimates for human survival. Amongst these cultural adaptations are the use of stone and clay as building materials, small windows, use of courtyards, mutual shading and underground construction (Rapoport). Building materials such as stone or mud have a significant advantage in desert environments due to their high heat capacity. These materials function as a heat sink, absorbing radiant and convective heat during the day without significantly transferring heat to the shaded interior of the building. An added advantage to this style of architecture is its ability to hold onto heat gained over the course of the day, which can help deal with the low overnight temperatures common to deserts. A similar tactic is seen in underground construction. Dwellings constructed underground can take advantage of the tremendous heat capacity of the soil itself to buffer interior temperatures from the outside environment. An example of this can be seen in the Matmata house found in parts of the Sahara. This structure takes the form of a series of subterranean rooms built around a central, pit-like courtyard. Upwards of 30 feet of earth shield the interior rooms from the heat of the sun, allowing the inhabitants to remain significantly cooler than they would be able to in an above ground structure (Rapoport). Surface Buildings in deserts are frequently constructed with few and small windows. This functions to reduce the amount radiant heat reaching the interior of the building directly. Placing windows high in the walls further serves this purpose. Permanent dwellings constructed by the Tuareg conform to these design standards; utilizing materials with a high heat capacity and minimizing opportunities for radiant and convective heat to enter the interior through windows or large doors (Nicholaison).

The tents employed by the Bedouin cannot employ the techniques described above to the same degree, but instead function in a fashion similar to that of dessert clothing. Like the long robes worn by the Bedouin, the tent absorbs radiant heat, but prevents it from reaching the cooler microclimate within the tent. Attaching extra rugs to the roof of the tent in particularly hot weather furthers its ability to block and absorb radiant heat by increasing the amount of material able to take in heat and the distance that heat has to diffuse to reach the interior of the tent. The ability to adjust the walls of the tent in accordance with the need for ventilation and the strength of the wind also is a significant advantage. Ventilation can be adjusted in order to maximize air flow through the interior of the tent, and therefore cooling, while minimizing convective heating. Even with the back walls of the tent all the way up, enough air should penetrate through gaps in the panels to create a current through the tent and out the front opening, removing hot air from the interior of the tent. The mat tents used by the Tuareg are thought to be vastly superior to the skin tents in the hot, dry season. They are better ventilated and, according to claims made by anthropologist Johannes Nicolaisen, "the mat covering is not heated to any great extent by the sun" (Nicolaisen, p. 375).

Desert environments are extremely difficult conditions under which to live. They combine high ambient heat with a lack of water, pushing the human ability to maintain thermal homeostasis to its absolute limit. This taxing environment has encouraged the use of cultural adaptations to provide a buffer between the environment and the physiologic mechanisms necessary to keep cool. The primary areas in which these adaptations are applied are clothing and architecture. Loose, light clothing is utilized to block the absorption of radiant and convective heat from the environment, while at the same time creating an environment that maximizes convective and evaporative heat loss by creating a cycling zone of cool air next to the skin, allowing thermoregulation while only minimally engaging homeostatic mechanisms. Desert architecture utilizes high heat capacity materials and the earth itself as a heat sink to keep indoor temperatures at reasonable levels. The microclimate created through these cultural techniques provides a buffer zone that protects desert populations from the harmful effects of overtaxing their bodies cooling mechanisms and allows them to survive under some of the harshest conditions on earth.

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