Response to Heat
Human maintain a relatively constant internal body temperature independent of environmental temperature through a complex mechanism of heat gain and heat loss. In hot climate, the fundamental response of human exposed to heat stress is heat dissipation. The process of heat transference between the body and environment includes radiation, conduction, convection and evaporation. It has been observed in all human to an almost equal degree, with the average number of sweat glands per individuals (approx 1.6 million) being fairly constant. However, the non-acclimatized individual, on exposure to heat stress exhibits significantly increased perspiration rates (Frisancho, 1993). Further, the effectiveness of the heat removal process depends, on the gradient of heat between the body core and the external environment which involves three factors:
- 1) The magnitude of heat gradient between the environment and body core.
- 2) The rate of heat exchange between the interior and skin surface.
- 3) The rate at which metabolic heat is produced.
- People from the hottest regions (South Asia, Africa, India and Australia) had the largest surface area to body mass ratios. Such a morphological configuration is ideally suited to the more energy-efficient dry heat exchanges, and to a reduced reliance upon evaporative cooling (Taylor, 2006).
- Another adaptive mechanism involves vasodilation, whereby blood capillaries near the skin surface widen to permit increased flow to the skin and hence enhanced peripheral heat conductance.
- Skin color is another adaptive mechanism to the distinct climatic conditions. Melanin pigment produced by melanocytes present beneath the epidermis provides protection from overexposure to ultra violet radiation which can cause genetic mutation in skin cell leading to skin cancer. Thereby, natural selection has favored dark skinned individuals in area near the equator where exposure to UV radiations is the most. Biotopes with high densites of UV radiation are also characterised by high temperature. In such biotopes a dark skin color would actually be disadvantageous, as it causes a strong heating of body surface, due to relatively low reflectance. This is explained by differences in numbers and function of the sweat glands among dark skinned individuals. The African groups have been able to maintain a lower body and skin temperature as compared to European light skinned people as a consequence of lower suppression of sweat rate than Europeans (Walter, 1971). Thus darkening of skin is of prime biological importance so not only in Negroid, but a functionally effective melanisation is present in South Indians and other ethnic groups also (Weiner, 1964).
Response to Cold
- Human physiological responses to cold combine factors that increase heat retention with those that enhance heat production (Jurmain et al., 2006).
- On exposure to cold stress vasoconstriction limits the flow of warm blood from core to the skin thereby lowering the skin temperature. Consequently reduction of temperature gradient between the skin surface and environment reduces the rate of heat loss. The reduction in heat conductance of the blood is also caused by deviation of the blood in the extremities from superficial vein to the deep veins. The countercurrent heat exchange between arteries and vein lower the heat conductance to the periphery.
- In addition, subcutaneous fat layer provides an insulator layer throughout the body. When vasoregulatory mechanisms are not sufficient to counteract heat loss, the organism adjusts by increasing the rate of heat production.
- Shivering augments the thermeogenesis of the muscle mass and the temperature of muscle is raised to approach that of the core, thus eliminating the temperature gradient heat loss (Frisancho, 1993) shivering also increase the metabolic rate to two three times the basal value which consequently release energy in the form of heat. In general, people exposed to chronic cold maintain higher metabolic rate than those living in warmer climates. The Eskimo living in the Arctic maintain rates between 13-45% higher that observed in non-Inuit. Himalayan population of India wear several layers of cloth to combat cold, but extremities remain exposed to cold stress. However, they are characterised by elevated resting metabolic rate and high level of blood flow to the extremity to maintain warm surface temperature during local exposure to cold (Little et al, 1977).
- Body size and proportions are also important in regulating body temperature. In general, within a species, body size increase with the distance from the equator. Two rules that pertain to such relationship between body size, body proportion and climate are: 1) Bergman rule: In mammalian species, body size tends to be greater in population inhabiting colder climates. Increased mass, thereby decreased surface area allows greater heat retention and reduced heat loss eg. Arctic region . 2) Allen’s Rule : In colder climates, shorter appendages, with increased mass-to-surface ratios are effective at preventive heat loss. Conversely longer appendages with increased surface area relative to mass permit heat loss eg. Sub Saharan Africans.
Response to High Altitude
- A high attitude environment exerts multiple stresses on human which include hypoxia, more intense solar radiation, cold, low humidity, wind, a reduced nutritional base and rough terrain.
- Of these, hypoxia exerts greater degree of stress on physiological functions and is not easily modified by cultural behavioural practices or responses. Hypoxia results from a decrease in partial pressure of oxygen in atmosphere proportionally to increase in the attitude which consequently leads to reduction in O2 Heamoglobin saturation. It interferes with the oxygen acquisition at the cardiopulmonary level and utilisation by the cells. Hypoxia induced anorexia and dehydration due to increased ventilation and low humidity at high attitude leading to weight loss. The multifaceted effect of hypoxia also manifests through increased rates of infant mortality, miscarriage and prematurity among people residing at higher elevation. Decreased foetal growth due to impaired maternal foetal oxygen transportation also results into birth of low birth weight babies. Thus, acclimatisation to high attitude hypoxia is a complex phenomenon that develops through the modification and synchronized interdependence of the respiratory, circulatory and cardio vascular system to improve oxygen delivery and utilisation. On exposure to high attitude low landers, acquire short term modifications or partial acclimatisation in response to hypoxia which include increase in respiration rate, heart rate, and production of RBC which contain oxygen-transporting protein heamoglobin while high attitude dwellers adapt to hypoxia during their lifetime, as they mature.
- During growth and development environmental factors constantly condition and modify the expression of inherited potentials. The environmental influences felt by the organisms depend on the type of stress imposed and especially on the age at which the individual is subjected to the stress. The respective contribution of genetic and environmental factors varies with the development stage of the organism, in general (Frisancho, 1993).
- The earlier the age or the longer the duration of stay at high altitude, the greater the environmental influence on body dimensions and respiratory functions. For instance, the altitude natives Anedean Indians have larger chests and greater lung capacity as well as more surface areas in the capillaries of lungs which facilitate the transfer of oxygen to the blood. Consistent with a presumed environmental effect, the children of high attitude Peruvians who grow up in low lands don’t develop larger chests. Peruvian who were born at sea level but grew at up at high attitude developed the same amount of being capacity as people who spent their entire lives at high attitudes (Ember & Ember, 2008). Thereby larger chests among Andeans which was considered to be a genetic adaptation, infact, presents acclimatisation which develops early in childhood and persists for the lifetime of an individual. The Spitians who inhabit high altitudes in the North West Himalayas showed large chest size in relation to stature indicating developmental adaptation to low oxygen pressure of high altitude (Singh et al. 1986). The larger chest circumference of the Bods of Ladakh as compared to lowland Indians also suggests a structural response to the greater lung function capacity and adaptation to high altitude hypoxia (Kapoor & Kapoor, 2005; Bhasin et al. 2008;). Rajis, a hunter-gatherer tribal population of Uttaranchal showed lower chest circumferences comparable to the mid-altitude population but lung functions comparable to those of other high-altitude populations. This leads to the conclusion that indigenous high-altitude populations may possess different genetic potential for thorax growth compared to low-altitude populations, possible related to ethnic differences in the rate of growth of thorax related to stature at high (Kapoor et al., 2009).