Understanding the human body temperature mechanism is very important for Interior Designers. It tells how the condition of human body and the space form determine the level of comfort and work productivity. In the longer term it also decides the well being of a person. Here in this article Body temperature mechanisms are described. In the follow-up articles (2) I intend to discuss spatial qualities and the ventilation systems that affect the body temperature mechanisms.
Climate affects our body system very profoundly. The climatic effects are primarily sensed by the skin. Five types of sensations are involved with the skin. The Touch-Pressure (mechanic-o receptors), Cold-Warmth feeling (thermo receptors), Pain and Itch. Cold is a consequence of contraction of blood vessels and warmth is felt due to dilation of blood vessels; both are felt by the same receptors.
Our body functions as a thermo equilibrium system. The thermal bearing capacity has upper and lower limits. The pain occurs at the upper limit of 52̊ C /126̊ F and has a lower limit of 3̊ C / 37̊ F. The Optimum or the comfort level temperature depends on the level of acclimatization. In certain acute work conditions like mines, metal smelting plants, cotton spinning-weaving plants, cold storages, the level of efficiency or productivity depend on the endurance level and adaptability of the body. A body may endure or adopt to certain abnormal conditions for a period of time, but there may occur side effects. The side effects may be realized in a different form and at a different time.
Our body gains heat from the atmosphere and also dissipates excess heat to it, to maintain a thermo equilibrium. The human body maintains itself at an average temperature of 98.4̊ F / 37̊ C. There are many minor variations in body temperature, which are considered normal. Body temperature is lowest in the morning and highest in the evening, within a range of 1.5̊ F / 1̊ C. Infants have a very imperfect mechanism for regulation of body temperature. A fit of crying may elevate and a cold wash may lower the body temperature. Aged persons have a low metabolism and so maintain a lower body temperature. It takes much longer for an aged person to gain or dissipate body heat. Female body temperature is slightly lower than Male. The type food one takes affect the body temperature. High protein foods increase the body temperature. The act of ingestion and food digestion raise the body temperature. Exercise increases the body temperature, because only 25 % of muscular energy is converted into mechanical work, rest comes out as body heat. Atmospheric conditions like, temperature, humidity and movement of air, affect the efficiency of heat exchange from the body, and so the body temperature.
There are three types of heat generating processes in the human body. Conversion of food matter into useful energy is a continuous heat generating process. Muscular activities like even sedentary work or sleeping, are heat generating processes. Lastly, certain infections and dysfunctions within the body, elevate or lower the body temperature by extra ordinary rate of heat generation or weakened heat dissipation mechanism. Of all the energy produced in the body only 20 % is utilized, rest 80 % is surplus heat.
Normal skin temperature is between 31̊ and 34̊C. As the air temperature approaches the skin temperature heat loss from the body gradually decreases, vasomotor regulation will increase the body temperature to 34̊C to maintain the heat loss, but if air temperature is higher, the convective heat loss may not work.
As long as temperature of the opposite surface or object (sun, fire, radiator) is below skin temperature, the body can lose heat by radiation. But once it reaches an equilibrium occurs, body will rather gain heat by radiation.
When the convective process is inoperative and radiation heat gain is positive, the body can maintain the thermal balance by evaporation. Evaporation can occur if air has velocity and appropriate humidity (low). Even in case of very high humidity conditions a high velocity air can remove the humidity.
A person exposed to constant high rate of sweating and permanent vaso-dilation can have lot of physical strain with loss of work efficiency.
The body must not only release all the excess heat that is generated from within the body, but all the excess heat as gained from the environment. Heat is lost from the body by radiation (60 %), evaporation (25 %), by convection and conduction (15 %).
Heat is lost through radiation, if there is a difference in temperature of opposing surfaces. Evaporation heat loss is controlled by the level of humidity in the air (dryer the air, faster the evaporation), temperature of the air, body and rate of air movement. Body dissipates heat through evaporation by perspiration, sweat and exhalation of air. Convection occurs when the air in the vicinity of skin becomes hot, expands, decreases in density, and elevates to allow cooler air in its place. Rate of heat convection from body depends on the difference in temperatures (skin & surrounding air) and rate of air movement. Conduction depends on the difference between the body temperature and the contact object.
The body continues to accelerate or decelerate the heat loss till it reaches an equilibrium. Heat loss is accelerated by several body functions like perspiration, high transfer of heat to the skin by increased blood circulation (vaso-dilatation). When these prove to be insufficient, sweating occurs. In hot climates the heat loss rate is lower due to unfavourable atmospheric conditions. But by lowering of the body heat generation (lower metabolic and muscular activity), the net amount of heat to be dissipated can be reduced. But this requires some time to take effect. On immediate basis when the heat loss is not balanced with heat gain `heat stroke' occurs. In cold climates the heat loss is higher, so heat balance is achieved by conservation of heat and by appropriate heat gain. Heat production is raised by certain reflex secretions (adrenaline, thyroxine), higher intake of food (increased metabolic activity) by reflex shivering (muscular exercise) and by sufficient insulative protection. The body may control the heat loss by vaso-constriction (lower blood supply), and depressed sweating.
Many physical, chemical and bacterial agents disturb the heat regulation mechanism and cause fever. These may be due to increased heat production or reduced heat loss, or both.
In reptiles and amphibians heat regulation mechanism is absent. Their body temperature rises or falls with the atmospheric temperature. Hence they are called cold blooded animals. In abnormal temperature conditions they regulate the body temperature by suitable habitat. In winter they go deep into burrows or in hibernation (minimize the metabolic heat generation). Mammals and birds are known as hot blooded creatures, because the heat regulation mechanism is well developed, and they are able to maintain a level of body temperature.
Comfort of an occupant in an environment also depends on subjective variables or individual factors:
- Acclimatization: exposed to new conditions a person shortly (approx. 30 days) acclimatizes own-self.
- Age and sex: Older persons take much longer to adjust to temperature change, and as a result slightly higher temperature. Women also have slower metabolic rate than men so prefer a little higher temperature.
- States of health:
Activity heat output in watts
Sleeping 70
Sitting, typing 130/160
Standing, working at a bench 160/190
Walking 220/290
Digging 440/580
Sustained hard work 580/700
BMR: Basal metabolism rate: It is the amount of heat given out by a person who is awake, but physically and mentally at rest in a comfortable condition of atmospheric temperature, pressure and humidity, 12/18 hours after a normal meal.
Normal BMR in an adult male is 40 Cal per sq. mt. of a body surface per hour. (females 37 Cal /sq. mt.). Average surface area of an adult male is 1.8 sq mts. In children BMR is high and as one ages it decreases. This is due to the fact that children have high surface area compared to their low weight. Generally higher the surface area greater is heat loss, but a large body also generates greater amount of heat.
In colder climates BMR is high to compensate the high rate of heat loss. In tropical climates BMR is purposely lowered by the body to retard the heat generation.
Muscular and energetic people have a high level BMR compared to people living a sedentary life. Nature of diet affects the BMR. High protein foods have high BMR. After 2 hours of food ingestion BMR rises and maintains the high level for 4 hours.
Ductless glands (adrenal medulla, adrenal cortex, thyroxine, anterior pituitary and insulin) discharges increase the BMR. Any dysfunction of these glands affects the level of BMR.
Moderate pressure change (sea level to hilly regions) does not change the BMR. But a fall of pressure by ½ the normal barometer pressure (which occurs in very high mountaineering or in high altitude non pressure air craft flights) reduces the BMR. However increase in oxygen pressure (anesthesia) does not raise the BMR.
For every 1 F rise in body temperature, as in case of fever, raises the BMR by 7 %. This is due to the fact that high body temperature increases the chemical processes of the body and so the BMR.
Light exercise + 30 to 40 %
Walking + 50 to 60 %
Severe Exercise + 100 %
Mental work (maths problem) + 3 to 4 %
Strong emotions + 5 to 10 %
Sleep - 10 to 13 %
Conditions which increase the BMR
Hyper thyroidism + 100 %
Fever
Diabetes insipidus
Leukaemia + 20 to 80 %
Conditions which lower the BMR
Starvation, malnutrition
Hypo thyroidism
Addison's disease
Lipoid nephrosis.
. In the Follow-up articles (2) I intend to discuss spatial qualities and the ventilation systems that affect the body temperature mechanisms.
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