BODY TEMPERATURE CONCEPT MAP

Temperature Regulation

At the extremes in the animal world are what in the popular language are referred to as warm-blooded and cold-blooded animals. The more precise scientific terminology is:

• Poikilotherms - Animals whose body temperature is labile, it moves with the temperature of their environment.
• Homeotherms - are at the other end of the spectrum, these animals have a variety of structural and physiological adaptations which maintain their body temperature at a very stable value in spite of wide fluctuations in their environmental temperature. Humans are homeotherms, except under some pathological conditions.

In this chaotic world, the temperature is always changing, so there are two basic issues in temperature regulation,

• 1. Staying Warm
• 2. Keeping Cool

The core temperature (at deep sites within the body) of the living vertebrate is a balance between thermal inputs and thermal losses. Heat transfer occurs with the same basic principles as diffusion, e.g. heat moves from regions of high temperature to low temperature. There are several sources of heat which include:

• 1. air/water temperature of the environment
• 2. radiant energy e.g. sunlight or radiant heaters
• 3. internal metabolic sources

In the more primitive reptiles which do not have complex physiologic mechanisms for thermal regulation, behaviour patterns permit a crude regulation of body temperature. Thus if air temperature is low, the lizard or Iguana will search out sunny areas to heat their body to optimum temperature and will engage in vigourous muscle activity to assist in elevation of core temperature. On the other hand, if local temperature is high they will seek shady spots or cool damp locations and avoid physical activity. These critters have very low capacity of the circulatory, nervous systems and other more subtle physiologic mechanisms to independently regulate core body temperature. At a seminar many years ago, I heard a researcher describe a model for temperature regulation in iguanas: It was an aluminium beer can half full of water. If he moved it from sun to shade when the iguanas he was observing moved, the temperature of the water in the beer can was a very good approximation of the core temperature of the live iguanas. Iguanas are poikilotherms! While humans, at least some humans, will seek shady spots during hot sunny conditions, this is not the prime means for regulating our core body temperature. We have a variety of sophistic ated physiologic mechanisms to enhance heat loss allowing a human to bask in the sun and absorb sufficient amount of external heat, to produce a lethal effect in an iguana, but obviously most humans survive. Humans are homeotherms!

Sources of Heat

• Metabolic Thermogenesis -- as glucose/lipids are metabolised by the Citric acid cycle and the other metabolic pathways there is "waste" heat produced as the chemical reactions occur. This metabolic source is greatest in skeletal muscle, since skeletal muscle constitutes the largest mass of metabolically active tissue in the body. The brain has a very high energy requirement and a large heat production related to all the action potential, synaptic, and membrane transport activity, but is relatively smaller in total mass and volume. The level and efficiency of metabolism e.g. metabolic rate is regulated by several hormonal and neuro-regulatory mechanisms. Thyroxine release from the thyroid gland, produces both short and long-term increases in generalised metabolism, with subsequent increases in thermogenesis. Likewise cortico-steroid release from the adrenal cortex trigger increases metabolic thermogenesis.
• Contractile Thermogenesis - Muscle contractile activity is a particularly large source for heat production. Large amounts of metabolic energy reserves are consumed in activating the muscle biochemistry, with subsequent "waste heat". The contractile response itself converts chemical to mechanical energy with large amounts lost as 'frictional' heat is stretching the elasticity of muscle/tendon structures. Muscle is large source of heat when engaging in a variety of 'useful' muscle activities, and a neurologically regulated heat source when reflex 'shivering' is produced by drops in core body temperature below the optimum. This results in a reflex activation of many large muscle groups in continuous transient contraction-relaxation cycles with consequent heat production, and restoration of appropriate core temperature.
• Lipolysis with oxidation of fatty acids requires oxygen and can produce quite large amounts of heat. Brown fat cells are especially adapted to thermogenesis; it appears that they produce heat with little production of ATP, hence an uncoupling of t he lipolysis from the usual end-product. Brown fat cells are relatively abundant in species which are highly cold-adapted, and in most species including humans as neo-nates. Urban humans who are usually reared in warm tightly temperature controlled environments tend to have reduced populations of Brown adipose tissue and hence reduced capacity for non-shivering thermogensis which has high survival value in cold environments. Long-term cold adaptation increases the amount of brown adipose tissue. Norepinephrine release triggers lipolysis, oxidation of fatty acids and heat production in both White and Brown Adipose tissue.

Structural Adaptation to reduce heat loss

Other mammals have dense fur or hair which acts as an effective layer of insulation to reduce heat loss through the skin. Man has residual amounts of body hair and retains the reflex which is present in truly hairy mammals to improve the insulating properties. That is a neurogenic reflex of the sympathetic nervous system to activate the pilo-erector muscles. This results in the body hairs being pulled to a position more nearly perpendicular to the skin surface, instead of lying flat to the skin, and thus increasing the thickness of the trapped layer of insulating air. In humans the amount of body hair is usually trivial, so all we see is the appearance of 'goose bumps' in response to cold stress as an evolutionary memory of when we had enough ha ir to do something useful in reducing heat loss.

All mammals have a superficial layer of fat associated with the skin which serves as an insulating layer. In marine mammals this 'blubber' is very thick and provides the means for their survival in a very cold environment. Humans who are cold adapted for long periods will develop thicker superficial fat layer to reduce heat loss. These differences have been measured in Inuit and others living and working in the arctic.

Analysis of the skeletal remains of Neanderthal humans who prospered during a period when ice ages were a dominant feature of the climatic pattern is interesting from the point of view of temperature regulation issues. While Neanderthals were larger and taller than modern man, the key difference appears to be a very thick hugely muscular body. This was beneficial in two ways, 1) a reduced surface to volume ratio, with reduced heat loss a consequence, and 2) a huge muscle mass with the capability of a similarly large thermogenesis capacity. Since we only have skeletal remains, one can only hypothesize that they probably also had large amounts of brown adipose tissue to assist their survival. It didn't hurt that this level of physical strength was useful in the hunting of mega-mammals dominant in that period. In addition the morphology of the skull with the sloping forehead and long nose, provided a heat and water conservation mechanism, exhaled air had a larger portion of its heat recaptured by the vascular bed of the long naso-pharynx, and more of the water vapour was recaptured as well. A good evolutionary adaptation to a harsh cold climate.

Neural and Cardiovascular Responses to Cold Stress

Thermal receptors exist in the skin, deep organs and in the central nervous system. They send their signals to the autonomic nervous system. The key site for determining perceived core temperature appears to be in the hypothalamus, but a variety of temperature adaptive responses can be triggered by peripheral receptors. When the thermal receptors indicate excessive loss of heat through the skin, the autonomic nervous system produces vasoconstriction of peripheral circulation to the skin, with shunting of most blood flow to the deeper tissue thus reducing heat loss. In man vasoconstriction can reduce heat loss by 1/6 to 1/3. One of the consequences of this adaptation is an increase in central arterial pressure as the resistance in the superficial vascular beds increases dramatically.

The diving reflex is a specialized version of this with protective potential that is most sensitive in human neo-nates. That is cold stimulus of the face will trigger reduced heart-rate (bradycardia) and shunting of the blood flow from the periphery t o the CNS and vital organs. This reflex is responsible for the survival without CNS damage of children falling through ice in very cold water with recovery occurring at a time when by usual criteria they should be brain dead.

Cold also triggers release of adrenaline and nor-adrenaline from the adrenal medulla. In addition there is a neurally regulated increase in thyroxine release. All of these result in increased metabolic activity. The net result is a neurally regulated non-shivering thermogenesis.

CNS perception of cold can induce reflex shivering to increase contractile thermogenesis.

Playing it Cool!

What are the mechanisms for coping when there is too much heat and there is a threat of increasing core temperature?

• Vascular compensation - The inverse of thermal conservation occurs, peripheral vasodilation, with increased blood flow in the skin and thus increased heat loss. There will often be some vasoconstriction and reduction in flow to the internal organs, rendering the individual in a 'hot' environment more susceptible to postural hypotension e.g. standing up quickly may lead to transient cerebral ischemia and loss of consciousness This is the underlying reason why people faint easily in hot stuffy environments. It has definite survival value since the temperature is cooler on the floor and in the prone position adequate blood will be delivered to the brain.
• Heat loss due to evaporation. Perspiration rate increases because of sympathetic activation of sweat glands. In cool air with low levels of exercise man loses about 1 litre of water/day, In high temperature with exercise the rate may increase to as high as 1.5 litres/hour. The increased loss of water results in surface evaporation and much greater heat dissipation because of the heat of vaporization of water. At rest about 20% of heat loss is by evaporative cooling, in exercise this rises to 75-80 %. This compensates for the increased thermal output of the active muscles and maintains core temperature within the homeothermic limits. Additional evaporative heat lose occurs with water evaporation at the alveoli of the lungs. With increased respiration rate during exercise this can be a quite large component of total heat loss. This is a much more important pathway in animals without the capacity for sweat production which man possesses, e.g. dogs who will pant to increase the rate of heat dissipation through the lungs and tongue.
• Endocrine compensation - Thyroid gland activity down regulates, and the stimulation of non-muscle thermogenesis is withdrawn. Hot temperature adapted individuals have reduced metabolic rates, and reduced circulating levels of the hormones which will up-regulate these activities.