A young hiker has been on a long walk. He is now sitting in front of a small campfire, drinking a hot cup of soup. It is dark and a frost is beginning to form on his tent. Explain the hiker’s heat gains and losses, and what mechanisms would come into effect to keep his temperature at the desired 37ºC.
Humans are subject to vast changes in environmental temperatures, but our complex biochemical systems have a major limitation in that enzymes only operate within a relatively narrow temperature range. Accordingly the human body have anatomical and physiological mechanisms that keep body temperatures within acceptable limits, regardless of environmental conditions. This homeostatic process is called thermoregulation and it involves constantly balancing heat-producing and heat-losing mechanisms. If the body temperature is not maintained within these acceptable limits serious physiological changes can occur. If the body temperature falls below 36ºC or goes above 40ºC this can cause disorientation, and a temperature above 42ºC can cause convulsions and permanent cell damage.
We continuously produce heat as a by-product of metabolism. When energy use increases due to physical activity, or when our cells are more active metabolically, additional heat is generated. The heat produced by biochemical reactions is retained by water, which accounts for nearly two thirds of body weight. Water is a very effective conductor of heat, so the heat produced in one region of the body is rapidly distributed by diffusion, as well as through the blood stream. If the body temperature is to be remain constant, that heat must be lost to the environment at the same rate it is generated. When environmental conditions rise above or fall below ‘ideal’ the body must control the gains and losses to maintain homeostasis.
In the example of the hiker his system responds to the varying temperature changes that occur in his environment and his body. The heat loss centre and the heat gain centre in the preoptic area of the anterior hypothalamus coordinate these changes within his body. It is important to note that behavioural changes also play a part in heat gain and loss for example if our hiker becomes hot he may remove some clothing to cool down and if he becomes cold he may put on some additional clothing or undertake some activity to create energy and therefore warmth (such as star jumps, huddling up etc.).
Our hiker has just been on a long walk and this prolonged and perhaps strenuous activity creates heat gain in the body due to the increase in metabolic rate. In order to maintain an optimum body temperature his system will have already been involved in a variety or heat loss mechanisms. The heat loss centre will have stimulated three major effects: The inhibition of the vasomotor centre causes peripheral vasodilation and warm blood flows to the surface of the body, the skin takes on a reddish colour and skin temperature rises. The heat generated can be dissipated to the surrounding environment via any exposed skin, this process is known as radiation. If the hike was vigorous, blood flow to the skin will have increased and in turn the sweat glands will have been stimulated to increase their secretory output. As the perspiration flows across the body’s surface evaporative heat losses accelerate. Finally, if the exercise is strenuous enough it may also have stimulated increased respiration, so as the respiratory centres are stimulated the depth of respiration increases and the hiker may even begin to respire through an open mouth rather than through the nasal passageways. This would also increase the evaporative heat losses through the lungs.
In addition heat loss may be achieved by conduction as the clothes in direct contact with the hiker’s skin take up some of the heat and via convection as air passing over any exposed areas of the hiker’s body (such as the face and hands) is heated and rises, cool air replaces it and convection currents are initiated. Convection can also cool the body through clothes as long as they are not windproof.
Once our hiker reaches his camp for the night he may remove some outer clothing to aid the heat loss following physical exertion. He may even sit down on a log or on the ground. Heat loss will continue until a tipping point is reached. The hiker’s optimum body temperature will have been reached but as it is getting dark and colder as the sun goes down he will rapidly begin to lose too much heat and a whole new set of negative feedback mechanisms will begin to promote heat gain. The function of the heat gain centre of the brain is to prevent hypothermia, when the temperature at the preoptic nucleus drops below acceptable levels the heat-loss centre is inhibited and the heat gain centre is activated.
The hiker can support this feedback mechanism with some behavioural modification: He is no longer active and so there is little physical activity to generate heat. He starts a fire and this external heat source is very effective. In addition he makes himself a cup of hot soup, this helps to raise the core temperature of the hiker’s body from within. It would also help is he were to minimise conductive heat loss by sitting on a blanket or mat rather than the ground.
The hiker’s system will begin to react to the new conditions. First it will attempt to conserve the heat it already has in the system. The sympathetic vasomotor centre decreases blood flow to his skin, thereby reducing losses by radiation, convection and conduction. As the blood flow is restricted his skin cools and may become paler in colour as the blood is not so near the surface. This does not damage the epithelial cells, which are designed to tolerate these changes for extended periods. The hiker’s body hair muscles will contract and his body hair will stand on end (piloerection), this traps air in a thick layer next to his skin, decreasing heat loss. Blood returning from the hiker’s limbs is shunted into a network of deep veins that lie within the insulating layer of subcutaneous fat and wrap around the deep arteries. Heat is thus conducted from the warm blood flowing outward to the limbs to the cooler blood returning from the periphery. This is known as concurrent exchange and it works to trap the heat close to the body core and restrict heat loss.
As the hiker makes his fire and drinks his soup his body will engage in active heat generation. The mechanisms for generating heat can be split into two main categories known as shivering thermogenesis and nonshivering thermogenesis. In shivering thermogenesis a gradual increase in muscle tome increases the energy consumption of skeletal muscle tissue throughout the body. The more energy consumed, the more heat is produced. Our hiker may begin to shiver and this increases the workload of the muscles elevating oxygen and energy consumption. The heat that is produced warms the deep vessels to which blood has been shunted by the sympathetic vasomotor centre mentioned above. Shivering is very effective and can elevate the body temperature by up to 400%.
Nonshivering thermogenesis involves the release of hormones that increase the metabolic activity of all tissues. The heat gain centre stimulates the suprarenal medullae via the sympathetic division of the autonomic nervous system and adrenaline is released increasing the metabolic rate of most tissues with immediate effect. Also the preoptic nucleus regulates the production of thyrotropin-releasing hormone (TRH) by the hypothalamus, which in turn stimulates the release of TSH (thyroid stimulating hormone). The thyroid responds to this release of TSH by increasing the rate at which thyroxine is released into the blood and elevating the rate of carbohydrate catabolism and also the rate of catabolism of all other nutrients.
As the hiker drinks his warm soup and it passes through his system the liver becomes active and heat is produced as a by-product. Metabolic rate and heat production are increased after eating. In addition the digestive organs will produce heat during peristalsis and during the chemical reactions involved in digestion.
As we can see there are a multitude of involuntary and also some voluntary mechanisms that work constantly to regulate the fluctuations in the hikers body temperature.