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All vertebrates create a certain amount of heat as a by-product of their metabolic reactions. In addition, the rate of metabolism is to an extent determined by the temperature of an organism’s body, so that temperature cannot vary greatly without metabolic consequences. This is because enzymes are affected by temperature: the colder their environment, the slower the reactions they catalyze will proceed. As temperatures rise, the reactions speed up, but only until a certain point at which the enzyme loses its structural integrity, or denatures, and can no longer function properly. So all animals must exist within a temperature range that allows their enzymes to function, and at a reasonable rate.


Vertebrates can be classified based upon the major source of heat that allows their bodies to remain within a reasonable temperature range, and according to whether or not they have the ability to actively regulate their temperature and keep it at an almost constant level. Ectotherms absorb most of the heat used to warm their bodies from the environment, while endotherms use the heat they generate themselves, during metabolic reactions. Poikilotherms have little ability to regulate their body temperatures by internal mechanisms, and are often referred to as “cold-blooded”. What this really means is that their body temperature varies with the environmental temperature.
Homeotherms, often referred to as “warm-blooded”, are able to maintain their internal temperatures in a very narrow range that is generally high and advantageous for their metabolic activities. Almost all endotherms are also homeotherms, and almost all ectotherms are poikiolotherms. Mammals and birds are the most obvious examples of homeothermic endotherms, while fishes and reptiles are examples of poikilothermic ectotherms. Since fishes live in a watery environment, the external temperature does not vary much, and therefore their body temperatures remain relatively constant, albeit low. Their metabolic processes are similarly slow. Reptiles must absorb heat from the environment, usually by basking on a rock to “get some sun”. This raises their body temperature to ensure a reasonable metabolic rate. At night, the temperature of their bodies falls, their metabolism slows, and they become almost completely inactive. Humans, of course, are endotherms and homeotherms, using our relatively high metabolic rates to create heat and to keep our body temperature at a very constant 370C (98.60F). Just how do we achieve this thermal constancy?
The hypothalamus, a portion of the brain we encountered in Chapter 14, contains a thermostat set to maintain the body temperature at 370C. It receives information about the temperature of various parts of the body from internal receptors, and about the external conditions from the temperature sensors located in the skin (part of the sense of touch, see Chapter 13). If it is very hot outside, and the body temperature begins to rise above the thermostat “set-point”, a variety of responses help restore the internal temperature to normal. Vasodilation (widening of blood vessels in the skin) increases the blood flow and accelerates the transfer of heat away from the body. In addition, sweat glands are stimulated to excrete sweat; this cools the body because internal heat is used to evaporate the liquid sweat. (Dogs rely more on the evaporation of saliva, which they expose to the environment by “panting”.) These mechanisms function in concert to ensure that the body remains at a safe temperature. They are, however, not foolproof. Sweating becomes less effective as the ambient humidity rises; on an extremely hot and humid day, sweat will be produced but will not evaporate, leaving you wet, but still hot! In addition, profuse sweating causes excessive water and electrolyte loss, which must be replaced. Likewise, vasodilation becomes less effective as the temperature rises, and if the external temperature is higher than body temperature, the body may begin to gain heat from the environment! In these cases, it is often advisable for humans to resort to extreme measures -- air conditioners, cool showers, or swimming pools!
The hypothalamus also acts when extreme cold causes the internal temperature to drop below normal. In this case, vasoconstriction (narrowing of blood vessels) reduces the blood supply to the skin, thus lowering the rate of heat loss. In addition, the insulating hairs of the skin become erect and trap warm air, and the layer of adipose tissue in the subcutaneous layer serves as an efficient barrier to heat loss. Overall metabolic rate rises as well, so more heat will be produced. This can be accomplished in a number of ways.
  • Involuntary contraction of muscles (“shivering”) creates heat, as do the voluntary movements we often perform when cold (walking, rubbing hands together, etc.).
  • Hormones such as adrenaline may be secreted, signaling the release of glucose into the blood and the increase of heart and respiratory rates.
  • In extreme cases, the thyroid gland may step up its production of thyroxine, which increases overall cellular respiration by directly stimulating mitochondria.
Of course, if it becomes too cold or low temperatures persist for too long, these mechanisms will not be adequate, and death will occur. As a final note, this explains why it is especially dangerous to get wet when cold. This can be thought of as sweating at the wrong time; heat is removed from the body as water evaporates from the skin. Hypothermia (a state of reduced body temperature with noticeable negative effects) can occur at temperatures as high as 500F if an individual is wet.

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