14 1 Introduction to Thermoregulation Our last unit will be one that affects all of the physiology we have learned about so far Thermoregulation Chemical interactions are temperature dependent Temperature affects the structure and movement of molecules Generally as the temperature goes down interactions happen at slower rates As temperature goes up interactions happen at faster rates All of the processes we have learned about from neurons firing action potentials to muscle contraction to long term potentiation to gas and nutrient exchange depend on interactions between chemicals Ligands bind receptors substrates bind enzymes chemicals react to form other chemicals gases and fluids move from one place to another we have spent a lot of time talking about how these processes are regulated within animals and comparing how they differ between species All of this is dependent on the temperature of the animal If an animal does not regulate its body temperature all of these chemical interactions will fluctuate with environmental temperature Animals vary in how much their body temperatures fluctuate based on where they live and their thermal strategy Some animals maintain their temperature within 1 C for their entire lives Others allow their bodies to fluctuate with their environments as much as 40 C between seasons Thermoregulation can be defined as the ability of an organism to keep its body temperature Tb within certain boundaries even when ambient temperature Ta is very different Musser and Carleton 2005 An animal s thermal strategy is the combination of behavioral biochemical and physiological responses that ensure Tb is within an acceptable limit Enzyme efficiency as measured by Km is highly conserved across animal taxa across a diversity of habitat temperatures enzymes function at similar rates This is illustrated in the graph below This observation reflects strong selective pressure for enzyme efficiency As these animals have diverged from one another and accrued new mutations those that decrease enzyme efficiency are strongly selected against Recall that Km is the substrate concentration at which an enzyme reaches half its maximum reaction rate 1 2 Vmax Maintaining enzyme efficiency in the face of different Ta has shaped mechanisms for thermoregulation as animals that fail to catalyze reactions at the right rates suffer from lower fitness Notice in the graphs above that the rate of an uncatalyzed reaction approaches a maximum rate as temperature increases However for a reaction that is catalyzed by an enzyme reaction rate declines if the temperature is too low or too high This is because the intermolecular forces covalent bonds ionic bonds hydrophobic interactions etc that hold enzymes together are also temperature dependent If the temperature gets too high these forces will weaken and the protein will denature and fail to catalyze the chemical reaction Similar Km for a given enzyme is seen across species that live in drastically different temperatures as demonstrated by the heat map on the globe at a given point in the year as well as within an animal as its own Ta varies throughout the day as shown in the desert landscape below and between seasons Temperature varies throughout the day and year less in coastal regions and is more variable further inland This is due to the high specific heat capacity of water Water can hold more heat before changing temperature than other fluid mediums like air Thus animals that live in aquatic habitats and terrestrial habitats close to the water experience narrower Ta ranges As temperatures increase the rates of reaction start to decline until at some critical upper temperature the animal dies There is a similar critical lower limit at which reactions become so slow the organism also dies These temperatures are not absolute fixed values but are affected by duration of exposure and potentially acclimation to a particular temperature The body temperature range over which life is sustainable is relatively narrow compared with variation of temperature on land On land most animals would be dead within a few days if they did not exert some control over their body temperature thermoregulation or evolve special adaptations to extend the temperature range over which they can survive Species vary widely as to what their lethal temperatures are as they have adapted to their particular habitat and acclimated to more recent changes such as seasonal temperatures Tb is a reflection of the thermal energy of all of the molecules in an animal s body This thermal energy can move from the animal to the environment and vice versa Like pressure and concentration thermal energy also moves down its gradient from areas of high temperature to low temperature The movement of thermal energy between an animal and its environment or heat transfer can be predicted by calculating how much heat is being absorbed vs released by the animal H Most of the heat in animals comes from the catabolism of macromolecules metabolism as these are exothermic heat releasing processes However heat is also exchanged with the environment via four processes radiation conduction convection and evaporation If the net gain from all of these processes is positive H Tb will increase If there is a net loss H heat will be transferred from the animal to the environment and Tb will decrease Animals thermoregulate by managing these processes Whenever two bodies are in physical contact they exchange heat via conduction Heat flows down the thermal energy gradient from the hotter object to the colder object The rate of heat flow depends on 4 things 1 Area of contact 2 Path distance 3 Thermal conductivity of the material 4 Thermal gradient external vs internal As the external temperature declines animals can compensate by increasing the thickness of the peripheral layer This increases the path distance that heat must travel which decreases heat loss via conduction For example the figure below shows how fur thickness varies between winter and summer on different parts of an adult reindeer Convection is the heat transfer that occurs by movement of fluids air and water There are two types 1 Free convection when there is no gross movement of the medium When air next to your body is warmed the molecules move more and the air expands As it expands its density decreases and it rises warm air rises cold air sinks When the warm air rises it will be replaced by cold air Thus although there may be no wind or gross
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