5.1.4 Thermoregulation

• Homeostatic mechanisms help organisms to keep their internal body conditions within restricted limits

• Temperature is a key factor that needs to be controlled
  • For example, the human body maintains a core temperature of 36.8 ± 0.5 °C
  • Core temperatures of 35 °C or lower and 38 °C or higher indicate hypothermia or fever respectively

• A stable core temperature is vital for enzyme activity
  • If the temperature of the tissue fluid surrounding cells is too high or too low it can negatively affect the rate of important enzyme-controlled reactions
  • For example, human enzymes have evolved to function optimally at a core body temperature of about 37 °C, so that is their optimum temperature (the temperature at which they catalyse a reaction at the maximum rate)

• Lower temperatures either prevent reactions from proceeding or slow them down:
  • At lower temperatures molecules move relatively slowly
  • As a result, there is a lower frequency of successful collisions between substrate molecules and active site of enzyme so less frequent enzyme-substrate complex formation occurs
  • The substrate and enzymes collide with less energy, making it less likely for bonds to be formed or broken (stopping the reaction from occurring)

• Higher temperatures speed up reactions:
  • Molecules move more quickly due to having greater kinetic energy
  • There is a higher frequency of successful collisions between substrate molecules and the active sites of enzymes
  • More frequent enzyme-substrate complex formation occurs as a result
  • Substrates and enzymes collide with more energy, making it more likely for bonds to be formed or broken (allowing the reaction to occur)

• However, as temperatures continue to increase, the rate at which an enzyme catalyses a reaction drops sharply, as the enzyme begins to denature:
  • Bonds (eg. hydrogen bonds) holding the enzyme molecule in its precise shape start to break
  • This causes the tertiary structure of the protein (ie. the enzyme) to change
  • This permanently damages the active site, preventing the substrate from binding
  • Denaturation has occurred if the substrate can no longer bind

Thermoregulation

Thermoregulation in endotherms

• Endothermic animals detect external temperatures via peripheral receptors (thermoreceptors found in the skin and mucous membranes)
  • There are receptors for both heat and cold
  • These communicate with the hypothalamus to bring about a physiological response to changing external temperatures

• The hypothalamus also helps to regulate body temperature by monitoring the temperature of the blood flowing through it and initiating homeostatic responses when it gets too high or too low
• Endotherms display a variety of cooling mechanisms, including:
  • Vasodilation
  • Sweating
  • Flattening of hairs

• They also display a variety of warming mechanisms, including:
  • Vasoconstriction
  • Boosting metabolic rate
  • Shivering
  • Erection of hairs

• Human skin contains a variety of structures that are involved in processes that can increase or reduce heat loss to the environment

Structures in human skin involved in increasing or reducing heat loss

Cooling mechanisms in endotherms

• Vasodilation
  • Heat exchange (both during warming and cooling) occurs at the body's surface as this is where the blood comes into closest proximity to the environment
  • The warmer the environment, the less heat is lost from the blood at the body's surface
  • One way to increase heat loss is to supply the capillaries in the skin with a greater volume of blood, which then loses heat to the environment via radiation
  • Arterioles (small vessels that connect arteries to capillaries) have muscles in their walls that can relax or contract to allow more or less blood to flow through them
  • During vasodilation these muscles relax, causing the arterioles near the skin to dilate and allowing more blood to flow through skin capillaries
  • This is why pale-skinned people go red when they are hot

• Sweating
  • Sweat is secreted by sweat glands
  • This cools the skin by evaporation which uses heat energy from the body to convert liquid water into water vapour
  • This means sweating is less effective as a cooling mechanism in humid environments, as humid air is less effective at evaporating water (due to a reduced concentration gradient)

• Flattening of hairs
  • The hair erector muscles (effectors) in the skin relax, causing hairs to lie flat
  • This stops them from forming an insulating layer by trapping air and allows air to circulate over skin and heat to leave by radiation

Responses in the skin when the body temperature is too high and needs to decrease

Warming mechanisms in endotherms

• Vasoconstriction
  • One way to decrease heat loss is to supply the capillaries in the skin with a smaller volume of blood, minimising the loss of heat to the environment via radiation
  • During vasoconstriction the muscles in the arteriole walls contract, causing the arterioles near the skin to constrict and allowing less blood to flow through capillaries
  • Instead, the blood is diverted through shunt vessels, which are further down in the skin and therefore do not lose heat to the environment
  • Vasoconstriction is not, strictly speaking, a 'warming' mechanisms as it does not raise the temperature of the blood but instead reduces heat loss from the blood as it flows through the skin

• Boosting metabolic rate
  • Most of the metabolic reactions in the body are exothermic (heat-producing) and this provides warmth to the body
  • In cold environments, the hormone thyroxine (released from the thyroid gland) increases the basal metabolic rate (BMR), increasing heat production in the body

• Shivering
  • This is a reflex action in response to a decrease in core body temperature (this means it is a nervous mechanism, not a hormonal one)
  • In this case, muscles are the effectors and they contract in a rapid and regular manner
  • The metabolic reactions required to power this shivering generate sufficient heat to warm the blood and raise the core body temperature

• Erection of hairs
  • The hair erector muscles in the skin contract, causing hairs to stand on end
  • This forms an insulating layer over the skin's surface by trapping air between the hairs and stops heat from being lost by radiation

Responses in the skin when body temperature is too low and needs to increase

Body temperature control table

Remember homeostasis involves the maintenance of a constant internal environment; temperature control is an example of negative feedback

Thermoregulation in ectotherms

• On land, environmental temperatures can vary greatly between seasons or even over the course of a single day
• Ectothermic animals need to avoid extremes of temperature
  • For example, if they get too cold their low body temperatures decreases the speed they are able to move at, which decreases their ability to catch prey or escape predators

• To heat up, ectotherms seek out the sun or warmer surfaces and rest or 'bask' in these locations as they warm, until their body temperature has been increased sufficiently
• To cool down, ectotherms seek shade or water
• This means the behaviour of ectotherms is more restricted by environmental temperatures compared to endotherms, meaning that they cannot easily colonise habitats that are very hot or cold
• In contrast, endotherms require much more energy to maintain their body temperature, meaning their metabolic rate (and therefore their food requirement) is much greater
• This, in turn, can restrict the behaviour of endotherms and means that ectotherms can actually survive better in environments where food is limited (they need less and can last longer without food)
• Lastly, aquatic ectotherms actually have relatively minor difficulty maintaining a stable internal body temperature as water temperatures are significantly less variable than those on land (this is due to the high specific heat capacity of water)