Thermoregulation (OCR A Level Biology)

• Homeostatic mechanisms help organisms to keep their internal body conditions within restricted limits
• Temperature is a key factor that needs to be controlled; the human body maintains a core temperature of around 37.5 °C
• A stable core temperature is vital for enzyme activity
  • Lower temperatures reduce the kinetic energy available for molecules and slow down chemical reactions
  • Higher temperatures speed up reactions up to a point, above which the rate of reaction drops sharply as the enzyme begins to denature

Thermoregulation

• Thermoregulation is the control of internal body temperature
• Animals can be split into two groups on the basis of their primary thermoregulation mechanism:
  • Endotherms
    • animals that possess physiological mechanisms for the maintenance of internal body temperature
    • e.g. mammals and birds
  • Ectotherms
    • animals that rely on behavioural mechanisms to ensure to maintain internal body temperature, e.g. by moving into or out of the sun, or huddling together for warmth
    • e.g. reptiles and amphibians

Thermoregulation in endotherms

• Endothermic animals detect external temperatures via peripheral receptors; these are thermoreceptors found in the skin and mucous membranes
  • Receptors detect heat and cold
  • Impulses are sent from the receptors to the hypothalamus
• The hypothalamus also contains receptors that monitor the temperature of the blood flowing through it
• Information from the receptors is processed by the hypothalamus, which then initiates homeostatic responses when temperature gets too high or too low
• Human skin contains a variety of structures that are involved in the regulation of heat loss

Human skin contains several structures that are involved with thermoregulation

Endotherm responses to high body temperature

• Vasodilation
  • During vasodilation the muscles in the walls of arterioles relax, causing dilation and allowing more blood to flow into skin capillaries
  • Heat is lost to the environment by radiation
• Sweating
  • Sweat is secreted by sweat glands in the skin
  • This cools the skin by evaporation
    • Heat energy from the body is used to convert liquid water into water vapour
  • Sweating is less effective as a cooling mechanism in humid environments there is a reduced water vapour concentration gradient between the skin and the air when humidity is high
• Flattening of hairs
  • The hair erector muscles in the skin relax, causing hairs to lie flat
  • This stops them from forming an insulating layer of air and instead allows air to circulate over skin, removing heat lost by radiation

Effectors that aid heat loss from the skin include muscles in the arteriole walls, sweat glands and hair erector muscles

Endotherm responses to low body temperature

• Vasoconstriction
  • During vasoconstriction the muscles in arteriole walls contract, causing the arterioles near the skin to constrict and allowing less blood to flow through skin capillaries
    • Instead, the blood is diverted through shunt vessels, which are deeper in the skin and therefore do not lose heat to the environment
  • Heat loss by radiation at the skin surface is reduced
• Increased metabolic rate
  • Most of the metabolic reactions in the body are exothermic and this provides warmth to the body
  • In cold environments the hormone thyroxine increases the basal metabolic rate (BMR), increasing heat production in the body
    • Thyroxine is released by the thyroid gland
• Shivering
  • Muscles contract and relax repeatedly in quick succession
  • The metabolic reactions required to power this muscle contraction releases heat energy 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 traps an insulating layer of air over the skin's surface, reducing heat loss by radiation

Effectors that respond to low body temperature include the muscles in arteriole walls, the thyroid gland, the muscles that cause shivering and the hair erector muscles

Body temperature control table

Temperature control is an example of negative feedback

Thermoregulation in ectotherms

• On land environmental temperatures can vary greatly between seasons, or even during a single day
• Ectotherms do not have the ability to respond to these changes via internal temperature regulation mechanisms, so they instead use behavioural mechanisms:
  • To warm up ectotherms may:
    • seek out the sun or warmer surfaces and 'bask' in these locations as they warm until their body temperature has been increased sufficiently
    • huddle together to retain heat that may have been gained from the sun earlier in the day
  • To cool down ectotherms may:
    • seek shade
    • move their bodies into water

• The behaviour of ectotherms is restricted by environmental temperatures, meaning that they cannot easily colonise habitats that are very hot or cold
• Ectotherms save a lot of energy by not regulating their body temperature internally, so their nutritional requirements may be much lower; ectotherms can survive in environments where food is limited 
• Aquatic ectotherms have little 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)