Thermoregulation (SL IB Biology)

• Thermoregulation is the control of internal body temperature
• Thermoregulation is an example of a negative feedback mechanism; when body temperature deviates from pre-set limits, the responses of the body act to reverse the change and bring temperature back to normal
• Negative feedback is brought about by:
  • Using receptors to detect any deviation from normal levels
    • External body temperature is monitored using peripheral thermoreceptors in the skin
    • Internal body temperature is monitored using receptors located inside the hypothalamus of the brain
  • Effectors respond to any deviation from normal levels
    • Controlling heat loss at the skin to the external environment
    • Modifying the generation of heat inside the cells by metabolism

Negative feedback mechanism diagram

Thermoregulation is an example of negative feedback; the 'factor' here is temperature, the 'stimulus' is a change in internal body temperature, and the 'corrective mechanisms' are the action of effectors that control heat generation and loss

• Examples of effectors involved with temperature change include:
  • The hypothalamus
    • Regulates secretion of a hormone called thyrotropin-releasing hormone
    • Thyrotropin-releasing hormone stimulates the pituitary gland to release thyroid-stimulating hormone
    • Thyroid-stimulation hormone stimulates the thyroid gland to release thyroxin
    • Thyroxin increases metabolic rate
      • Altering the level of thyroxin alters heat generation by cell metabolism, aiding regulation of body temperature
  • Muscle tissue
    • Shivering in the muscles raises the metabolic rate of muscle cells, releasing heat energy
  • Adipose tissue
    • White adipose tissue stores lipids in a layer beneath the skin and around the internal organs, providing insulation that aids temperature regulation
    • Brown adipose tissue can generate heat energy before shivering begins in the muscles; this is known as non-shivering thermogenesis

• Internal body temperature is a key factor that needs to be controlled in homeostasis
  • A stable core temperature is vital for enzyme activity, e.g. human enzymes have evolved to function optimally at a core body temperature of about 37 °C
    • Lower temperatures either prevent reactions from proceeding or slow them down:
      • At lower temperatures molecules have little kinetic energy, so collisions are infrequent and few enzyme-substrate complexes form
    • Temperatures that are too high can cause enzymes to denature, meaning that they lose their tertiary structure and enzyme-substrate complexes can no longer form
• Endotherms are animals that maintain a constant internal body temperature, e.g. mammals and birds
• Mammals and birds can regulate their body temperature using:
  • Physiological mechanisms, such as shivering and altered metabolism
  • Behavioural mechanisms, such as seeking the shade of an underground burrow, or sunbathing

Thermoregulation in humans

• Endothermic animals detect external temperatures via peripheral receptors, e.g. thermoreceptors found in the skin
  • There are receptors for both heat and cold
  • These communicate with the hypothalamus to bring about a physiological response to changing external temperatures
• Human skin contains a variety of structures that are involved in processes that can increase or reduce heat loss to the environment

Skin structure diagram

Human skin contains structures that are involved with monitoring and responding to temperature change

Human responses to an increase in temperature

• Vasodilation
  • Arterioles (small vessels that connect arteries to the skin 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
  • The increased blood flow to the skin means that more heat is lost to the environment by radiation from the skin surface
• 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
• Flattening of hairs
  • The hair erector muscles in the skin relax, causing hairs to lie flat
  • This stops the hairs from forming an insulating layer of air and allows air to circulate over skin, meaning that heat energy lost by radiation can be moved away from the skin surface

Increasing heat loss via the skin diagram

The skin responds to high temperatures with vasodilation, sweating, and relaxation of hair erector muscles

Human responses to a decrease in temperature

• Vasoconstriction
  • 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 deeper in the skin and therefore do not lose heat to the environment
  • The reduction in blood flow to the skin surface means that less heat energy is lost by radiation
• 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
    • Humans have very little hair on their skin, so this response is less effective than it would have been in their evolutionary ancestors

Reducing heat loss via the skin diagram

The skin responds to low temperatures by vasoconstriction and the contraction of hair erector muscles

• Shivering
  • Muscles contract and relax rapidly
  • The metabolic reactions required to power shivering generate sufficient heat to warm the blood and raise the core body temperature
• Uncoupled respiration in brown adipose tissue
  • The reactions of respiration are usually said to be 'coupled' with ATP production, meaning that most of the energy released from carbon compounds is used to generate ATP
  • The 'uncoupling' of respiration from ATP production means that all of the energy released from metabolism is released as heat, and ATP is not produced
  • This can occur in brown adipose tissue where lipids are metabolised to release heat energy
    • This process occurs mainly in newborn infants, who cannot shiver so rely on this non-shivering thermogenesis
• Boosting metabolic rate
  • Most of the metabolic reactions in the body release heat
  • The hormone thyroxine is released from the thyroid gland, and acts to increase the basal metabolic rate (BMR), increasing heat production in the body

Thermoregulation negative feedback diagram

Thermoregulation is an example of negative feedback