exam questions

• Describe how thermoregulatory mechanisms are controlled to help marathon runners avoid heat stress(4)

  • thermoreceptors in the hypothalamus or skin detect increase in temperature
  • thermoregulatory centre in hypothalamus is stimulated
  • hypothalamus sends impulses to sweat glands
  • increased blood flow to surface of skin by vasodilation/contriction of shunt vessels
  • decreased metabolic rate
• describe how the brain reduces the activity of the sweat glands after exercise(2)

  • thermoreceptors detect a decrease in temperature
  • hypothalamus/thermoregulatory centre sends fewer impulses to sweat glands
• explain the role of the nervous system in bringing about the increase in temperature of the fingertip(5)

  • thermoreceptors detect an increase in temperature
  • thermoregulatory centre in the hypothalamus detect temp increase
  • therefore impulses sent along the sympathetic nervous system
  • which leads to constriction of shunt vessels
  • therefore causing vasodilation of arterioles
  • so more warm blood flows near surface of skin
• Describe how muscle, present in blood vessels in the skin, helps to increase heat loss from the body(4)

  • muscles in shunt vessel contract
  • muscles in arterioles relax-vasodilation
  • to redirect blood away from deep arterioles into surface arterioles
  • to increase blood flow into capillaries in surface of skin
  • so more heat lost through radiation
• explain the importance to certain organisms of maintaining a constant body temperature(4)

  • to ensure conditions remain optimal for enzyme action/cell function
  • excess heat denatures enzymes/changes tertiary structure/changes shape of active site
  • enzyme-substrate complexes cannot form and the enzyme cannot catalyse the reaction
  • the reaction will stop- less sufficient metabolic reactions
• state what is meant by the term 'negative feedback loop'(1)

  • where a change triggers a response which reduces the effect of a change back within normal limits
• describe the mechanism by which thyroxine is responsible for thermoregulation(3)

  • it binds to thyroid hormone receptor(tf) so the receptor(tf) cannot bind to the start of a gene-coding for a protein that increases metabolic rate
  • this allows RNA polymerase to bind to the start of the gene/switches on the gene
  • transcription/expression of the gene will increase
  • the protein which increases metabolic rate is produced, increasing body temperature
• describe the homeostatic mechanism that will regulate the mountaineers body temperature near the top of the mountain(5)

  • thermoreceptors in the hypothalamus and skin detect the decrease in body temperature
  • the hypothalamus will send impulses along motor neurons to effectors
  • results in vasoconstriction of blood vessels/arterioles that supply skin capillaries
  • erector pili muscles contract causing hair to stand up and form an insulating layer/trap air
  • this reduces heat loss through radiation to the environment
  • sweat glands release less sweat to reduce heat loss via evaporation
• use knowledge of skin structure and blood vessels to explain why gangrene(tissue death by frostbite) is more likely in extremeties eg fingers or toes(2)

  • vasocontriction of blood vessels in cold conditions leads to less blood flowing to capillaries in extremeties
  • this results in less oxygen/glucose reaching the cells making up tissue in the extremeties
  • less respiration occurs/less energy is released in these cells which could result in cell death
• explain why positive feedback loops cannot be considered a homeostatic mechanism(2)

  • the stimulus produces a response that enhance the effect of the original stimulus
  • therefore it cannot maintain a constant internal environment in living organisms