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Homeostasis is essential for proper functioning of organisms because:

  • Enzymes which control biochemical reactions within cells, and other proteins are sensitive to changes in pH and temperature. Any change reduces efficiency of enzymes and can prevent them from working altogether.
  • Changes to water potential of blood and tissue fluid may cause cells to shrink and expand as a result of water entering or leaving by osmosis.
  • Organisms which can maintain a constant internal environment are more independant of the external environment. Have a better chance of finding food.

Control mechanisms

  • Set point - desired level at which system operated. Monitored by:
  • Receptor - detects deviation from set point and informs
  • Controller - coordinates information from various receptors and sends instructions to appropriate
  • Effector - brings about changes needed to return nervous system to set point. This return creates
  • Feedback loop - informs receptor of changes to system

Regulation of body temperature in ectotherms

  • Exposing themselves to the sun
  • Taking shelter
  • Gaining warmth from the ground
  • Generating metabolic heat
  • Colour variation

Regulation of body temperature in endotherms - response to cold environment

  • Vasoconstriction - diameter of arterioles near surface of skin is made smaller. Reduces volume of blood reaching skin surface through capillaries.
  • Shivering - muscles of body undergo involuntary contractions that produce metabolic heat
  • Raising of hair - hair errector muscles in skin contract, thus raising hair on skin.
  • Increased metabolic rate - more of hormones that increase metabolic rate are produced. Metabolic heat including respiration is increased
  • Decrease in sweating
  • Behavioural mechanisms e.g. huddling and sheltering

Regulation of body temperature in endotherms - response to warm environment

  • Vasodilation - diameter of arterioles near skin surface become larger. Allows warm blood to pass close to skin surface through capillaries. Heat from blood is radiated away from body.
  • Increased sweating - to evaporate water from skin surface, requires energy in form of heat.
  • Lowering of body hair - hair errector muscles in skin relax and the elasticity of skin causes them to flatten against the body.
  • Behavioural mechanisms e.g. seeking shade

Control of body temperature

Within the hypothalamus there is a thermoregulatory centre consisting of two parts:

  • A heat gain centre - activated by fall in blood temperature. Controls mechanisms that increase body temperature.
  • A…


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