Module 5 - 5.1.1

• Homeostasis


  The maintenance of a constant internal environment despite changes in external and internal factors
• Factors that must be maintained within a small range of variables

  • CO2 concentration
  • Body temperature
  • Blood glucose
  • Salt
  • Water potential
  • Pressure
• Homeostasis mechanism


  1. Deviation from optimum detected by sensory receptors

  2. Signal transmitted via hormonal or neuronal system

  3. Coordination centre (usually brain) transmits signal to effectors

  4. Effectors bring about response that reverses initial change

  5. Sensory receptors detect reduced stimulus
• Negative feedback


  A deviation from the optimum is detected and a mechanism reverses the change
• Positive feedback


  A deviation increases the original deviation, so is a mechanism whereby a change is increased
• Positive feedback systems are more rare in the body, but are used to effect larger changes over a short period
• Positive feedback example


  • During pregnancy the dilation of the cervix causes the posterior pituitary gland to release oxytocin which increase uterine contractions which stretch the cervix more, causing more oxytocin to be secreted, increasing the dilation of the cervix
• The action of neurons also involves positive feedback with an influx in sodium ions depolarising the cell, causing voltage-gated sodium ion channels to open
• Ectotherms


  Not able to control their body temperature using their metabolism; they rely on external sources of heat and their body temperature fluctuates with the external temperature
• Ectotherms controlling body temperature

  1. Basking in sun to gain heat

  2. Changing body orientation to increase/decrease surface area exposed to sun

  3. Curling into ball or increasing ribcage size to change body size

  4. Increasing breathing rate to evaporate water and cool body

  5. Using burrows or crevices with stable air temperature

  6. Pressing body against warm ground to increase temperature through conduction

  7. Increasing metabolic rate through movement
• Ectotherms


  • Some have physiological adaptations like darker colours in colder climates and ability to control heart rate
• Endotherms


  Control their body temperature within a small range of variables, largely independent of external temperatures, using their internal exothermic metabolic activities to keep them warm, and energy-requiring physiological responses to cool them down
• Endotherm thermoregulation mechanism


  1. Thermoregulatory centre in hypothalamus monitors blood temperature

  2. Peripheral temperature receptors in skin and muscles allow quicker response

  3. Physiological responses controlled by negative feedback from hypothalamus

  4. Heat loss/gain centre in hypothalamus activates autonomic motor neurons to effectors in skin and muscles
• Endotherm behavioural responses to temperature changes

  • Basking in sun
  • Orienting body to increase/decrease sun exposure
  • Pressing against warm surfaces
  • Exercising to increase metabolic rate
  • Rolling into ball to reduce surface area
  • Remaining dry when cold
  • Bathing in water/mud
  • Finding shade
  • Remaining inactive
  • Spreading out body
  • Licking skin/fur to use evaporation for cooling
• Endotherm physiological responses to temperature changes

  • Sweat gland secretion and evaporation
  • Hair/feather flattening to reduce insulation
  • Vasodilation/vasoconstriction to direct blood flow
  • Respiratory rate control in liver to release/retain heat
  • Skeletal muscle shivering to generate metabolic heat
• Endotherms use a significant part of their energy intake to maintain their body temperature, need more food as a result, and use for growth a lower proportion of the energy and nutrients gained from food
• Ectotherms use less of their energy in respiration, more of the energy and food can be converted to growth, they need to find less food, and they can survive long periods without food
• Ectotherms cannot be active at low temperatures, which can be a predatory disadvantage