Homeostasis and feedbackEdexcel International A Level Biology: Subtopic test

1. 1
   The core body temperature of a healthy human is maintained close to 37 °C, even when the temperature of the surroundings changes from day to day. A physiologist records the core temperature of a volunteer every minute for several hours and finds that it fluctuates by a small amount above and below 37 °C.

   (a)

   Which term describes the maintenance of a constant internal environment within narrow limits?

   [1 mark]

   • AEquilibrium
   • BOsmoregulation
   • CExcretion
   • DHomeostasis

   (b)

   What happens in a negative feedback loop after the effector responds?

   [1 mark]

   • AThe change is amplified further from the set point
   • BThe receptor stops detecting the factor
   • CThe change is reversed and the factor returns towards the set point
   • DThe coordinator becomes a receptor

   (c)

   Explain why homeostasis is described as maintaining dynamic equilibrium.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   During childbirth, the baby's head presses on the cervix at the base of the uterus and stretches it. Stretch receptors in the cervix send impulses to the brain, which causes the release of the hormone oxytocin into the blood. Oxytocin stimulates the muscle of the uterus to contract more strongly, which pushes the baby's head harder against the cervix.

   (a)

   Which term describes the control system shown in the birth of the baby?

   [1 mark]

   • ANegative feedback
   • BPositive feedback
   • CReflex arc
   • DOsmoregulation

   (b)

   What brings this process to an end?

   [1 mark]

   • AOxytocin binds to receptors on the cervix and inhibits itself
   • BThe contractions get weaker as feedback increases
   • CThe hypothalamus detects a normal oxytocin concentration and stops its release
   • DThe baby is born, which removes the stretching of the cervix

   (c)

   Explain why positive feedback is not used to maintain body temperature.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A marathon runner's core body temperature was monitored throughout a training run. Before the run it was 37.0 °C. It rose to 38.6 °C after 40 minutes and then stayed between 38.6 °C and 38.8 °C for the rest of the run. Her sweat rate rose from 0.05 dm³ per hour at rest to 1.2 dm³ per hour during the run, and the blood flow through her skin rose from 0.25 dm³ per minute at rest to 2.5 dm³ per minute.

   (a)

   Explain how the hypothalamus brings about the changes in sweating and skin blood flow shown in the data.

   [3 marks]

   (b)

   The runner's body temperature rose during the first 40 minutes even though negative feedback was operating. Explain why it rose, and why it then stayed between 38.6 °C and 38.8 °C.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A group of volunteers took part in a trial in climate chambers. In the first part they rested in a chamber at 5 °C for 30 minutes. In the second part they cycled hard for one hour in a chamber at 30 °C. Their core body temperature was monitored throughout both parts of the trial.

   (a)

   Explain how the body responds to a fall in core temperature when the volunteers rest at 5 °C.

   [6 marks]

   (b)

   Explain why it is important that core temperature is kept within narrow limits during the hard cycling.

   [6 marks]

   Total for question 4: 12 marks

End of questions