1. 1. LEVELS OF BIOLOGICAL ORGANISATION AN INTRODUCTION TO HOMEOSTASIS

14. 2. HOW HOMEOSTASIS WORKS THE ROLE OF FEEDBACK SYSTEMS

15. HOMEOSTASIS: A definition: the maintenance of a stable internal environment despite fluctuations in the external environment

16. COMPONENTS OF HOMEOSTASIS A system requires three components for homeostasis: - A receptor; - A control centre; - An effector. These components do specific jobs that allow regulation of the internal environment.

17. FEEDBACK SYSTEMS May be negative or positive Discuss how these devices work: A toilet cistern when you flush A hot water cylinder when you run the shower

18. WHAT UPSETS THE BALANCE? Various factors apply stress on the internal environment: Internal factors - within the body, eg pain, high or low blood pressure External factors - outside the body, eg extremes of temperature

19. NEGATIVE FEEDBACK... causes the system to respond to reverse the direction of change... which tends to keep things constant… so it allows the maintenance of homeostasis. eg, if there is a fall in calcium in the blood, the parathyroid glands sense the decrease and secrete more parathyroid hormone, thereby increasing calcium release from the bones;

21. NEGATIVE FEEDBACK IN INCUBATOR temperature Temp too high Temp too low Temperature sensor Thermostat system with switch Heater (switched off) Temperature sensor Heater (switched on) Thermostat system with switch Incubator temperature at 32 C Rise in Temp falls i Temp rises i

22. NEGATIVE FEEDBACK A FRIDGE Fall in Temperature sensor Thermostat system with switch cooling (switched off) Temperature sensor cooling (switched on) Thermostat system with switch Fridge temperature varies to setting C Drop in temperature Rise in temperature Drop in temperature

23. POSITIVE FEEDBACK… increases the variable in the same direction… a destabilising effect that does not result in homeostasis. Positive feedback is used when rapid change is needed. eg in childbirth the hormone oxytocin is produced to stimulate and enhance labour contractions eg clotting mechanism when skin is cut