1. CONTROL IN THE BODY A2 TO BE ABLE TO DESCRIBE THE MAIN CONTROL MECHANISMS AND IMPORTANCE OF HOMEOSTASIS.

2. HOW ARE ORGANISMS AND SYSTEMS CONTROLLED?  Most animals and plants are complex, made of millions of cells.  Different parts of organisms perform different functions.  Its essential that info can pass between different parts to co- ordinate their activities.

3. Carrying on……………  Sometimes the purpose of the info transfer is to regulate levels of some substance within the organism. e.g……………  Sometimes the purpose may be to change the activity of some part of the organism in response to an external stimulus. e.g………………

4. CHEMICAL MESSENGERS AND NERVES..In both animals and plants, chemical messengers called HORMONES help to transfer info from one part to another to achieve co-ordination. In many animals, including mammals, NERVES transfer info in form of electrical impulses.

5. HOMEOSTASIS  A vital function of control systems is to maintain a stable internal environment.  This is called HOMEOSTASIS.  Internal environment means the conditions inside the body, in which cells function.

6. Homeostasis  Why is homeostasis important?  What factors need to be kept at constant levels, why?

7. Homeostasis…..  All cells need to be in the best physical and chemical conditions possible if they are to function properly.  Keeping every cell at the optimum temp, with enough water and glucose is a complex task that involves many systems in the body.

8. Homeostasis………………….  Many cells are highly specialised and are physiologically demanding if they are to perform to their optimum level.  Many features of the environment affect the functioning of the cells and have to be controlled in their effect.

9. Homeostasis………………………  Biologists call this overall control mechanism HOMEOSTASIS  Literally - steady state.  It refers to the fact that conditions inside the body, especially in the tissue fluid, need to be maintained within certain limits.  Some are kept within narrow limits and others vary over a wider range.  E.g……………………………………………………………..

10. Responding to a change  Change in conditions  Detected  Corrected

11. POINTS FOR FOCUS IN HOMEOSTASIS.  What causes the change? A rise in air temp may cause an increase in body temp.  What detects the change? If body temp starts to rise it is detected by hypothalamus in brain.  How is change corrected? To reduce its temp the body sweats and vasodilation takes place.

12. Examples of Homeostasis  From AS you have looked at controlling the water potential of blood.  Body temp was looked at briefly in the SA:VOL ratio section of AS.  Control of blood acidity was studied in mod 3.

13. AT A2 WHAT WILL YOU STUDY?  You will look at homeostatic mechanisms in the human body that keep blood glucose concentration and body temperature within set limits.

14. Stimulus And Response.  Homeostasis involves mechanisms that correct changes in the body.  Changes are usually due to a stimulus that can be INTERNAL or EXTERNAL.  TASK  Describe this situation, the response and the effect in the control of blood sugar.

15. Stimulus and Response  Write a simple stimulus –response pathway.

16. Stimulus and Response StimulusReceptorCoordinatorEffectorResponse Normal levels Receptor detects increase Coordinator stimulates effector Response reduces levels back to normal

17. NEGATIVE FEEDBACK SYSTEMS.  Receptors and effectors work together in homeostatic control systems in FEEDBACK SYSTEMS.  These are usually negative feedback systems.  Explain positive and negative feedback control in homeostasis