2.89 Homeostasis: A* understanding for iGCSE Biology

Homeostasis is one of the more difficult topics for students to understand in the iGCSE specification.  I have posted already about the skin and its role in thermoregulation so I suggest you read that post again to get the details….

https://pmgbiology.wordpress.com/2014/05/29/skin-a-understanding-for-igcse-biology/

In this post, I am going to try to explain the concept of homeostasis in much more general terms, then in later posts, look at the two examples mentioned in the syllabus.  Here goes….

Homeostasis 

Homeostasis is one of the life characteristics shared by all organisms.  Living things all inhabit a world in which the external environment changes from hour to hour, from day to day, from month to month.  Even organisms living in the most stable aquatic environments may be subject to changing oxygen concentrations, changing water pH, changing light intensities and so on.  This changing external environment poses a challenge for life since how can life processes operate at optimal levels in all these differing conditions.   Life has solved this by allowing organisms to keep their internal environments much more constant than the ever-fluctuating external environment.

A definition to learn:

“Homeostasis is the set of processes occurring in an organism to maintain a constant internal environment”

Examples of Homeostasis in Humans

A whole variety of factors are maintained at constant values in the body by homeostasis.  For example (there are many more….):

  • Blood pH
  • Blood temperature
  • Blood dilution
  • Blood oxygen concentration
  • Blood carbon dioxide concentration
  • Blood glucose concentration
  • Blood pressure

This introduces the first area of common confusion in students’ exam answers.  For some reason many students think that homeostasis is a word for the maintenance of body temperature in humans.  I hope you can see it is a much more general term than that.

But…. the systems that maintain a constant body temperature in endothermic animals are one example of homeostasis.  In fact this example (thermoregulation) is one of the two from the list above that you need to understand for your exam.  The other one you might be asked about is osmoregulation (the maintenance of a constant dilution of the blood).

All homeostatic control systems have some common features.  The variable that is going to be regulated needs to be measured somewhere in the body.  A change in this variable is called a stimulus and is measured by a cell called a receptor.  The measured value needs to be compared with a “set value” and this is done by an integrating centre that then controls an effector.  The effector is an organ that can bring about a response.  But what kind of response do you want in the process of negative feedback?

Screen Shot 2015-02-20 at 11.24.00

A common process involved in homeostasis is negative feedback.  This is quite tricky to define but in fact it is a really simple idea…….  If you want things to stay the same, any change must be corrected. That’s negative feedback in a nutshell.

For example a school might want students walking round the campus at a sensible speed:  not to fast to knock people over, not to slow or people are late for lessons…  Imagine a particular group of children who start to run around the place, causing mayhem and injuries to fellow students.  Well this will first be detected by the system.  There may be a particular teacher who comes out and sees the students running, the school nurse might report an increase in cuts and bruises.  However it happens, a change in the system (a stimulus) is detected.  There will be an integrating centre in this control system too, probably in the form of a stern deputy head.  She will compare the measured speed to her own “set value” of how fast students should move.  And she will initiate a response:  probably a loud telling off to the entire school in assembly, lots of dire warnings about future conduct and an after school detention for all the rule breakers.  The net response of this will be that students will start moving slower around the school….  Eventually of course people will start moving too slowly and will be late for lessons.  How do you think the system will react to this new stimulus?  This process where the response tends to reduce the stimulus is called negative feedback.

To give you a biological example in thermoregulation, look at the diagram below:

thermoregulation-2_med

If the body temperature goes up, the system responds by lowering the body temperature.

If the body temperature drops, the system responds by raising body temperature.

Both examples of negative feedback!

feedback_f

The end result of negative feedback is that the value will fluctuate around the set value (see this graph also showing the effect of negative feedback in thermoregulation)

Any questions, problems, queries:  please comment using the box below the post or tweet me @Paul_Gillam and I will do my best to help…….

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