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Homeostasis

91604Demonstrate understanding of how an animal maintains a stable internal environment

Introduction and key words


Key words list
Some good resources (including PowerPoints) here.

This assessment can be taken in a number of ways but more than likely you will be looking at how the humans body keeps a consistent environment in terms or physical exertion, being in an extreme environment or diet. This most likely will not be open book so it is very important that you understand this topic very well. Usually I tell my students if you can explain it to another person without referencing notes you have the level of understanding required.
The following 3 sentences must be understood before you walk into your test (:
  1. The main purpose and components of the homeostatic control system. What it does and what structures it uses and why.
  2. The mechanism of this control system, i.e. how and why it responds to the normal range of environmental fluctuations, the interaction and feedback mechanisms between parts of the system
  3. How balance is re-established following the potential effect of one specific disruption - you will not see this before the assessment. What occurs in the system to return the fluctuation back to the normal internal physiological state.
  4. For Excellence you need to explain an example of a negative feedback being broken (this is important). 


What is Homeostasis?


Homeostasis is the physiological consistency of the body despite external fluctuations. All complex multicellular organisms maintain a stable internal environment using their organ systems. As you can see in the picture (click on it to see a bigger version) there are a lot of systems working together. Don't stress though as you assessment will only be looking at one of these. 














From a student of Mrs Drysdale (thanks)
Overview of homeostasis in the human body homeostasis
A overview of homeostasis and how it is maintained in the human_body
A link to work through notes on how the body maintains homeostasis and then try the homeostasis_quiz
Animation that gives an overview of homeostasis and blood sugar levels
The hypothalamus and it's role in temperature regulation
Use this interactive animation to try to maintain homeostasis
This link gives you an overview of homeostasis and the different homeostatic systems in humans
This link is an overview of the two systems along with great diagrams and quizzes homeostatic_systems
This is a great website that overviews all of components and their interactions in thermoregulation & Blood glucose management

Intro and basics


A good introduction to the topic (make sure you play the animations)

The homeostatic control systems in Animals have three components: 
1. Some sort of receptor (sense organ) to detect a change. In the case of thermoregulation this would be skin receptors (click on image below to see the skins layers in detail.)

2. A centre of control (usually a brain or a section of the brain)
3. An effector (muscle cells, organs) to produce a response that is appropriate to the change.
These all work together in what is called a feedback system. The regulation of this is called homeostasis. This may be + or - depending on the example.
There will be a method of communication between these layers.
Here is a good video explaining the Endocrine System, and below that is a table covering all the communication pathways.

Route Travelled

Specific pathways (nerve axons)

Via blood

Time to reach target cell

Milliseconds

Seconds

Cells reached

Only those at the end of axons

Entire body (only some cells respond)

Strength of the signal indicated by

Frequency of impulses

Concentration of hormone

Duration of Signal

Millisecond for each impulse

Minutes to hours or days


The - (negative) feedback system

 are by far the most common found in the human body. 
They form a looped feedback system that restores the condition to a steady state. 
The main idea is that the stimulus from one part of the body produces a response that will stop or reduce (make smaller) the original stimulus.

Picture above is from Peter Shepard (Maurice Wilkins)



+ (positive) feedback

is rare in the human body. They form a looped system that changes the body from its original position. Normally one part of the body enhances another parts effect which can cause a escalation of the condition. This can be bad as it is not controlled that well.  Some examples are human shock which can reduce blood pressure.

Maintaining your homeostatic environment
Hormone and nevous systems work together to create a homeostatic environment.


Intro to the standard



As above, your teacher will run this as an internal assessment and the standard requires the following to be covered. Each part below will link to relevant resources.

· a discussion of the significance of the control system in terms of its adaptive advantage

· an explanation of the biochemical and/or biophysical processes underpinning the mechanism (such as equilibrium reactions, changes in membrane permeability, metabolic pathways)

· an analysis of a specific example of how external and/or internal environmental influences result in a breakdown of the control system.


A control system that maintains a stable internal environment (homeostatic system) refers to those that regulate:

The hypothalamus

An introduction
A fantastic link
animation on the hypothalamus


Humans have control systems that regulate: (possible question subjects)

body temperature (thermoregulation)

Thermoregulation introduction link

As an intro to think about thermoregualtion



Homeostasis is the control of internal body conditions so that body processes can work efficiently.

Control of body temperature is called thermoregulation.

Normal body temperature is 37ºC. This is the temperature at which enzymes work best.

The thermoregulatory centre in the brain has receptors to monitor the temperature of the blood flowing through it. Receptors in the skin send impulses to the centre about skin temperature.


If body temperature is too high:

If body temperature is too low:

  • Blood vessels in the skin dilate (get wider) so more blood flows to the surface of the skin. 
  • Blood vessels in the skin constrict and narrow so less blood flows to the surface of the skin.
  • Sweat is produced, which cools the body as it evaporates. 
  • Muscles shiver.  They need more energy so respire more and some energy is released as heat.
More water and salts, in the form of ions, are lost by sweating when it is hot. These have to be replaced by taking drinks and food.

Homeostasis of body temperature involves many types of effector systems including physiological (changes in skin blood flow, cooling mechanisms = sweating, heating mechanisms = shivering) and behavioural (sun basking, retreating to shade, changes in posture) – all controlled by the set-point of temperature sensing nerve cells in the hypothalamus (the thermometer).


Click the image below to see the larger version

The thermoregulatory centre normally maintains a set point of 37.5 ± 0.5 °C in most mammals. However the set point can be altered in special circumstances:
• Fever. Chemicals called pyrogens released by white blood cells raise the set point of the thermoregulatory centre causing the whole body temperature to increase by 2-3 °C. This helps to kill bacteria, inhibits viruses, and explains why you shiver even though you are hot.
A mild fever is an immune response to stop a bacteria based infection, although remember a fever that stays too high can damage the cell activity permanently. This link gives just a bit more information about how homeostasis works to protect the body and how sometimes a fever can be helpful.

blood pressure





osmotic balance (osmoregulation)

Negative feedback - osmoregualtion
level of blood glucose (modified from Peter Shepard's excellent PowerPoint). 
An excellent animation covering glucose regulation

Glucose regulation



One simple example of hormonal homeostatic control is the control of blood sugar level by insulin and glucagon produced by endocrine cells in the pancreas.

Insulin stimulaes uptake of glucose from the blood by tissues for use or storage. This lowers blood glucose concentration.

Glucagon stimulates the release of glucose from glycogen stored in the liver. This raises blood glucose concentration.


Why regulate it?

•Some tissues can use a range of energy sources such as fats and even amino acids but several important tissues in the body can only really use glucose so these tissues have a need for a constant supply of glucose to function properly. 
•These tissues includes red blood cells and immune cells 
•Brain and the nervous system also rely on glucose which explains why when blood glucose levels fall below about 2.5 mM
that people get seizures and can go into a coma as the brain doesn’t function properly. 
•Therefore maintaining a certain level of  glucose is a matter of life and death.

How it gets into the cells?

•Glucose can’t get across the membrane of cells unless specific transporters are in the membrane to provide a channel for the glucose to move through.
•These are specific for glucose and so are called glucose transporters. They do not use energy so will only transport glucose from areas of high glucose concentration to areas of low glucose concentration (i.e down a concentration gradient). 
•Therefore if a cell is using glucose then levels in the cell drop and the glucose will move from the outside of the cell to the inside.
•In liver cells stimulated by glucagon there will be lots of glucose produced inside the cells from glycogen therefore the flow of glucose will be from inside the cell to the outside.
•In brain and liver there are always glucose transporters in the plasma membrane

What happens between meals?
•Glucose levels fall and glucagon is released 
•Glucagon binds to receptors that are found on cells in the liver •This stimulates the release of glucose from the liver where it has been stored as long polymeric chains of glucose called glycogen.
What happens if glucose gets too low?

Because the brain and blood cells need glucose the body has developed emergency measures when blood glucose get dangerously low (known as a “hypo”) as might happen if a Type-1 diabetic administers to much insulin.

•Symptoms appear (called “hypoglycemic awareness”) including fatigue, irritability, nervousness, depression, flushing, memory loss, loss of concentration, headaches, dizziness, fainting, blurring of vision, ringing in the ears, numbness, tremors, sweating and heart palpitations. 
•These are stimulated by signals from the brain that is sensing the low glucose and part of this response is to increase adrenaline levels in an attempt to increase blood glucose levels
•The warning signs are very useful for a Type-1 diabetic as the hypoglycemia can easily be overcome by taking in some glucose. 
•Unfortunately the ability to detect hypoglycemia sometimes get lost by diabetics (a condition termed “hypoglycemic unawareness”).

After a meal?

•Glucose levels rise and insulin is released 
•Insulin binds to receptors that are found on cells in the liver, in muscle and in fat cells 
•This stimulates the uptake of glucose into these tissues so blood glucose levels go down 
•Glucose taken up by liver is mostly stored as glycogen 
•Some of the glucose going into fat cells contribute to the accumulation of fat in these cells

What happens when insulin binds to its receptor?

•Insulin moves through the blood stream until it finds its specific receptor on the surface of the liver cells, muscle cells and fat cells. 
•The receptor is a protein that spans the membrane 
•The binding of insulin causes an allosterically induced change in the shape of the intracellular portion of the receptor which activates an enzymatic activity. 
•The receptor is now said to be activated and as shown in later slides this brings about changes inside the cell. 
•This is in effect allowing the hormone on the outside of the cell to regulate functions inside the cell even though the hormone has not entered the cell. This is called “transduction” and the whole process is often called signal transduction.

levels and balance of respiratory gases in tissues.


Work thorough the following animation here

 

   The biological ideas related to the control system includes the:

·       purpose of the system


·       components of the system


·       mechanism of the system (how it responds to the normal range of environmental fluctuations, interaction and feedback mechanisms between parts of the system)


·       potential effect of disruption to the system by internal or external influences.


Environmental influences that result in a breakdown of the control system may be external influences such as extreme environment conditions, disease or infection, drugs or toxins, or internal influences such as genetic conditions or metabolic disorders.


PowerPoints that will help


Waikato University Web Day PPT

Homeostasis www.hartnell.edu/faculty/nwheat/30.%20Homeostasis.ppt

Homeostasis www.cneccc.edu.hk/.../bio/.../HOMEOSTASIS_IN_MAMMALS__AR.P

Notes - Homeostasis.ppt bcsd.k12.ny.us/high/recchia/Notes%20-%20Homeostasis.ppt



Notes - PDF that may be of help




Videos




Notes


HOMEOSTASIS - Extended notes.

At all stages of the life of a mammal the cells of the body are provided with a constant supply of the things they need.

There is a buffering of the fluctuation of the environment so that the cells in a mammal may live even though the conditions outside the body are not good.

A control system (i.e. a homeostatic system) that maintains a stable internal environment refers to those that regulate one of:
  1. body temperature
  2. blood pressure
  3. osmotic balance
  4. level of blood glucose
  5. levels and balance of respiratory gases in tissues. 
Because the cells have a fluid bathing them whose chemical composition and temperature is VERY CONSTANT these cells are able to function equally well in the tropics or the arctic, in the ocean, in fresh water or in the desert.

Environmental influences that result in a breakdown of the control system may be:
  1. external influences such as extreme environment conditions, disease or infection, drugs or toxins
  2. internal influences such as genetic conditions or metabolic disorders.

HOMEOSTASIS = maintenance of constancy of the INTERNAL ENVIRONMENT

where the `internal environment' = the INTERCELLULAR FLUID (the medium in which body cells are bathed) - also known as INTERSTITIAL or TISSUE FLUID.

BASIC PRINCIPLES OF HOMEOSTASIS

1.         Whenever a condition (e.g.. temp; glucose level in blood etc.) deviate from a set point or NORM (e.g.. 37 C; 90mg glucose per 100cm  blood) the corrective mechanism is  triggered by the very entity which is to be regulated, ie. homeostasis involves a  self-adjusting mechanism of the control process being built into the system.

2.         In the case of e.g.. glucose regulation an increase in the amount of glucose triggers a  process to decrease it.  Conversely, a decrease in the glucose level triggers a process  to increase it.  In both cases the result is a reasonably constant level of glucose. When a change in an entity brings about the OPPOSITE EFFECT this is known as a NEGATIVE FEEDBACK mechanism.

Sometimes the corrective mechanism leading to NEGATIVE feedback breaks down  with the result that a deviation from the norm initiates FURTHER deviation.  This is known as POSITIVE FEEDBACK.

  e.g.. Once the temperature regulating mechanisms fail ,the metabolic rate goes on climbing even if the environmental temp. is no longer increased.  This is because every time the metabolic rate increases it generates more heat which increases the metabolic  rate a bit more, and so on.

It is difficult to think of any household situation in which positive feedback  systems operate.  When a baby is born, contractions of the womb become progressively stronger as the head is pressed down into the vagina (birth canal); this is a positive  feedback system that results in the expulsion of the baby from the mother's womb.

Another physiological example can be seen in the generatic of a nerve impulse where Na+ (sodium ions) crossing the nerve cell membranes stimulate further Na+ to cross.   (see later)  This process only lasts for a brief moment.

 It is clear from the examples of POSITIVE FEEDBACK (above) that (i) under normal conditions it is UNCOMMON since (ii) the end result  is a further increase or      further decrease in the entity concerned and hence CONSTANCY IS AVOIDED.

3.         Homeostasis must necessarily involve FLUCTUATIONS, small though these may be.      

            Only by deviating form the NORM can the mechanism be brought into play.

4.         The feedback system must have:

·    RECEPTORS (or SENSORS) capable of detecting the change;

·    a CONTROL MECHANISM (or MONITOR) capable of initiating the appropriate corrective measure;

·    EFFECTORS which can carry out these corrective measures.

As a simplified example, consider the movement of fluid through a pump - to make this a physiological system, assume that the fluid is blood and the pump is a heart.  The function to be controlled homeostatically is the rate of outflow of blood from the heart, so this is the output and there must be a sensor that measures the rate of outflow.  This sensor transmits its measurements to the monitor, which compares the actual with the required output; the monitor sends signals to the pump - the heart muscle - and so adjusts the rate of pumping when the output is different from the set level.            

And finally the levels you will be targetting      

NZQA Exemplar                                                                                                                                                                                                                                                                                                                                                                          

Achievement

Merit

Excellence

Uses biological ideas to describe a control system by which an animal maintains a stable internal environment.

Biological ideas related to the control system includes description of:

·        the purpose and components of the system

·        the mechanism of the system i.e. how the components work together e.g. how it responds to the normal range of environmental fluctuations, interaction and feedback mechanisms between parts of the system

·        how balance is re-established following a specific disruption to the homeostatic system by internal or external influences.

 

 



 

As for Achieved and,

Uses biological ideas to explain how or why an animal maintains a stable internal environment to include:

·        the mechanism of the system to show how or   why the components work together

·        how a specific disruption results in responses within a control system to re-establish a stable internal environment.

 



 



 

As for Merit and,

Links biological ideas about maintaining a stable internal environment in an animal to include at least one of:

·        a discussion of the significance of the control system in terms of its adaptive advantage

·        an explanation of the biochemical and/or biophysical processes underpinning the mechanism (such as equilibrium reactions, changes in membrane permeability, metabolic pathways)

·        an analysis of a specific example of how an external and/or internal environmental influences result in a breakdown of the control system.

 

Subpages (1): Homeostasis Key Words
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