HOMEOSTASIS

Staying within limits

Limits

Staying

Limits Keeping enzymes

happy

Maintaining a

balance

What is Homeostasis?

• Homeostasis is one of the key characteristics of living things

• It is the condition of maintaining a relatively stable internal environment regardless of the conditions in the external environment.

• The ‘internal environment’ consists of the body fluids and cells while the ‘external environment’ is that outside the body.

• Homeostasis = staying the same.

Why do we need Homeostasis?

• We need to keep key variables in our bodies within a narrow range so they can function optimally – for example so that enzymes can perform a variety of chemical reactions.

• These variables include body temperature, pH (percent H+ ions), water, gas concentrations (CO2) salt concentrations (Na+, Cl-, Ca+) and glucose levels.

• In animals, homeostasis is achieved through the action of nerves and hormones.

• In plants, homeostasis is achieved through the action of hormones only as plants do not have a nervous system.

• If homeostasis is not maintained the organism will be unable to function properly and is likely to die.

How does Homeostasis work?

•If our temperature decreases, homeostasis brings it back to normal by initiating certain mechanisms:

•Our muscles begin to shiver in order to generate heat.

•We to dress in warm clothes to protect from the chill.

•The hairs on our skin stand erect (goose bumps) in order to help insulate us.

•We eat more hot food, or drink hot beverages to help warm us.

•In general our appetites increase in winter to supply our bodies with more energy and to increase our body fat so we can stay warmer.

•Blood supply to the surface of our skin can be reduced by the constriction of blood vessels.

Keeping warm

How does Homeostasis work?

•If our body temperature increases, homeostasis brings it back within the normal range by initiating certain mechanisms:

•Sweating is induced so that air passing over wet skin can cool us.

•Animals without sweat glands start to pant – another form of evaporative cooling.

•We are motivated us to wear less and seek out shady areas.

•We tend to eat less food, drink more cold drinks and exercise less.

•The amount of blood flow to the skin increase to help cool the blood down.

Staying Cool

How does Homeostasis work?

•Whether we want to stay warm, keep cool, reduce the pH of our blood, increase the water content of our blood, or stabilize any other function under homeostatic control, a series of steps must take place.

•Temperature control will continue to be used in this PowerPoint as a standard example.

•For homeostasis to function there must be a stimulus telling our bodies the temperature has changed. A detector must recognize this.

•There must also be a response by our bodies in order to correct the change in temperature. An effector must initiate the response.

•Homeostasis allows a change to take place. If it is a counter change, it is known as a negative feedback system.

•If the change has an amplifying effect rather than a counterbalance, then it is known as a positive feedback system.

The Process

Negative Feedback•A negative feedback system causes the body to respond so that a reversal in the direction of a change occurs. This tends to keep the internal environment at a constant regardless of the external environment, thus maintaining homeostasis. 

Fall in body temperature …change detected by

RECEPTOR: Hot & cold thermo

receptors (nerves) in skin

Transmits information

to..

CONTROL CENTRE:

CNS

By nervous or hormonal control

activates…

EFFECTOR

RESPONSE

Active muscles warm the body,

which is detected by the…

Initiation of reverse

change in temperature as muscles

begin to shiver

Negative Feedback Mechanisms

negativefeedback

input sensor control effector output or centre or receptor response

communicationsystem

Another way to show the model

Negative Feedback•Many electrical appliances around the home function as a negative feedback system.  The homeostatic model can be applied to a heater

Room drops below 200C

RECEPTOR: thermometer

Transmits information

to..

CONTROL CENTRE:

thermostat set to 200C

Activates…

EFFECTOR: heater

RESPONSERoom warms up

again

Initiation of reverse

change in temperature

Negative Feedback

200C

Temp. too high

Heater turns off

Temp too low Heater

turns on

Positive Feedback•A positive feedback system causes the body to respond so that an increase in the direction of a change occurs. This can have both beneficial and harmful consequences and can sometimes be known as the multiplier effect. It works like a chain reaction.•If temperature was under positive control then we would easily overheat or over cool.•Nerve cells (neurons) use positive feedback to propagate electrical potentials by allowing more Na+ ions to enter into the nerve cells. Once one has entered, more channels in the cell membrane open up so that the electrical message is pushed along the nerve cells creating a signal. Therefore the cell does not revert to its normal state.

RECEPTOR

EFFECTOR

RESPONSE

EFFECTOR

Role of the Nervous System

•The nervous system coordinates the responses between the receptors and the effectors via the hypothalamus in the brain and the network of nerves around our entire bodies.

•Receptors detect a change from external stimuli. We have light receptors in our eyes, gravity and sound receptors in our ears, chemical sensors in our noses and tongues, pressure and pain sensors on our skin.

•These sensors send an electro-chemical signal along many interconnected nerve cells (peripheral nervous system) to the hypothalamus (central nervous system) in the brain. Here, the effect is coordinated and transmitted via the peripheral nervous system.

Role of the Nervous System

• Ability of nervous system both to stimulate and inhibit means that the responses to changes that disrupt homeostasis can be finely and rapidly controlled.

In brief… Homeostasis is..

• .. the condition of a relatively stable internal environment.

• .. essential for cell functioning• .. can be disrupted by agents such

as disease and trauma.• Most body systems play various

roles in homeostasis.

ThE ENd