Transcript Homeostasis - NCEA Level 3 Biology

Homeostasis
Homeostasis
• The term is derived from the Greek word
meaning ‘to stay the same’
• It is used to describe the remarkable ability
of organisms to maintain relatively
constant internal conditions in the face of
very different and often changeable
external conditions.
Homeostasis
• In everyday life, an organism must
regulate respiratory gases, maintain fluid
and salt balance, regulate nutrient supply
and energy, maintain body temperature,
protect itself against disease and repair
damaged tissues.
• All these responses must be coordinated.
Principles of Homeostasis
• Organisms need to maintain a cellular
environment that is optimal for cell
function.
• Homeostatic control systems have 3
functional components:
– A receptor – to detect change
– A control center
– An effector – to direct the appropriate
response
Components of a Homeostatic
System
• For homeostasis the body must be able to
detect changes in the environment and
bring about a response.
stimulus
response
Receptor
Effector
Communication
System
The Communication System
• There are 2 kinds of communication
system:
– Nervous
– Endocrine
The Communication System
Nervous System
Route Travelled
Specific pathways
(nerve axons)
Endocrine System
Via blood
Seconds
Time to reach target Milliseconds
cell
Only those at the end Entire body (only
Cells reached
of axons
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
Principles of Homeostasis
• There are 2 feedback systems in
homeostasis:
– Negative Feedback Mechanisms
• More common. A movement away from the
steady state triggers a mechanism to
counteract the change.
– Positive Feedback Mechanisms
• Rare. Leads to a response escalating in the
same direction. Used in labour, lactation,
fever and blood clotting.
Negative Feedback Mechanisms
• When a receptor detects a change from
the set point, this is communicated to
effectors, which respond by bringing
about changes tending to correct the
disturbance.
• The greater the disturbance from normal,
the greater the tendency to correct it.
• This is negative feedback.
Negative Feedback Mechanisms
• E.g. the maintenance of O2 and CO2 levels
in the tissue fluid during exercise.
• NB the more the blood CO2 concentration
rises above the normal, the more strongly
the breathing muscles are stimulated to
respond, thus the greater the rate of CO2
excretion.
Feedback Control of Breathing
During Exercise
Increased
rate of CO2
excretion
Normal
blood CO2
(set point)
Increased
breathing
Rise in CO2
in blood
Increased
muscular activity
Negative Feedback Mechanisms
• NB homeostatic systems cannot prevent
any change in the internal environment.
• What happens is that conditions fluctuate
above and below the set point.
• This is because there must be a delay
between the detection of a change and the
response that corrects it.
• The longer the delay, the greater the
departure from the set point and the
stronger the corrective response, leading
to “overshoot”.
Over-correction in feedback control
brought about by delayed response
Positive Feedback Mechanisms
• Positive feedback is described as a selfamplifying cycle in which one change,
leads to even more significant changes in
the same direction.
• Positive feedback is not a corrective
mechanism activated by the body when it
strays from the set point, but rather, it is a
way for the body to produce significant
changes in a relatively short period of
time.
Positive Feedback Mechanisms
• E.g. a woman in labor.
• The baby's head pushes on the cervix,
sending signals to the brain, which then
produces oxytocin, which stimulates
contractions, which then push the baby's
head even more tightly against the cervix,
and the loop then repeats and becomes
more and more intense until the baby has
been born.
Positive Feedback Mechanisms
Summary of Key Ideas
• Living organisms have the ability to keep
their internal conditions different from their
external environment.
• The regulation of conditions is called
homeostasis.
• In more complex organisms cells are
bathed in a liquid that forms an internal
environment.
Summary of Key Ideas
• In animals with closed blood systems (e.g.
vertebrates) the internal environment is
the tissue fluid, which is separated from
the blood by the capillary walls.
• Tissue fluid and blood are collectively
called extracellular fluid.
• Disturbances in the internal environment
are detected by receptors, and corrections
are made by effectors.
Summary of Key Ideas
• In most animals there are 2 communication
systems linking the receptors and effectors:
nervous and endocrine.
• Homeostasis involves negative feedback,
in which the greater the disturbance from
the set point the greater the tendency to
correct it.
• Positive feedback is where one change,
leads to even more significant changes in
the same direction.