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1. LEVELS
OF
BIOLO
GICAL
ORGANISAT
ION
AN
IN
TR
OD
UC
TI O
N T
O H
OM
EO
ST A
SI S
2. HOW
HOMEOSTA
SIS
WORKS
TH
E R
OL E
OF F
EE
DB
AC
K S
YS
TE
MS
HOMEOSTASIS:
A definition:
the maintenance of a stable internal environment despite fluctuations in the external environment
COMPONENTS OF HOMEOSTASIS
A system requires three components for homeostasis:
- A receptor;
- A control centre;
- An effector.
These components do specific jobs that allow regulation of the internal environment.
FEEDBACK SYSTEMS
May be negative or positive
Discuss how these devices work:
A toilet cistern when you flush
A hot water cylinder when you run the shower
WHAT UPSETS THE BALANCE?
Various factors apply stress on the internal environment:
Internal factors - within the body, eg pain, high or low blood pressure
External factors - outside the body, eg extremes of temperature
NEGATIVE FEEDBACK...
causes the system to respond to reverse the direction of change. ..
which tends to keep things constant…
so it allows the maintenance of homeostasis.
eg, if there is a fall in calcium in the blood, the parathyroid glands sense the decrease and secrete more parathyroid hormone, thereby increasing calcium release from the bones;
NEGATIVE FEEDBACKIN INCUBATOR
temperature
Temp too high
Temp too low
Temperature
sensor
Thermostat
system with
switch
Heater (switched off)
Temperature
sensor
Heater (switched on)
Thermostat
system with
switch
Incubator temperature at 32 C
Rise inTemp fallsi
Temp risesi
NEGATIVE FEEDBACKA FRIDGE
Fall in
Temperature
sensor
Thermostat
system with
switch
cooling (switched off)
Temperature
sensor
cooling (switched on)
Thermostat
system with
switch
Fridge temperature varies to setting C
Drop in temperature
Rise in temperature
Rise in temperature
Drop in temperature
POSITIVE FEEDBACK…
increases the variable in the same direction…
a destabilising effect that does not result in homeostasis.
Positive feedback is used when rapid change is needed.
eg in childbirth the hormone oxytocin is produced to stimulate and enhance labour contractions
eg clotting mechanism when skin is cut