1. LEVELS OF B

IOLOGICAL

ORGANISATION

A N I NT R O D U C T I O

N TO H

O M E O S T A S I S

2. HOW HOMEOSTASIS

WORKS

T H E RO L E O

F FE E D B A C K S

Y S T E M S

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 SYSTEMSMay be negative or positive

Discuss how these devices work:A toilet cistern when you flushA 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

temperatureTemp 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 CDrop 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