The document discusses homeostasis and how organisms maintain stable internal conditions. It describes three main points: 1. Homeostasis involves keeping internal environments like water levels, blood sugar, and temperature in balance. It occurs through stimulus-response mechanisms. 2. Stimuli are detected by receptors and signals are sent to coordinating centers which trigger effectors to respond. Responses are coordinated through the nervous and endocrine systems. 3. The nervous system provides fast control via electrochemical signaling along neurons. The endocrine system provides slower control through chemical messengers called hormones transported in the bloodstream.

2.  Maintenance of a relatively stable internal
environment (keeping everything in balance
or at equilibrium)
 Minor fluctuations around an optimum level
 Includes:
 Water balance
 Blood sugar levels
 Body temperature
 And regulation of many other substances

3.  Homeostasis happens via a series
of steps known as the stimulus
response model.
 Organisms must first detect a
stimulus before they can respond.
 This is the job of receptors.
 Receptors are specialised areas
that respond to certain stimuli
like light, heat sound or pain.
 The message is then sent to an
organ, gland or muscle that acts
as an effector.
 The effector then causes a
response

5.  Stimuli in the external
and internal
environments are
detected by receptors.
Electrical impulses travel
to the CNS where
coordination takes
place. Impulses are sent
to effectors that carry
out a response.

6.  Hormones are released
directly into the
bloodstream and
transported
throughout the body.
Target tissues involved
in regulating cell
activities respond.

7.  Oxygen from the air
diffuses from the lungs into
the capillaries and is carried
to all cells.
 Carbon dioxide from the
cells is carried tothe lungs
in the blood and diffuses
into thealveoli from where
it is exhaled.
 Removal of CO2 helps
regulate pH.

8.  Blood distributes
warmth, hormones, O2,
nutrients including
glucose, fatty acids and
amino acids to cells and
removes wastes including
CO2.

9.  The products of
digestion – simple
molecules – are
absorbed into the blood
and lymph vessels in the
wall of the intestine
from where they are
supplied to other parts
of the body.
 Undigested material is
eliminated.

10.  Nitrogenous wastes
(urea), excess water
and salts are
excreted from the
body in urine.
 Water balance
(osmoregulation) is
regulated in this
way.

11.  Evaporation of sweat from
the surface helps regulate
temperature.
 The skin is a barrier between
the internal and external
environments and reduces
the chance of entry of micro-
organisms.

13.  Feedback is a process whereby some
proportion of the output signal of a system is
passed (fed back) to the input.
 This is often used to control the dynamic
behaviour of the system.

14.  Positive feedback is a feedback system in
which the system responds to an alteration of a
body system/s in the same direction as the
alteration.
 In contrast, a system that responds to an
alteration of a body system/s in the opposite
direction is called a negative feedback system.
 The term "positive" means responding in the
same direction as the original alteration
whereas "negative" means responding to the
opposite direction.

15.  Characterised by a stimulus (condition)
 Stimulus is detected by a receptor
 This message is sent to a coordinating centre in the
body
 A message is sent from there to an effector (muscle
or gland), which carries out a response

16. Internal body Detected by
temperature rises thermoreceptors in skin
Hypothalamus
compares
against optimum
Increased blood flow to
skin, sweat glands,
Adrenal gland activity
slows

18.  Nervous System
 Rapid response
 Electrochemical messages
 Endocrine System
 Slow response
 Messages transmitted via the blood stream
 Messengers may be peptide hormones or steroid
hormones

19. Fast Control

20. Nelson p 250

21. Nerve impulse travel along defined
pathways
Sensory neurones (PNS)
receptors
Interneurones (CNS)
brain and
spinal cord
Motor neurones (PNS)
Nelson p 237

22.  Motor Neuron:
 Transmit impulses from the central
nervous system to:
▪ Muscles
▪ Glands
 Most motor neurons are stimulated by
interneurons, although some are
stimulated directly by sensory neurons.
 Sensory Neuron:
 Respond to different stimulus receptors:
▪ Touch Smell
▪ Taste Sight
▪ Sound
 Interneuron:
 Found exclusively within the spinal cord
and brain
 Stimulated by signals from:
▪ Sensory neurons
▪ Other interneurons

23. Martin & Kinnear p 302

24.  Receptors are nerve cells capable of detecting
changes in the environment
 Chemoreceptors – detect smell , taste, oxygen
levels, glucose levels, etc
 Mechanoreceptors – detect pressure, touch , sound
vibrations, position and posture of body
 Photoreceptors – detect light eg. retinal cells or
body surface cells in invertebrates
 Thermoreceptors – detect heat and cold, located in
skin and hypothalamus of brain
 Pain receptors – are free nerve ending in skin

25. •Resting nerve cell is
negatively charged
•Excitation due to a
stimulus results in
an electrical impulse
• Impulse travels
along the nerve in a
‘Mexican Wave’
•Myelinated nerve
cells transmit nerve
impulses more
rapidly Grivas & Carter p 246

26. Nelson p 244

27.  Nerve impulse travels along axon to the synaptic knob of the
neurone
 This causes the movement of synaptic vesicles to the end of the
knob
 There neurotransmitters are released from the vesicle into the
synaptic gap
 Neurotransmitters pass across the synaptic gap and bind briefly
to the neurotransmitter receptors on the next neuron
 This then stimulates an action potential in the next neuron

29.  Automatic reflex response
 Quick Response - as the impulse does not go
all the way to brain, just to the spinal cord
 Important survival mechanism –protection

30.  Synaptic gaps occur between sensory
neurones, between interneurones and
between motor neurones
 Synapses allow a variety of interconnections
between nerve cells to be made
 Synapses ensure that nerve impulses are
transmitted only in one direction

31. Slow but sustained control of the body

32. Martin & Kinnear p 308

33.  Chemical substances that are released directly into blood
stream
 Target & ‘excite’ particular cells and organs to respond in
some way
 Target tissue may be a long way from the gland that secretes
the hormone
 Only minute quantities of hormones are produced
 Pituitary gland – master gland - produces many hormones
that affect hormone production by other glands

34. Nelson p 252