Transcript 8.Homeostatic Mechanisms

12 Biology
Chapter 5&6- Homeostasis &
regulatory mechanisms
Homeostasis is the maintenance of the internal
environment in a relatively stable state in the
face of changes in either the external or internal
environment.
Organisms only survive, grow and reproduce
when their external environment provides
adequate levels of nutrients, water, oxygen, carbon
dioxide and suitable physical conditions such as
light and temperature.
These requirements usually must stay within
narrow tolerance limits for an organism to
function efficiently.
Can you think of an example of an organism exceeding a
tolerance limit in some way?
Organisms must also regulate their internal
environment in the face of internal & external
factors which may occur according to their
activities.
Can you think of an example of an organism doing this?
.
• Homeostasis occurs in
ALL living organisms,
Uni-cellular & Multicellular.
Uni-cellular organism
don’t have an internal
environment. They are
cells directly placed in
fluids of their external
environment, their cell
membrane regulates their
cytoplasm.
An advantage for multi-cellular organisms is that their
cells are protected from the organisms external
environment by the extracellular fluid.
This internal environment allows conditions inside the
organism to be maintained for efficient cell
functioning.
What is the environment external to a cell?
How are cells protected from this surrounding
fluid?
External environment
The medium surrounding
an organism
Internal environment
The extracellular fluid: is
the fluid that surrounds
cells in multi-cellular
organisms.
For optimal functioning, cells regulate:
Concentration of particular salts
Temperature
Nutrient levels
Waste levels
PH
• Tight regulation of extracellular fluid & a
stable internal environment is vital for
optimal cellular function in multi-cellular
organisms.
Examples of processes used to stabilise the
internal environment include:
 Lungs & exchange of carbon dioxide &
oxygen
 Animal circulatory systems
 Removal of wastes
 Root absorption of water & minerals
• Vertebrates have many complex
homeostatic mechanisms which are
ultimately controlled by the hormonal
& nervous systems.
• Can you think of another system which
is vital for delivering the messages of
the nervous and hormonal system?
The stimulus response model
• LINKhttp://www.phys.unsw.edu.au/biosnippet
s/
• Changes or stimuli are detected by
receptors. The stimulus must reach a
threshold of intensity of the specific
receptor, before receptors can
stimulate effectors to produce a
response.
• These act to restore the variable to it’s
original state.
• The response produced reduced the effect
of the original stimulus, therefore provides
negative feedback to that stimulus.
• Eg. A rise in body temp=physiological
changes and behavioral responses to restore
temp to original level (shivering & putting
on clothing)
Misalignment detectors:
These detectors detect when a particular
variable is “out of line” or out of its
optimal range. Eg. Oxygen content in
blood
Disturbance detectors:
These detectors warn of problems before
they arise. They detect changes that are
likely to to cause change in a variable.
Homeostasis is achieved by three
important mechanisms:
STRUCTURAL – the organism has
particular physical features to maintain
homeostasis.
FUNCTIONAL – the metabolism of the
organism is able to adjust to changes.
BEHAVIOURAL – the actions of the
organism individually or with others help
the organism to maintain homeostasis.
Stimuli to response
stimulus
• Stimuli are
environmental factors
that organisms can
detect and to which
they can respond.
• Stimuli are detected by
means of specialised
effectors to produce a
response.
response
receptor
nerves
Control centre
Hormones
nerves
Effector
Types of signals
Physical Stimuli
• Light
• Heat
• Touch/mechanical
Chemical Stimuli
• Nutrient moleculesglucose
• Hormones
• Neurotransmitters
• Pheremones
Homeostatic Mechanisms
The most complex organisms to regulate their
internal environment are the mammals and
birds.
The mechanism used by these organisms is
called ‘The Stimulus-Response mechanism’
The 3 main types are:
1. Simple Stimulus-response
2. Negative Feedback systems
3. Positive Feedback systems
Stimulus-Response
• The general pattern of a stimulus–
response mechanism is the withdrawal
reflex.
Negative Feedback System
• Negative feedback systems are stimulus–
response mechanisms that act to restore
the original state. The response produced
reduces the effect of the original stimulus;
that is, the response provides feedback that
has a negative effect on the stimulus.
Feedback System
Some systems controlled by homeostasis
Control of
Requires regulation of
nutrient levels
(e.g. glucose)
•nutrient intake
•digestive and circulatory system functions
•storage and mobilisation of nutrients
•behaviour
body temperature
•general metabolism
•blood flow to tissues
•muscle activity and sweating
•behaviour
water and salt
balance
•excretion of water and salts to maintain correct
osmotic concentration of internal body fluids
•behaviour
metabolic rate
•lung ventilation and circulation to deliver adequate
oxygen to tissues
•nutrient intake and storage
•behaviour
Regulating responses to stimuli
 To coordinate all the
different activities a
multicellular organism
will integrate and
coordinate the activities
of their cells.
 There are two major
systems for this:
1. ENDOCRINE SYSTEM
(hormones) and
2. NERVOUS SYSTEM
(nerves).
FEEDBACK LOOPS
(*write onto a diagram)
This involves a six step process:
 Stimulus : a change from ideal conditions
Receptor : the cells or tissue that detects the change
Transmission : method by which the message is carried
Effector : a gland or muscle which causes the response to
happen
Response : an action that occurs due to the effect of the
response
Feedback : the consequence of the response on the stimulus
FEEDBACK LOOP: BLOOD
PRESSURE
RECEPTORS
in
the muscles of blood
vessels note the
change
STIMULUS
Blood pressure
falls
TRANSMISSION
message
sent to
brain
FEEDBACK
Blood vessels
Constrict and blood pressure increases
EFFECTOR
RESPONSE
Heart rate increases
Brain sends message
to heart & blood
vessels
Controlling Blood Pressure
MAINTENANCE OF BODY
TEMPERATURE
The ability to control body temperature is
extremely important if animals are to
survive.
*(Recall from Area of Study 1 that enzymes have optimal
temperatures for their activity.)
If the enzyme, or the cell it is in, is too cold,
collisions between enzyme and substrate
occur infrequently, and metabolic processes
slow to rates which may no longer support
life.
FEEDBACK LOOP: KEEPING WARM
RECEPTORS
in
the hypothalamus detect
change
STIMULUS
Body temp
falls
TRANSMISSION
Skin muscles
erect hairs
FEEDBACK
Pituitary gland secretes
thyroxine
EFFECTOR
RESPONSE
Skeletal muscles
shiver
Blood vessels in skin
constrict
FEEDBACK LOOP: COOLING DOWN
RECEPTORS
in
the hypothalamus detect
change
STIMULUS
Body temp
RISES
TRANSMISSION
Blood vessels
in skin
dilate
FEEDBACK
Muscle contractions
are reduced
EFFECTOR
RESPONSE
Pituitary gland secretes
Less thyroxine
Sweat glands release
sweat
Regulating blood glucose
Negative feedback by hormones
 The control of blood
glucose levels involves
two hormones:
1. insulin, which controls
the upper limit and
2. glucagon which controls
the lower limit.
The normal range is 3.6 to
6.8 mmol/L
Insulin and Glucagon
• The hormone Insulin controls the uptake
by cells of glucose from the blood. Beta
cells in the pancreas control insulin levels.
• The hormone Glucagon acts on the liver to
release more glucose into the blood.
*do on board
FEEDBACK LOOP: Fall of blood glucose
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•
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Blood sugar falls
Receptors in the pancreas detect change
Alpha cells release glucagon
Beta cells suppress insulin production
Liver converts glycogen to glucose
Muscle glycogen converted to glucose
Fat tissue broken down for energy
Blood sugar level rises
*do on board
FEEDBACK LOOP: Blood sugar level rises
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Blood sugar rises
Receptors in the pancreas detect change
Beta cells increase insulin production
Alpha cells decrease glucagon production
Fat tissue increases conversion of glucose to fat
Skeletal muscle increases uptake of glucose
Liver synthesises glycogen
Blood sugar level falls
Diabetes – high blood glucose
High blood glucose
Caused by eating
Lower blood glucose
Pancreas
(beta cells detect then
secretes more insulin)
Liver increases glycogen synthesis
Removing glucose from blood
Diabetes – low blood glucose
Blood glucose
drops (due to
exercise, starving)
Blood glucose
level rises.
Beta cells detect glucose level
Alpha cells in pancreas
produce glucagon
Liver breaks down
stored glycogen and
glucose released into
bloodstream
Other examples of Regulation
Blood CO2 levels
• Respiration is controlled involuntarily & is under the control of
the respiration centre in the medulla.
• The level of carbon dioxide (CO2) in the blood is one of the main
stimuli that can alter the rate of respiration.
• CO2 also binds with hydrogen, making blood more acidic
(lowered pH).
• Exercise is an example of when blood CO2 would increase. When
exercising, your muscles require more energy, meaning your
tissues consume more oxygen than when at rest.
• More oxygen consumption=more carbon dioxide production.
• If blood CO2 increases, chemoreceptors become stimulated which
sends signals to the breathing centre, which sends nerve impulses
back to the muscles of breathing (diaphragm & intercostals),
causing them to relax quicker  increasing breathing rate.
• This aims to increase oxygen in the blood and reduce CO2=
homeostasis.
Summary
• Negative feedback systems produce stability.
They are stimulus– response mechanisms in
which the response produced reduces the effect
of the original stimulus.
• Disturbance detectors respond to changes in the
environment that are likely to cause a change in
the precise factor of the internal environment
that is being controlled.
• Misalignment detectors monitor the precise
factor of the internal environment that is being
controlled.
(read pages 113-119 for other systems)
Summary
• Homeostasis is the maintenance of a
constant internal state despite
changes in the external environment.
• There are two major systems
involved in homeostasis: nerves and
hormones
Regulatory pathways
In both hormonal and nervous
systems signals are passed
from one cell to the next by
chemical communication.
 Hormones and
 Neurotransmitters
SIGNAL TRANSDUCTION
• Signal transduction refers to any process by
which a cell converts one kind of signal or
stimulus into another.
• This often involves an ordered sequence of
biochemical reactions inside the cell, that are
carried out by enzymes and linked through
second messengers resulting in what is thought
of as a "second messenger pathway".
• Such processes are usually rapid.
Signal Transduction
• Signal transduction refers to the
ways that receptors convert
incoming signals into information
that leads to an appropriately
coordinated response.
• Receptors- convert incoming signals
into messages usually carried by
nerves or hormones.
• In many signal transduction
processes, the number of proteins
and other molecules participating in
these events increases as the
process eminates from the initial
stimulus, resulting in a "signal
cascade" and often results in a
relatively small stimulus eliciting a
large response.
Cell signalling
The signalling and response process is called the signal
transduction pathway and often involves many enzymes and
molecules in a signal cascade which causes a response in the
target cell.
These pathways are categorised on the distance the signal travels
to reach its target cell.
1. Endocrine signalling – carried long distances by circulatory
system
2. Paracrine signalling – released in immediate vicinity of target
cells.
3. Autocrine signalling – cells produce and react to their own
signals.
4. Pheromone signalling- chemical signals between members of
the same species that travel through the external environment.
Signal Transduction pathway
Applying the stimulus response model to the
cell.
1. The signal binds to receptor molecule.
2. Receptor molecule changes shape or
confirmation.
3. Initiates a molecular cascade of secondary
messenger molecules to finally an effector
molecule.
4. Effector molecule initiates the cellular
response.
How do we detect changes in the
environment?
 Homeostasis and regulation are carried out by hormonal and nervous
system. In animals responses are based on sensory information received
from all parts of the body. Individual effector organs involve muscle and
glandular tissue.
 Our bodies have a number of different receptors that detect change:
1. Chemoreceptors – CO2 levels and hence pH
2. Mechanoreceptors – pressure/touch
3. Photoreceptors - light
4. Thermoreceptors – temperature
5. Baroreceptors – blood pressure
6. Proprioreceptors – stretch and tension
7. Olfactory receptors – detect airborne and dissolved chemicals
Types of Receptors
• Chemoreceptors - detect chemicals
– Olfactory lining in nose; taste buds; oxygen concentration
receptor in aorta; osmoreceptors in hypothalamus; glucose level
receptors in pancreas; pH/CO2 receptors in medulla, aorta and
carotid arteries
• Mechanoreceptors – detect pressure and movement
– Ear; touch & pressure receptors in skin muscles, joints and
connective tissue; muscle length receptors in skeletal muscle;
muscle tension receptors in tendons; joint receptors; venous
pressure receptors; arterial pressure receptors; lung inflation
receptors; lung deflation receptors; lung irritant receptors.
• Photoreceptors – detect light
– Eye.
• Thermoreceptors - detect temperature
– Heat receptors and cold receptors in skin; body temperature
receptor in hypothalamus.
Feedback systems
• Homeostasis or regulation therefore involves
fluctuations around a set-point.
• The size of the fluctuations depends on the
sensitivity and location of the sensory receptors,
the tolerance of the control centre to variation
from the set-point, and efficiency of the
response mechanism.
• Most biological feedback systems are negative
feedback systems which operate as proportional
control systems – the size of the response is
proportional to the size of the stimulus.
Quiz
• Define Homeostasis
• What are the two major systems involved in
homeostasis?
• What are the characteristics of hormones?
• What is the difference between Protein and
Steroid hormones?
Answers
• Homeostasis is the maintenance of a constant
internal state despite changes in the external
environment.
• There are two major systems involved in
homeostasis nerves and hormones
• Hormones are proteins that are released in glands,
they are specific, slow working substances.
• Protein hormones are made of amino acid
(polypeptides) and Steroids are fatty acid based
(cholesterol)
Nervous and Endocrine Systems
• The nervous and endocrine systems work together to
coordinate the actions of all other systems of the body
to produce behavior and maintain homeostasis.
• The endocrine system produces chemical messengers
that are transported through the circulatory system. It
requires seconds, minutes or hours.
• The nervous system is more rapid, requiring only
thousandths of a second.
• In general, the endocrine system is in charge of body
processes that happen slowly, such as cell growth.
• Faster processes like breathing and body movement are
monitored by the nervous system.
Endocrine System
(hormonal)
Hormones:
 are chemical messengers released in response to a stimulus
detected by a receptor.
 are substances produced by one cell that have an effect on
another. Only effect target cells with specific receptors for the
hormone.
 can regulate the growth or activity of specific cells.
 can transmit their signal by altering specific biochemical reactions
in cells.
 are released from glands directly into the blood stream. (no ducts)
 are slow working, BUT long lasting.
 take time to reach the target area, but once there the response
can be fast or slow.
Two Main Types of Hormones
 PROTEIN hormones – made of chains of amino acids
(polypeptide chains) which are too big to pass through the
cell membrane. They must bind to the membrane using
specific receptor sites triggering metabolic response.
 STEROID (fatty acid based) hormones are small and lipid
soluble, they can pass through the cell membrane and enter
the cytoplasm. The hormone binds with receptor molecules in
the cytoplasm which then diffuse into the nucleus. Once in
the nucleus they then affect gene expression on the
chromosomes (DNA). This initiates enzyme synthesis and a
biochemical pathway.
Protein Hormones
 Adrenaline,
ADH,
thyroxine
and growth
hormones.
Steroid hormones
 Testosterone,
oestrogen,
progesterone
and
corticosteroids
Endocrine/hormonal Organs
Endocrine System
• The foundations of the endocrine system are the hormones and
glands.
• As the body's chemical messengers, hormones transfer information
and instructions from one set of cells to another.
• Although many different hormones circulate throughout the
bloodstream, each one affects only the cells that are genetically
programmed to receive and respond to its message.
• A gland is a group of cells that produces and secretes, or gives off,
chemicals. It selects and removes materials from the blood,
processes them, and secretes the finished chemical product for use
somewhere in the body.
• Some types of glands release their secretions in specific areas. For
instance, exocrine glands, such as the sweat and salivary glands,
release secretions in the skin or inside of the mouth.
• Endocrine glands, on the other hand, release more than 20 major
hormones directly into the bloodstream where they can be
transported to cells in other parts of the body.
Endocrine System and Negative
Feedback
• When hormone levels reach a certain normal or necessary amount,
further secretion is controlled by important body mechanisms to
maintain that level of hormone in the blood.
• Regulation of hormone secretion may involve the hormone itself or
another substance in the blood related to the hormone.
• For example, if the thyroid gland has secreted adequate amounts of
thyroid hormones into the blood, the pituitary gland senses the
normal levels of thyroid hormone in the bloodstream and adjusts its
release of thyrotropin, the pituitary hormone that stimulates the
thyroid gland to produce thyroid hormones.
• Another example is parathyroid hormone, which increases the level
of calcium in the blood. When the blood calcium level rises, the
parathyroid glands sense the change and decrease their secretion of
parathyroid hormone.
This turnoff process is called a negative feedback
system.
Glands, Hormones & Action
Gland
Hormone
Action
Thyroid
Thyroxine
Stimulates metabolic
process
Ovaries
Oestrogen
Maintenance of female sex
characteristics
Testes
Androgens
Maintenance of male sex
characteristics
Pituitary
Anti diuretic hormone (ADH)
Promotes water retention
by kidneys
Pituitary
Follicle stimulating Hormone
(FSH)
Stimulates production of
egg & sperm
Pancreas
Insulin
Lowers blood glucose
levels
Pancreas
Glucagon
Raises blood glucose
levels
Hypothalamus
Releasing hormones (RH)
Hormones that stimulate
the pituitary
Nervous System
Peripheral
Somatic
Sensory
Central
Autonomic
Motor
Brain
Sympatheti
c
Generally increases
energy use and
prepares the body
for action by
increasing heart and
metabolic rate.
Neurotransmitter is
usually adrenaline.
Spinal cord
Parasympathetic
Enhances activities that
conserve energy, such as
digestion and slowing
heart rate. Restores
resting state.
Neurotransmitter is
acetylcholine.
Nervous System
Sensory Input
• Receptors are parts of the nervous system that sense changes
in the internal or external environments.
• Sensory input can be in many forms, including pressure, taste,
sound, light, blood pH, or hormone levels, that are converted
to a signal and sent to the brain or spinal cord.
Integration and Output
• In the sensory centers of the brain or in the spinal cord, the
barrage of input is integrated and a response is generated.
• The response, a motor output, is a signal transmitted to
organs than can convert the signal into some form of action,
such as movement, changes in heart rate, release of hormones,
etc.
Sensory neurons
Category of
sensory receptor
Chemoreceptor
Mechanoreceptors
Photoreceptors
Thermoreceptors
Other
What they detect
O2 , CO2, pH, ions, complex
organic molecules (eg:
hormones,
neurotransmitters)
Sound, touch, pressure,
gravity
Light, infrared radiation
Heat, cold
Electric fields, magnetic
fields
Sensory receptors
Signal transmission
There are three basic steps involved in the way
signals are sent in the nervous system :
 generation of an impulse (action potential),
 conduction of the action potential and
 transmission across a synapse.

Neurons
• The neuron is the functional unit of the
nervous system.
• Humans have about 100 billion neurons in
their brain alone!
• Although they vary in size and shape, all
neurons have three parts:
– Dendrites receive information from another
cell and transmit the message to the cell body.
– Cell body contains the nucleus, mitochondria
and other organelles typical of eukaryotic cells.
– Axon conducts messages away from the cell
body. Axon terminals form synapses with
other neurons or with a muscle or a gland.
Relationship between different
types of neurons
The Nerve Message - Electrical
• In a resting nerve (one that is not responding to
stimulus), a small difference exists between the
electrical charge on the inside and outside its cell
membrane. The outside of the cell membrane of
the axon is positive compared with the inside.
• Stimuli of various kinds can activate neurons so
that they transmit nerve impulses along their
axons. Such a nerve cell is said to be ‘excited’.
• Nerve impulses involve changes in the charge
across the axon membranes.
• As the impulse moves along the axon, a change
occurs in the permeability of the membrane so
that positive ions move into the cell.
• This results in the outside of the membrane
becoming negative compared with the inside.
The change in permeability travels along the
neuron.
• After a nerve impulse has been transmitted by a
neuron, the original distribution of ions across
the cell membrane is restored.
Myelin sheath
• Acts as insulator to minimize metabolic
expense while maintaining rapid conduction
of electrical impulse through neurons.
• Sheaths are formed by glial cells:
– oligodendrocytes in the central nervous
system
– Schwann cells in the peripheral nervous
system.
• The myelin sheath in peripheral nerves
normally runs along the axon in sections
about 1 mm long, punctuated by unsheathed
nodes of Ranvier which contain a high density
of voltage-gated ion channels.
• An action potential is conducted more rapidly
along a myelinated axon because it ‘jumps’
from one node to the next.
The Nerve Message - Chemical
• Neurons communicate with one
another via synapses, where the
axon terminal of one cell impinges
upon a dendrite or soma of another
(or less commonly to an axon).
• The human brain has a huge number
of synapses. Estimates vary for an
adult, ranging from 100 to 500
trillion synapses.
• The space between two cells is
known as the synaptic cleft. For
signals to cross the synaptic cleft
neurotransmitters are required.
• The time for neurotransmitter action
is between 0.5 and 1 millisecond.
• Acetylcholine is an example of a
neurotransmitter, as is
norepinephrine, although each acts
in different responses.
The Synapse
How do neurotransmitters work?
• Neurotransmitters are stored in small synaptic vesicles clustered at
the tip of the axon.
• Arrival of the action potential causes some of the vesicles to move
to the end of the axon and discharge their contents into the
synaptic cleft.
• Released neurotransmitters diffuse across the cleft, and bind to
receptors on the other cell's membrane, causing ion channels on
that cell to open and prompting transmission of the message along
that cell’s membrane.
• Some neurotransmitters cause an action potential, others are
inhibitory.
• Once in the cleft, neurotransmitters are active for only a short time.
• Enzymes in the cleft inactivate the neurotransmitters. Inactivated
neurotransmitters are taken back into the axon and recycled.
What is a reflex arc?
• The brain generally coordinates responses to
information from receptors.
• Sometimes however the body cannot wait for
the transmission of information from the spinal
cord to the brain and back from the brain to an
effector. This is where the reflex arc comes in….
• The reflex arc is an automatic, involuntary
reaction to a stimulus.
• Examples of reflex arcs include balance, the
blinking reflex, and the stretch reflex.
Example of a reflex arc
Touching a sharp object
• Receptor detects pain
• Sensory neurons carry signal to spinal cord
• Interneurons in the spinal cord directly stimulate
effectors (e.g. skeletal muscle) using motor neurons
• Spinal interneurons also transmit pain signal to the brain
This is why you drop the object and then a moment later
feel the pain
• Brain then coordinates the rest of your voluntary and
involuntary responses to pain.
Reflex arc
Reflex arc
Reflex arc
Sensory
neurone
Receptors in skin
cells
Grey matter
reflex arc bbc
Relay
neurone
Motor neurone
Effector
(muscle)
Pheromones
Pheromones are:
 released outside the body to stimulate other organisms.
 chemicals released by an animal that acts as a signal to
other animals of the same species; usually sexual
attractants or alarm signals.
 used by different animals for various purposes.
Examples include:
-social organisation and control (e.g., bee colonies);
-scent marking and territoriality (e.g., ring-tail possums
and other mammals);
-alarm signals (e.g., some insects);
-mating signals and inducing mating activity
(mammals).