Download Notes Homeostasis Stimulus Response Model

VCE Biology Unit
Homeostasis
Before the Program
1.
Briefly explain why you think your body responds in specific ways to different external
conditions, such as sweating in the heat or shivering in the cold.
2.
In biological terms, briefly explain why you think it is important for many animals and plants
to maintain a fairly constant internal environment.
3.
List as many examples as you can of the internal physical and chemical conditions that you
think humans would need to keep fairly constant in order to remain healthy.
During the Program
1.
What is the origin and literal meaning for the word homeostasis?
2. Complete the sentence:
In biological terms, homeostasis refers to the ability of organisms to
despite variations in
external conditions.
3.
What is the constant level of an organism’s internal environment known as?
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VCE Biology Unit
4. Fill in the blanks:
Homeostasis mechanisms help to maintain within narrow limits such things as:

Blood
and

/

and
balance
concentrations

Blood

Body
5.
What are some of the body systems maintained by homeostasis?
6.
What is the biochemical trigger for breathing in animals such as humans?
7.
What do enzymes do for chemical reactions within the body?
8.
What can changes in internal conditions (such as pH) do to enzymes?
9.
What is the collective name for the parts of an organism that detect external and internal
stimuli?
10. List the five external receptors of the human body.
11. What is the collective name for the parts of an organism that carry out homeostatic responses?
12. Complete the sentence:
The two types of pathways for homeostatic responses are n_______________ and h _____________
13. Complete the sentence:
The
is the control centre for homeostasis and is linked to
the endocrine system via the
gland.
14. What is the term that relates to a homeostatic response that ultimately negates (reverses) a
change in internal or external conditions?
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VCE Biology Unit
Homeostasis
DEFINING HOMEOSTASIS
[Gk. homos, same or similar + stasis, standing]

Homeostasis is term used to describe: The condition of a relatively
s__________ internal environment which is maintained within a narrow
range. (tolerance limits)

The internal environment is the fluid which bathes the cells.

The processes and activities that help to maintain homeostasis are referred to as
homeostatic mechanisms

Homeostasis is maintained despite fluctuations in the external environment.
Homeostasis is achieved by the combined effect of mechanisms including the
following:
 Structural: Specialized structures and organs that help an organism to survive in
its particular environment. E.g._______________________________________
 Functional : Metabolism of an organism is able to detect and adjust to
environmental changes. E.g.
__________________________________________
 Behavioural: Organism alter their behaviour to help them survive in their
particular environment. E.g.
________________________________________________
If one of these mechanisms fails death may result.
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VCE Biology Unit
Body systems and their contribution to homeostasis
Body system
Nervous
Main components
CNS
PNS
Homeostatic function
To transmit information about
the external and internal
environments and to effect
appropriate responses
To send chemical messengers
around the body in order to
regulate body function
Removal of Nitrogenous waste
Regulates pH of blood
Regulates blood salt
concentrations
To absorb oxygen from the
external environment, and
remove carbon dioxide
To transport cellular
requirements and chemicals to
cells and to transport wastes
from cells to appropriate organs
for removal. To distribute heat
throughout the body
Endocrine(hormonal)
Ductless glands
Excretory
Kidneys, ureters, bladder,
urethra
Respiratory
Trachea, bronchi, Lungs
(alveoli)
Cardiovasular
(Circulatory)
Heart, arteries, capillaries,
veins
Digestive system
Alimentary canal
(oesophagus, stomach,
small intestine, colon)
pancreas and liver.
To break food down into
molecules small enough to be
absorbed. To facilitate
absorption of these molecules.
Integumentary
Skin and connective tissue
Reduces entry of pathogens
Role in thermoregulation
Sensory role

In order for homeostasis to be maintained organisms need to be able to
d____________ changes in their internal and external environment and
respond to them in a coordinated way.

Changes are detected by r__________________ inside and on the surface of
the organism

In animals, the ___________________ and
__________________________are responsible for coordinating the
mechanisms of homeostasis.

Both nervous and hormonal control systems involve a
_______________________ _____________________ ___________________
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VCE Biology Unit
Scenario one: Identify the various parts of the stimulus response model for the
following scenario:
You are running around Albert Park lake and your hard working muscle cells are using
up lots of energy. This means that carbon dioxide is being produced and its levels are
starting to increase in the blood. This increase is detected by chemoreceptors in your
aorta and carotid artery and a message is sent to the brain. The brain interprets the
information and sends a message via nerves to your diaphragm and intercostal
muscles which will increase your breathing rate…..
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VCE Biology Unit
Feedback mechanisms:

Feedback is when the response causes a change in the original stimulus.

Feedback can be negative feedback or positive feedback
Most control mechanisms in the body involve negative feedback
Negative feedback:
Negative feedback is process in which a change in a variable/factor is detected
which causes a response to occur in the body that reverses the direction of the
change i.e. The response to the stimulus opposes or eliminates the stimulus.
e.g. An increase in temperature is the change detected. The body puts
mechanisms in place to cause a fall in temperature.
e.g. Blood glucose level falls and the change is detected. The body puts
mechanisms in place to cause an increase in blood glucose levels to within
acceptable limits.
e.g. Blood pressure increases and mechanisms are put in place to reduce blood
pressure to within acceptable limits.
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VCE Biology Unit
Positive feedback (“Vicious Circle”):




Less common than negative feedback.
Not used by regulatory systems to maintain homeostasis.
Important in controlling a physiological response that must be completed quickly
When the detection of the stimulus leads to a response that exaggerates or
enhances the original stimulus.
E.g. Labour: Initial stimulus is the distortion of the uterus by the growing
fetus. This causes the release of hormones that lead for further distortion of
the uterus as contractions begin and heighten
E.g. Suckling of a baby causes the mother’s body to release a hormone,
which further stimulates milk production thus increasing suckling of the baby
E.g. Movement of Na+ into the neuron leads to voltage regulated gates
opening and an influx of even more Na+
 Positive feedback loops are stopped when the accelerated physiological
response is complete.
Control of Blood Pressure: Negative feedback under nervous control
Scenario Two: Increase in blood pressure
Fill in the stimulus response model for the following scenario
Pressure receptors in the wall of blood vessels detect and increase in blood
pressure. Sensory neurons send a message to the CNS. The brain interprets the
information and sends a message via motor neurons to the smooth muscle in the
wall of arteries. This result in the dilation of the blood vessels and a reduction in the
blood pressure.
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VCE Biology Unit
Scenario Three: Multiple Effectors
While ice fishing in Alaska you fall through the hole you have
been sitting near into the ice cold water. Luckily you are still
hanging onto your fishing pole and you get hauled out by your
friend. Thermoreceptors in your hypothalamus detect a fall in
core temperature. You are still alive but very pale and
shivering. Explain this in terms of the stimulus response
model.
Stimulus
Receptor
Feedback
…..THIS IS NEGATIVE
FEEDBACK
Transmission of
message by_______________
Control Centre
Effector
Response
Transmission of
message by_______________
Effector
Response
Effector
Response
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VCE Biology Unit
Scenario Four: Drinking a glass of water
An increase in water in your blood causes a decreased concentration of salts in your
blood. Osmoreceptors in the hypothalamus of the brain detect the decrease in salt
concentration and send nerve impulses to the posterior pituitary gland, decreasing
the amount of ADH (Anti diuretic hormone) released into the blood. This results in a
decreased reabsorption of water from the kidney collecting tubules back into the
body, which increases the salt concentration of the blood and produces large
amounts of dilute urine. As blood salt concentration in the blood increases, this is
again detected by the hypothalamus, which reduces its stimulation of the posterior
pituitary gland and so the levels of ADH in the blood increase.
Draw the homeostatic pathway for increased water intake in the body. On your
pathway indicate what the messengers are and whether positive/negative feedback
occurs.
Receptor
Stimulus
Transmission of
message by_______________
Feedback
Effector
Response
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VCE Biology Unit
Scenario Five Eating a lolly
An increase in glucose concentration in your blood is detected by
chemoreceptors on the pancreas. This causes the β cells in the
pancreas to release more insulin, a hormone, which sends a
message to the liver to take up more glucose from the body and
store it as glycogen.
Draw the homeostatic pathway the above scenario. On your
pathway indicate what the messengers are and whether
positive/negative feedback occurs.
Receptor
Stimulus
Transmission of
message by_______________
Feedback
Effector
Response
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