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Alford Academy
Higher Biology
Unit 3: Control and Regulation
Physiological HomeostasisChapter 33
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Name: ________________ Teacher: ________________
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Physiological Homeostasis
Homeostasis literally means staying the same. Physiological homeostasis is
the maintenance of the bodys internal environment within certain tolerable
limits. The community of cells and the tissue fluid that surrounds them are
collectively known as the internal environment.
The following are examples of factors which are controlled by homeostatic
mechanisms:-
Water concentration (Osmoregulation)
Blood sugar levels (Glucose concentration)
Temperature (Thermoregulation)
1) Why does the core body temperature need to be precisely controlled?
2) Why does the water concentration of the blood need to be precisely
controlled?
3) Why does the sugar level of the blood need to be precisely
controlled?
Negative feedback control
The maintenance of homeostasis is by the method of negative feedback. This
means that a change from the norm/set point in one direction automatically
brings about a change in the opposite direction. In order to bring this about
receptors and effectors are needed.
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Receptors detect a change from the Norm or Set-Point of the bodys internal
environment. These receptors send nervous or hormonal messages, which arereceived by effectors. The effectors respond which counteracts the original
change, returning the system to the set-point.
Osmoregulation
The water content of the blood is detected by osmoreceptors in the
hypothalamus. If the water content of the blood is low the hypothalamus
triggers the release of Antidiuretic Hormone (ADH) by the pituitary gland.
ADH stimulates the kidney tubules to become more permeable and increases
the volume of water reabsorbed into the blood.
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The Kidney
Urinary system Nephron
1. In which part of the nephron does filtration
take place?
2. In which part of the nephron does reabsorption takes place?
3. If you drink a lot of water what happens to the volume and concentration of
the urine you produce afterwards?
4. If you are short of water, perhaps from sweating a lot, what happens to the
volume and concentration of the urine you produce?
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Read Torrance page 279 (old), page 307 (new).
Use the following words to complete the exercise: Hypothalamus, less,
pituitary gland, more, sweating, collecting ducts,
When the water concentration of the blood falls, due to for
example, osmoreceptors in the of the brain are
stimulated.
These osmoreceptors stimulate the to release ADH (anti-
diuretic hormone) into the blood. When ADH reaches the kidney it acts on the
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tubules and , making them permeable to water.
Due to this, water which would normally have passed along the collecting duct to
become urine is instead reabsorbed into the blood, increasing the water
concentration of the blood to the normal level.
This return to the set point is detected by the osmoreceptors and ADH
production is reduced. This in turn reduces the permeability of the kidney tubules
so that water is reabsorbed into the blood; instead it will pass
through the collecting duct and become urine.
TYK page 280 (old) or page 308 (new)
1a) What is meant by the term physiological homeostasis?
bi) Outline the principle of negative feedback control.
bii) Why is such control of advantage to an organism?
2ai) Where in the human body are the osmoreceptors found that respond to a
decrease in water concentration of the blood?
ii) Which part of the body releases an increased concentration of ADH under
these circumstances?
b) By what means does ADH bring about its effect so that water is conserved
by the body?
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3. Compare the relative volume and concentration of urine produced when the
water concentration of the blood is
(i) above the set point
(ii) below the set point
Control of Blood Sugar Level
All respiring cells require a supply of glucose. The nervous system is sensitive to
any reduction in the normal glucose level. A rise in blood glucose level can also be
dangerous.
The supply of glucose varies in mammals because they do not eat continuously and
the quantity of carbohydrate intake changes from meal to meal. There may be
long periods when no glucose is absorbed from the gut into the blood.
Cells require a constant supply of glucose for respiration. A system which
maintains a steady glucose level in the blood despite intermittent supplies from
the gut is required.
If blood sugar levels rise above the set-point, cells in the Pancreas called Islets
of Langerhans detect this change. These cells respond by producing Insulin. This
hormone is transported to the liver in the bloodstream where it activates an
enzyme which catalyses the reaction Glucose Glycogen, this brings the bloodsugar level back to normal.
If blood sugar level drops, different cells in the Islets of Langerhans release
Glucagon. Again this hormone is transported to the liver where it activates a
different enzyme which catalyses the reaction, Glycogen Glucose, bringing theblood sugar level back to normal.
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Glucose is stored in the liver as Glycogen. This store can be removed from or
added to.
Diabetes Mellitus
This is a condition in which some or all of the Insulin secreting cells are non-
functional. This causes a rise in blood sugar level causing sugar to be excreted in
the urine, as there is too much to be reabsorbed. This can cause weight loss and
wasting of tissues, however, it can be controlled by Insulin injections and
controlled diet.
Adrenaline
In an emergency, the body may need additional supplies of glucose to provide
energy for fight or flight. Adrenal Glands, which are positioned above the
kidney, release Adrenaline. Adrenaline then overrides normal homeostatic control
of blood sugar level. Insulin is inhibited and Glycogen Glucose is promoted.Once the emergency is over, secretion of adrenaline is reduced and blood sugar
level returns to normal.
Homeostatic control of blood sugar
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Complete the following diagram using information from the previous pages and
your Torrance textbook.
TYK page 283 (old) or page 311(new)
1. Hormones are chemical messengers released directly into the bloodstream
by endocrine glands.
Complete the table on the next page using one or more answers from
the list below it
Hormone Endocrine gland that Effect of hormone
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Normal glucose concentration in
blood
in blood glucoseafter eating food.
Detected by receptor cells
in the
Pancreas produces ____
insulin and glucagon
Excess glucose stored as
glycogen in the
in bloodglucose after exercise.
Glycogen converted into
__________in the liver
Pancreas produces
insulin and glucagon
Detected by receptor
cells in the
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secretes it
Adrenaline
Insulin
ADH
Glucagon
List
A promotes conversion of excess glucose to glycogen
B increases permeability of kidney collecting ducts
C promotes conversion of glycogen to glucoseD decreases blood sugar level
E prepares the body to cope with an emergency
2a) With reference to concentration of sugar in the bloodstream, state the
circumstances that lead to:
(i) glycogen being converted to glucose
(ii) glucose being converted to glycogen
(3) (i) In the homeostatic control of blood sugar level, which organ is the
receptor and which is the effector?
(ii)Suggest why such a corrective mechanism is described as a form of
negative feedback control.
Now answer AYK Question 2 page 289 (old), page 318(new).
Control of body temperature
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Ectotherms
These are animals that are unable to regulate their body temperature. This means
that their body temperature varies directly with that of the external
environment. Includes all invertebrates, fish, amphibians and reptiles.
Endotherms
These animals are able to maintain their body temperature despite changes in the
external temperature. All birds and mammals are endotherms. They have a high
metabolic rate, which generates heat energy. Regulation of their body
temperature is brought about by homeostatic control.
Homeostatic control in endotherms
The hypothalamus acts as a receptor in regulation, by detecting fluctuations in
temperature. These receptors are better known as thermoreceptors. The skin
also possesses thermoreceptors,
which can detect the temperature of the external environment. This information
is relayed to the hypothalamus.
The hypothalamus transmits nervous impulses to the skin which acts as an
effector. The hypothalamus also sends messages to other effectors which can
alter the metabolic rate.
Thermoregulation
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Role of skin
Correction of overheating
Vasodilation- dilation of arterioles under skin allow more heat to be lost by
radiation.
Increased sweating- brings body temperature down as body heat is used to
evaporate the sweat.
Correction of overcooling
Vasoconstriction-constriction of arterioles under skin means less heat is
lost by radiation.
Decreased sweating-means more body heat is conserved
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Capillaries receive large volume of
overheated blood
Arterioles become dilated
Nerve impulses from
hypothalamusArteriole Venule
Much heat lost by radiation
Arteriole Venule
Capillaries receive small volume
of blood
Arteriole becomes constricted
Nerve impulses from
hypothalamus
Very little heat lost by radiation
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Contraction of hair erector muscles-hairs rise allowing a layer of air to be
trapped between the skin and external environment. Air is a poor conductor
of heat and acts as a good insulator.
Other corrective mechanisms
Include increase in shivering and metabolic rate when the temperature drops. And
a decrease in metabolic rate when the temperature rises.
Extreme conditions
If exposure is prolonged for example to freezing temperatures, negative
feedback breaks down and can result in death.
Now answer TYK questions 1-4 Page 288 (old), page 317 (new)
Now answer AYK question 5 page 289 (old), page 318 (new).
Control of heat by an ectotherm
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1. Describe the changes in the lizard body temperature and in the
environment temperatures as shown by the graph.
2. Explain how posture (A) helps the iguana to warm up.
3. Explain why the iguana takes up the elevated posture (B) when it is warm
enough.
4. Suggest what the iguana could do if its body temperature rose too high.
5.The graph below shows a graph of oxygen consumption and air temperature of an
endotherm and exotherm. Decide which is for the ectotherm and which is
for the endotherm (X or Y) and give reasons for you choice.
X=________________Y = _________________
Explanation: _______________________________________
Physiological Homeostasis
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Activity I can find
this in my
notes
I can
do
this
State the meaning of the term "Homeostasis"
State why each of these must be maintained within
particular levels: glucose content of blood; water content
of blood & cell chemicals; temperature
State what is meant by the term "Negative Feedback"
Briefly describe the mechanism of negative feedback in
general terms using points: changes in body's conditions;
receptor sites; messages sent; effectors; initiation of
correcting mechanism; final outcome
Describe how the negative feedback process operates toreturn the water content of blood to normal when the
body has: a) too much water b) too little water
Describe how the negative feedback process operates to
return the blood sugar levels to normal if the body has: a)
too much glucose b) too little glucose
State when the body must release extra adrenaline
Describe the role of adrenaline in glucose metabolism
Describe how the negative feedback process operates to
return the body temperature to normal when:a) the body
is too hot b) the body is too cold
Define the terms "endotherm" and "ectotherm" and give
examples of each
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