Download Prac - Homeostasis Activity and Negative Feedback

Science – Maintaining Control of Your Body (Homeostasis)
The nervous and endocrine systems work together to coordinate the actions of all other systems
of the body to produce behaviour 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.
Homeostasis is the dynamic balance within an organism, where many variables including
temperature, oxygen levels, water balance, blood pressure and glucose levels are maintained
within a specific range for optimal cellular efficiency. The state of internal and external variables
are monitored by sensory cells in various parts of the body and adjustments are made as
necessary. This is known as a feedback system. Most biological feedback systems are negative
feedback systems which means that the activity of the effector opposes the stimulus. This means
the effector produces the opposite effect of the stimulus.
TASK ONE Summarise the information in the two paragraphs above into a table in the space below
Negative Feedback
To illustrate the components involved in negative feedback, we can use the example of a driver
trying to stay near the speed limit. The desired value of a variable is called the set point. Here, the
set point is a speed of 55mph; in controlling body temperature, the set point would be 37°C. The
control centre is what monitors the variable and compares it with the set point. Here, the control
centre is the driver; for body temperature, it would be the hypothalamus of the brain. If the
variable differs from the set point, the control centre uses effectors to reverse the change. Here,
the effector is the foot on the accelerator pedal; in controlling body temperature, it would include
the glands that sweat and the muscles that shiver.
TASK TWO Summarise the information in the paragraph above into a flow chart in the space below
During exercise our breathing rate increases to bring more oxygen into the body, maintaining
homeostasis by ensuring muscle cells are adequately supplied with oxygen. Circulatory changes
during exercise include an increase in heart rate and cardiac output, and a rise in blood pressure.
Blood is redirected away from the internal organs such as the gut and towards exercising skeletal
muscle and the skin. Our glycogen stores begin to break down to increase blood glucose levels.
We perspire more, and this increased perspiration cools the body and maintains the body
temperature as the heightened metabolic state (due to the exercise) produces heat. The hairs on
our skin lay flat to allow air to pass across the skin increasing evaporation of sweat. The blood
vessels just below the skin dilate to increase blood flow at the surface (vasodilation), making us
appear flushed. This also increases heat loss, and cools the internal environment.
Homeostatic control of temperature after exercise
Aim
i) To investigate the effect of moderate exercise on body temperature, carbon dioxide
output and pulse rate.
ii) To infer how negative feedback mechanisms control temperature.
Materials (x8 sets)
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cobalt chloride papers
(blue)
adhesive tape
x2 100mL conical flasks
6 drinking straws
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rubber band
distilled water
100mL measuring
cylinder
0.4% NaOH
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2 pipettes
stop watch
paper towel
0.5% phenolphthalein
Safety Rules
Be careful when using alkalis and acids. If they spill on your hands, wash your hands with water
immediately.
Wash the conical flasks and drinking straws thoroughly after reading to eliminate experimental
errors.
After the experiment, dispose of the solutions in specified containers. Do not pour them down the
sink unless instructed to by a teacher.
Taking Measurements
Pulse rate: The pulse can be found on the left side of the lower neck, on the inside of the
elbow, or at the wrist. Measure your pulse in the wrist in the radial artery on the thumbside of the wrist. Place your index and middle fingers lightly on your wrist until you feel
your pulse, or heart beat. Do not use your thumb because it has a strong pulse of its own.
Alternatively the carotid pulse on the side of the neck can be taken by placing the first two
fingers on the side of the Adam’s apple, in the groove in front of the muscle running
vertically down the neck. Count the number of pulses in 15 seconds and then multiply by 4
to obtain beats per minute (bpm).
Sweating: Tape a piece of cobalt chloride paper on the forehead of the subject. The paper
is blue when dry but turns pink when wet. Note the time taken for the blue colour to
change to pink.
Breathing rate: This is the number of times air is drawn into the lungs per minute. It can be
measured by placing your finger under the subject’s nose so that you can feel the breath.
Count the number of breaths per minute. Repeat for another minute and record the
average reading. Normal respiration rates for a person at rest range from 15 to 20 breaths
per minute. After exercise, the breathing rate will range from 60 to 80 breaths per minute.
CO2 output: This is measured by titration. CO2 dissolves in water, forming carbonic acid.
Measuring how much NaOH is needed to neutralize this acid provides an indication of CO 2
output. If the amount of NaOH needed increases, then the amount of CO 2 released by the
body has increased. Phenophthalein indicator is colourless in acid and changes to pink in
alkali.
Method
At rest
1. Work in groups of three. One person is the experimenter who takes the readings. The
second person is the subject whose pulse, breathing, sweat and CO2 output are to be
measured. The third person is the recorder who records the results.
2. Ask the subject to sit down quietly. Measure their pulse rate by placing two fingers on the
wrist or at the neck. Measure and record the breathing rate.
3. Place cobalt chloride paper on the subject’s forehead. Note any changes in the colour of the
cobalt chloride paper, which indicates the amount of sweating. Also note the skin colour.
4. Cover a bench with paper towel and place two conical flasks on it. Pour 10mL of distilled
water into one flask. Add 5 drops of phenolphthalein indicator to the water and swirl the
flask. Note the colour of the water.
5. Use the rubber band to fasten 6 drinking straws together. Ask the subject to blow air
through the straws into the water in the conical flask for 30 seconds.
6. Add the sodium hydroxide solution drop by drop until the colour of the solution changes and
remains pink for 30 seconds. Use the second conical flask as a control for comparing the
colour. Note: Make sure the subject breathes in through the nose and breathes out through
the mouth into the straws.
7. Rinse the straws and conical flasks thoroughly after each titration.
8. After 2 minutes, repeat all the measurements. Record your results in Table 1.
Exercise
9. Ask the subject to run for 15 minutes.
After exercise
10. As soon as the exercise is completed, ask the subject to sit down. Measure their pulse rate,
breathing rate and CO2 output, and note changes in the cobalt chloride paper. Also note the
skin colour.
11. Repeat the measurements at 2 minute intervals (2, 4, 6 and 8 minutes) after exercise.
Record the results in Table 1.
12. Clean your equipment and pack up.
13. Collate the class results in Table 2. In Table 3, calculate the percentage increase in heart
rates for each group and the mean.
14. Draw three line graphs to show the changes in measurements before and after exercise.
Draw one for temperature, one for pulse rate and one for CO2 output.
Results
Table 1 – Individual Results
Name of Subject: ______________________
Rest
Time (minutes)
0
Exercise
2
Recovery
0
2
4
6
8
Pulse rate per minute
Breathing rate per minute
Carbon dioxide output
(drops of NaOH)
Table 2 – Class Results
(B = before exercise, A = after exercise)
Group 1
Group 2
Group 3
Group 4
Group 5
Group 6
Mean
B
B
B
B
B
B
B
A
A
A
A
A
A
A
Pulse rate per
minute
Breathing rate per
minute
Carbon dioxide
output
(drops of NaOH)
Table 3 – Class results (% increase in heart rate)
Group
1
2
3
4
5
6
Mean
Heart rate at rest
Heart rate after
exercise
% increase in heart
rate
Discussion
i)
What happened to the subject’s body temperature immediately after exercise?
ii)
What happened to the pulse rate immediately after exercise? __________________
iii)
Explain the difference in the pulse rate after exercise. _________________________
iv)
What factors apart from exercise would affect heart rate? _____________________
v)
Did the subject sweat a lot after exercise? Use your understanding about ‘Negative
Feedback’ to explain why you would expect them to sweat.
vi)
Why do blood vessels near the skin dilate after exercise? ______________________
Redraw and complete the flow chart below to summarise the findings from this experiment.
Stimulus
Increase in ...
Receptor
Internal changes detected by thermoreceptors in the hypothalamus.
External changes detected by receptors in ...
_
Control Centre
..............................................
.......
Effectors
Act to ...
Adrenal and thyroid glands
produce hormones to
........................ metabolic rate
Blood vessels near the
skin...
Sweat glands....
Response
Decrease in ...
Conclusion
State the findings of your experiment and explain how negative feedback mechanisms
control body temperature.
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