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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) cobalt chloride papers (blue) adhesive tape x2 100mL conical flasks 6 drinking straws rubber band distilled water 100mL measuring cylinder 0.4% NaOH 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. ___________________________________________________________________________ ___________________________________________________________________________ ___________________________________________________________________________