During hard exercise a runner's leg muscles respire more quickly to release the energy needed for contraction. Aerobic respiration produces carbon dioxide a...
Question 1 Report
During hard exercise a runner's leg muscles respire more quickly to release the energy needed for contraction. Aerobic respiration produces carbon dioxide as a waste product, so the concentration of carbon dioxide dissolved in the blood begins to rise. The body monitors this concentration closely and acts to bring it back within its normal range once the exercise is over. In the investigation shown, a runner exercised for a fixed period and then rested, while the concentration of carbon dioxide in her blood was measured at regular intervals. The graph below plots blood carbon dioxide concentration against time in minutes, with the exercise period and the recovery period marked above the curve. The concentration climbs during the exercise, reaches a peak, and then falls back during recovery. Use the graph and your own knowledge of control systems to answer the questions.
(a) State the part of the brain that monitors the composition and temperature of the blood. [1]
(b) Using the graph to help you, describe how negative feedback returns the raised blood carbon dioxide concentration to normal during the recovery period. [3]
Answer Details
The graph shows blood carbon dioxide concentration rising during exercise and falling back during recovery, which is an example of negative feedback.
(a) The part of the brain that monitors the composition and temperature of the blood is the hypothalamus (the brain stem is also accepted for CO2 monitoring). [1]
(b) Negative feedback returns the raised CO2 to normal during recovery as follows [3]: the rise in carbon dioxide is detected by receptors in the brain [1]; the breathing rate and depth (and heart rate) increase [1]; so more carbon dioxide is removed/exhaled at the lungs, returning its concentration to normal [1]. Negative feedback means the correction acts to reverse the change and switches off once the normal level is restored, which is why the curve levels out at the end.
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