The cardiac output is the amount of blood pumped out of the left side of the heart in one minute. This would therefore increase during exercise. Cardiac output is the product of stroke volume and heart rate. Therefore stroke volume also increases during exercise, this is the volume of blood pumped out of the heart at each beat. This works with the heart rate as an increase stroke volume stretches the aorta and carotids (due to the high pressure caused by more blood) which signal the cardiac inhibitory centre to slow the heart rate and this prevents the heart from working too fast.
Nerve impulses from the brain also increase heart rate during exercise. As we exercise and our muscles respire more, more carbon dioxide is produced. Chemoreceptors in the carotid artery detect a change in blood pH due to an increased concentration of carbon dioxide and these send impulses to the brain. Other receptors include stretch receptors in the aorta, as well as in the carotid artery and vena cava, which detect distension of the vessel due to increased blood flow. These receptors send impulses to the cardiovascular centre in the medulla oblongata in the hindbrain. From here the cardio-acceleratory centre sends impulses to the sino-atrial node (SAN) via the sympathetic nerve to speed up the heart rate. The SAN is the pacemaker and is located in the wall of the right atrium. It determines the basic rate of heartbeat as a wave of excitation spreads out from it across both atria causing them to contract. This wave reaches a similar group of cells known as the atrio-ventricular node (AVN), which lies between the two atria. The wave passes from here along the purkinge fibres, which make up the bundle of His, and these fibres lead along the septum to the ventricles where they radiate upwards causing the ventricles to contract. This allows blood to flow into the arteries. This is how the SAN controls heart rate. But if the heart rate had to be slowed down, receptors would send impulses to the cardiac inhibitory centre, which would send impulses to the SAN via the vagus nerve to slow down heart rate.
Hormones also are responsible for the increase in heart rate during exercise. Adrenalin is the major one and it is released from the adrenal glands before and during exercise to prepare the body for action. Adrenalin and noradrenalin, which is also secreted from the adrenal glands, speed up heart rate by binding to specific receptor sites and causing an increase in the rate at which the SAN sends out its wave of electrical activity.
But as all these factors increase heart rate, breathing rate is also increased. This is because as more blood is pumped to the muscles and the body, more blood is pumped to the lungs where it becomes oxygenated and this oxygenated blood can then be passed to the muscles, which use the oxygen to respire. But breathing rate increases, as more air is needed to travel to the alveoli in the lungs where the gas exchange of oxygen and carbon dioxide occurs with the blood.
What was also noticed in the table of results was the nature and number of anomalous results. There were only 2 anomalous results found in the table of results and on the graph none were found as all the points are very close to the line. This is all discussed in the evaluation.
Evaluation:
The overall success of the experiment was satisfactory but there is room for improvement. This is because the experiment was accurate and suitable enough to prove the main trend in that the heart rate increased as the amount of exercise carried out increased, but the results obtained could be more accurate and consistent. Firstly, the suitability and accuracy of the procedures undertaken and the consistency of certain variables will be discussed to find sources of error and how they can be improved or eliminated:
- I believe that a major source of error and limitation in this experiment was the method used to take the pulse. Firstly, the pulse on the underside of the wrist was not always easy to find. This was a bigger problem when the pulse was taken for 15 seconds after the exercise as it was necessary to achieve a heart rate that was valid for immediately after the exercise and time may have been wasted finding a pulse and during this time, the heart rate would have fallen. Once found, the pulse was sometimes very weak or difficult to monitor and this would greatly have affected the results if mistakes were made here. It was also hard to keep track of the pulse at times as the heart beat so fast; this was especially true for the last step of the exercise with 40 steps. There is also ambiguity over the right amount of time during which the pulse should be taken. Before the exercise the pulse was taken for 30 seconds and after exercising it was taken only for 15 seconds. To achieve a more accurate heart rate in beats per minute, the pulse should be taken for a longer period of time, a minute maybe. This would be better as it overrides the problem of the pulse changing during the minute. But there is a problem with this. When the pulse was taken for 15 seconds after the exercise, I noticed that during the 15 seconds the pulse decreased, in some cases considerably, and if the pulse was taken for longer, it would decrease even more and the value for heart rate immediately after exercising would be less accurate. Therefore what may be the best solution for this is to take the pulse for 1 minute before the exercise but only for 5 seconds after the exercise.
However, I believe that the best way to deal with all the problems surrounding the pulse and the method by which it was taken is to use a heart monitor instead. Now there is no need to find or even take a pulse, as the heart monitor would provide a value for heart rate before and immediately after exercise. This would ensure utmost accuracy.
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In this experiment, three different people were used. We were all of the same gender and roughly the same age and body mass. But for utmost accuracy I would have preferred to use people of exactly the same age, body mass, health, fitness as well as gender if this was all possible. It is impossible to get identical people but if all these variations were as constant as possible, the experiment would be more reliable. This is because each of these factors affects heart rate and changes to it due to exercise. For example, a well-trained person has a lower heart rate compared to a person who is not as fit. One way to find out how consistent these factors were between different people is to check how close the resting heart rates are. In our experiment, the resting heart rate varied by 7 beats per minute, which is a considerable difference. It may have been more suitable in this case to calculate the change in heart rate due to exercise but there is a problem with this due to the curved nature of the line of best ft on the graph. As the heart rate increases at a decreasing rate, starting off with a higher heart rate will mean that there is less change in heart rate compared to starting with a lower heart rate. Therefore the best solution to all this may be to try and keep the resting heart rate of the different people as similar as possible and thereafter to measure the change in heart rate due to exercise as if the resting heart rates are similar anyway there should not be that much variation in the change.
- Another factor, which would have affected resting heart rate, is adrenalin. It was mentioned in the analysis that in preparation of exercise adrenalin is released, which increases heart rate. Before the experiment this may have taken place in anticipation of exercise. In the results it was mentioned that before each exercise was carried out, “we waited until the resting heart rate was within 5 beats/minute of the original one”. However long we waited it would have been impossible to achieve the same resting heart rate before each exercise was carried out and the measurement was made more complicated due to the difficulty in pulse taking already mentioned. In some cases it was also noticed that the resting heart rate dropped form the original one taken at the start of the experiment and this would have been due to the adrenalin release at the start of the experiment. There is no way to control this adrenalin release but a way to keep the resting heart rate before each exercise as similar as possible it by using the heart monitor mentioned earlier. Using it, the person can start the exercise at precisely the right time when the heart rate is stabilised very close to the original one.
- The type of exercise carried out and the way it was carried out was nothing like as planned as noted in the modifications. I believe that what decreased the reliability of the experiment carried out compared to the one planned is that it relied more on human accuracy. For example, the intensity of the exercise was maintained constant by using a metronome and by making sure everyone stepped up and down at exactly the same time. However, if we had rowed instead, the intensity would have been maintained constant more reliably as it depended on an electronic machine, which is more consistent and reliable than humans who are susceptible to human error. The fact that this technique and exercise relied more on humans was an inherent problem with this technique.
These are the problems in this experiment, which I believe limited, and were sources of error in the results. I am quite sure that the 2 anomalous results found in the table of results were due to the problems discussed, especially the difficulty in achieving an accurate value of heart rate with the method of pulse taking used. The anomalous results could have easily been caused by a delay in finding the pulse or a mistake when taking it. Using an electronic heart monitor would evade any possibility of human error or limitation. By also looking at the table of results, a considerable deviation in heart rate between the replicates of different people is noticed and this would decrease the reliability of the experiment. However, by improving the experiment in the ways discussed would minimise this deviation, especially by experimenting on more similar types of people.
There are a number of ways of expanding this investigation to achieve a more reliable and concise understanding of how exercise really affects heart rate. Firstly, for this particular experiment, more people, more replicates (such as 3 times at each number of steps) and a greater range of number of steps (such as 2, 4, 6, 8, …50 steps) would increase the reliability of the results and would produce a more accurate graph. However, the problem with these changes is that it is unrealistic that the people could do this much exercise. But the whole experiment could be split up into a number of days but the conditions under which the exercise is carried out (especially the temperature) could be maintained constant by making sure the exercise took place indoors and the heating in the room could be controlled appropriately. If the resting heart rate at the start of each day was the same, there should not be a major problem. But this could form the basis of an extension to this experiment in investigating how exercise affects heart rate in different conditions such as different temperatures. Another way to extend the investigation is to use people of different age, sex, fitness and health to monitor how the affects of exercise on heart rate vary. Different types of exercises could also be investigated, so that different muscles are used or more muscles are made to work harder and this may affect heart rate differently. In this experiment, only the amount of exercise was varied whereas the intensity remained constant. Further investigations could vary other factors of exercise such as the intensity or time while others are kept constant.
Overall I was quite happy with the results of this experiment as the general trend was recognised and the results were quite accurate. But by improving the experiment in the ways mentioned earlier, I would hope to decrease the number of anomalous results and the deviation amongst replicates and gets the points on the graph even closer to the best-fit line and thereby increases the reliability and accuracy of the experiment. Expanding the experiment considerably would also make it more suitable and allow a more thorough examination of how exercise affects heart rate.
