Effects of Exercise on Cardiac Output.

Sasha Caddy, 22/12/03

RM11.

Effects of Exercise on Cardiac Output

From the two graphs there are distinctive trends and patterns. In the first graph, which shows the averages of both male and female affected heart rates per minute according to the levels of activity performed, there is an average resting heart rate for both males (74 bpm) and females (78 bpm), there is then a slight rise when undergoing the mild exercise of walking: male (77 bpm), female (86 bpm), then there is a decrease in heart rate in both counts when resting again: male (74 bpm), female (79 bpm - the females average is nearly back to its original resting pulse rate). The average heart rates at anticipation are a higher and slightly steeper rise: male (87 bpm), female (93 bpm), than the mild exercise. Then finally when sprinting there is a much greater and steeper curved line as the heart rates both increased tremendously: male (151 bpm), female (142 bpm).

However, the difference between resting and exercise is that when exercising, the heart's pulse rate is incredibly high compared to the resting heart rate where it is a lot lower. This is due to the body as a whole, using oxygen and glucose more quickly when exercising, so the blood must deliver it more rapidly, therefore the heart needs to pump a greater volume of blood each minute when sprinting than it does when resting. The male heart rate from resting to exercise has more than doubled, whereas the female heart rate has less than doubled the rate, this could be due to a number of reasons, such as the females were probably not running at a harder pace than the males, so there is a larger increase in the male's heart rate.

Although, the difference between the male's and female's heart rate per minute throughout these levels of activity is that at first the female's resting heart rate is 4 beats per minute higher than the male's, this may be because the males have a higher level of fitness overall than females so their pulse rate is lower. An explanation for this could be that the oxygen consumption of like-sized fit and unfit (trained and untrained) individuals will approximately be the same at rest or at any given level of activity. However, the more fit people e.g. males, will be able to achieve a greater maximal oxygen consumption due to regular aerobic exercise. As a person participates in regular aerobic exercise, the heart, lungs and muscles become more efficient at using oxygen. Resulting in the heart pumping more blood with each stroke, the lung capacity of each inhalation increases, and the muscle fibres extract more oxygen from the blood. The effect of exercise on the heart is quite obvious when heart rates are compared between the fittest and unfit individuals. The trained individuals will have a lower heart rate at rest (perhaps as low as 50 beats per minute in some cases) and during e.g. walking, than the untrained individuals (who might have a resting rate of 80 bpm). During low levels of exercise, the unfit person will experience a large increase in heart rate, while the fit person's heart rate will not rise nearly as much, from the graph you can see that this is the same for the female and male averages when carrying out mild exercise as the female's heart rate has increased more than the male's. Also after the mild exercise, there is another stage of resting, at the end of resting the pulse rates are taken. As you can see the male's heart rate per minute has returned back to the original resting rate, whereas the female's second resting heart rate is 1 beat per minute higher than the first. This could be because as the male's heart rate didn't increase nearly as much as the female's during mild exercise, it is easier for the male's heart rate to return back to 'normal'. During anticipation the heart rates both increase more than the mild exercise, although the female's anticipation heart rate is slightly higher (an extra 1 beat per minute), this is because adrenaline is being released from the adrenal glands (situated above the kidneys) into the bloodstream, where it flows to the heart causing it to beat faster.

At exercise both male and female heart rates per minute have increased greatly. However, from the graph you can see that the female's heart rate has crossed over the male's, so that the male's heart rate is 9 beats per minute higher than the female's. This could be due to the males active muscles working much faster than the females so therefore more oxygen and glucose is being used, causing a sudden increase in heart rate. The females active muscles possibly couldn't work at the same pace as the males, therefore less oxygen and glucose is used, so there is a less increase in heart rate per minute than the male's heart rate.

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In the second graph, which shows the averages of both male and female affected heart rates per minute according to the level of activity performed and time spent (minuntes) after exercise, there is a tremendous high heart rate for the male's (151 bpm) and female's (142 bpm) resulting from the exercise, then after one minute there is a steep fall in the heart rate per minute to male's (121 bpm), female's (124 bpm). Which is followed by a gradual decrease in heart rate for the following two to five minutes; male's (111 bpm - 94 bpm), female's (112 bpm - 88bpm). Slowly leveling off to resting pulse rates. However, the difference between exercise and five minutes spent after the exercise is that after the sprint the body starts to repay the metabolic debts which leads to faster and deeper breathing due to the need of large amounts of oxygen. Once oxygen has been supplied to the muscle cells, the metabolic rate has been repaid and any damage repaired, the heart rate can then start to return back to its resting rate, this can take over 5 minutes as shown by the graph.

The difference between the male and female heart rates from sprinting to 5 minutes after exercise, is that the female's heart rate didn't increase as much as the male's heart rate so there is less of an oxygen debt, therefore there is a steady decrease. As the males have a 151 beats per minute at exercise (9 beats per minute higher than females) it will take the male's heart rates slightly longer to return to their average resting heart rate. There is a steeper decrease in the male's heart rate than the female's from sprinting to 1 minute after exercise, I think this is because the majority of the males fitness levels may be higher than the females, therefore immediately after exercise the muscles in the males and the rest of their body relaxes and starts to repay the metabolic debt built up during sprinting a lot faster than in females. So the heart pumps more oxygen with each stroke to the areas in the body which are low on oxygen e.g. muscle cells, much quicker, so the heavy breathing rate in males will begin to decrease because the heart, lungs and muscles in trained males have become more efficient at using oxygen through regular exercise. Which results firstly in a much steeper decrease in heart rate, followed by a steady decrease back to the resting heart rate.

Here is a much more detailed biological knowledge of what is occurring throughout the body before, during and after exercise and also the effects of exercise, which support my results.

Exercise is a physical activity, carried out especially to develop or maintain fitness. Although skeletal muscle contraction is a main feature of exercise, many other systems in the body are activated to support this process. Such as the heart, which pumps increased volumes of blood to supply oxygen and nutrients and remove carbon dioxide and metabolic wastes. The respiratory system handles an increased workload, exchanging oxygen and carbon dioxide between the blood and the atmosphere. Also the nervous system and various hormones e.g. adrenaline, have important roles during exercise, by integrating the body's response to exercise and regulating the metabolic changes that occur in muscles and other tissues. The body's oxygen consumption increases with the intensity of exercise. If we keep on increasing the intensity or work required, we will eventually reach a limit, known as the body's maximal oxygen consumption, which corresponds to the highest energy production or metabolic rate of the body. The metabolic rate is ten times the resting rate in the untrained, but is up to twenty times in trained people. The maximal metabolic rate might be limited either by the muscles and the rate they produce and use energy for contraction and relaxation; or by the heart and the rate it pumps the blood around the body. The lungs and the rate they bring oxygen into the blood and expel carbon dioxide does not limit the maximal metabolic rate because even during maximal exercise, when the amount of air breathed in increases a considerable number of times over the rest level, the lungs can still work a lot faster, so the lungs just appear to have excess capacity.

Before the exercise we were told that we have to sprint two lengths of the sport's hall, this led to the impulses transmitted from the cardiovascular centre to alert the sympathetic nervous system then to the adrenal glands. Which then respond by secreting a hormone, known as adrenaline, into our bloodstream which surges through our veins causing our pulse to increase due to the overcoming of emotions e.g. excitement, nervousness. The effects of adrenaline is to prepare our body for the coping of demands before they actually happen. For our bodies to be able to cope with the demands there is an increase in cardiac output which is brought about by an increase of heart rate and stroke volume triggered by the impluses received from the brain in anticipation of the sprint (the cardiovascular centre also increases the heart rate in anticipation of activity). This also increases a general constriction of arterioles except those serving the skeletal muscle and the heart, so that blood under high pressure is diverted to the places in the body that need it most, such as the active muscles. Anticipation also brings about an increase in the ventilation rate so that oxygen can be delivered to the muscles more quickly.

During the sprint or contraction of muscle fibres, the metabolic rate increases due to a shortage of adenosine triphosphate (also known as ATP which is a nucleotide that supplies energy to cells). There is only enough ATP in our muscles at one time to provide for several seconds of maximal exertion. Therefore, ATP is made from a storage form (phospho-creatine) on the go, however this storage lasts only for another several seconds. This decrease in the stored ATP activates the enzyme which begins the further breakdown of the muscle glycogen into glucose, which is used to make more ATP. Carbon dioxide builds up in the skeletal muscle tissues resulted from the increased metabolic rate leading to the arterioles serving these active muscles to dilate causing an increase in blood flow through them. One of the cardiovascular system and respiratory system critical roles are to get rid of the heat produced by increased metabolism during exercise. To achieve this the increased blood flow to the skin results in direct transfer of heat to the environment as well as loss of heat during evaporation of sweat. Substantial heat is also transferred to the atmosphere in exhaled air during breathing. This increase in body temperature makes the tissues more sensitive to carbon dioxide. The continued rapid movement of the limbs stimulates stretch receptors in the skeletal muscles and tendons, which transmit impulses to the cardiovascular centre leading to a further increase in cardiac output. The chemoreceptors in the medulla, carotid artery and aorta bodies detects any changes in the level of carbon dioxide concentration in the bloodstream and responds by adjusting the ventilation rate. Anaerobic respiration may also occur in certain body cells sometimes deprived of sufficient oxygen. This occurs in the muscle cells, which need more oxygen during this strenuous exercise than the circulatory system can supply. Meanwhile, insufficient oxygen is delivered to the muscles to keep pace with their demands, resulting in the muscles respiring anaerobically with the process of lactic acid being formed. When the muscle cells respire anaerobically they build up an oxygen debt. This lactic acid gathers during the sprint and afterwards it is oxidised by way of the Krebs cycle or by circulation to the liver where it is converted back to glycogen, oxygen is needed for this.

Immediately after the sprint, the muscles stop working and relax, the body begins to repay the metabolic debts built up during the sprint and repairs any damage done, the heavy panting (a faster and deeper breathing) is due to the muscle cells needing large amounts of oxygen to make up for this debt, so impulses are triggered. The panting supplies the muscle cells with large amounts of oxygen. The breathing rate before, throughout and after the exercise is adjusted to meet the body's changing needs. The stretch receptors in the lungs detect the stretching due to heavy breathing and so the impulses suppress inspiration. The lactic acid gathered during the sprint causes local vasodilation (dilation of a blood vessel) on the arterioles. Which stimulates the aortic and carotid bodies, meaning the stressing of the ventilation responses, began by the carbon dioxide. Within the time spent after the sprint (5 minutes), there is a continued increase of carbon dioxide because of the oxidation of lactic acid and the way the lactic acid was buffered in the bloodstream. In this process lactic acid dissociates into lactate and hydrogen ions:

lactic acid lactate + H

The hydrogen ions then combine with hydrogencarbonate ions to form carbonic acid:

H + HCO H CO

The carbonic acid then splits into water and carbon dioxide:

H CO H O + CO

Meanwhile, the greatly increased metabolic rate results in a rise in body temperature, which is compensated by the process of the body's cooling.

So therefore, the effects of exercise can vary considerably with the intensity of exercise and with each person's physical fitness. The cardiac output is effected, which is the amount of blood pumped out of the left side of the heart in one minute. The cardiac output depends on two things:

the stroke volume - the amount of blood that the left ventricle pumps out each time the heart beats. Stroke volume is controlled. The more the cardiac muscle is stretched, the more strongly it contracts. So, the more the ventricle fills during diastole, the more strongly it contracts in systole. The amount of blood entering each side of the heart is mostly determined by the pressure in the vein leading to that side.
the heart rate (or cardiac frequency) - the number of times the heart beats per minute. The heart rate is controlled by various nerves which can increase or decrease the heart rate according to the needs of the body, for example the nerves of the autonomic nervous system control the atrioventricular node (AV node) and the sinoatrial node (SA node), which is the heart's pacemaker. Stimulation of these nerves can quicken or slow the heartbeat, during vigorous exercise the body needs more blood so the nervous system stimulates the SA node, which increase the rate of its impulses. The impulses keep the heart chamber contracting at a faster speed, thus pumping more blood. Whereas, the AV node conducts impulses slowly.

Therefore, the calculation for cardiac output is as follows:

Cardiac Output = Stroke Volume x Heart Rate

At rest, the cardiac output is approximately 5 litres (or cm ) per minute (so the heart rate is usually regular and constant), whereas during exercise the stroke volume and heart rate both increase to such an extent that the cardiac output may reach over 20 litres (or cm ) per minute. This is due to the increase in the flow of blood to the lungs and muscles, picking up and delivering more oxygen as there is a need for more food and oxygen by the muscles, so the heart rate may increase to triple the resting rate. The cardiac output can be adjusted when it is being exercised to meet the needs of the body.

Another effect of exercise is nerve impulses from the brain. The accelerator nerve (part of the sympathetic nervous system) increases the heart rate (controlled by the autonomic nervous system) by sending impulses from the cardiovascular centre down the nerve to the sinoatrial node (SA node) and increases the impulses travelling to the heart muscle, stimulating stronger contractions. When exercising, cellular respiration increases in the muscles which produces more carbon dioxide. The vagus nerve sends impulses from the cardiovascular centre down this nerve to the SA node in order to slow down the heart rate (part of the parasympathetic nervous system), this is achieved by the parasympathetic neurones from the vagus nerve and sympathetic neurones reaching the SA node. Then the parasympathetic neurones release acetylcholine, which slows the heart down. Sympathetic neurones release noradrenaline which increases the heart rate. The two systems work against one another: an accelerator and a brake. The balance determines heart rate. So therefore, when exercise begins there is a change in the pattern of signals reaching the heart in the autonomic nervous system, as the sympathetic activity increases, the parasympathetic activity decreases, so the heart beats faster.

Hormones are another effect of exercise. Although adrenaline increases the heart rate, it is probably not an immediate reaction, in this case anticipitation is the immediate reaction. Adrenaline is a hormone which is synthesised in the adrenal glands and is released into the bloodstream when under levels of stress or before the need of action, which then travels to the heart. By increasing the rate at which the SA node sends out its waves of electical activity the heart muscle beats faster with more strength due to the adrenaline in the bloodstream.

There are also changes in the volume of blood returning to the heart which is another effect. The volume of blood returning to the heart during exercise increases because as the active muscles are being used they respire faster and therefore use up more oxygen, which leads to a decrease in the levels of oxygen. The oxygen decrease causes the veins which transport the blood back to the heart from the body to widen, causing the walls of the heart to stretch more than usual. Resulting in the heart beating faster with more strength due to the need to pump the high levels of blood back out of the heart faster.

Exercise also effects the rest of the circulation, when there is an increase in vein pressure filling the heart, this increases the stroke volume. The movement of the body squeezes these veins rhythmically in the limbs, this pumps blood along increasing the pressure in the veins. Which is partly due to the increase in cardiac output. However, when there is increased muscle contraction this means a faster rate of respiration and so more oxygen is needed to supply the working muscle cells. So therefore, the blood flowing through the capillaries in the muscles is increased, this tends to be only when the muscles are active, when the muscles are resting the capillaries are closed. Away from the heart, blood is directed to where it is needed. If more blood flows to one organ, less must flow to another. During exercise, more blood is transported to the skeletal muscles and skin, whereas less is transported to the organs that make up the digestive system or kidneys. A constant supply of oxygen to the brain is continually needed so that the blood supply to the brain is not affected no matter how severe the exercise is.

Therefore, the effects of exercise which were mentioned above increases cardiac output by increasing the size of the left ventricle. Meaning, during exercise the stroke volume rapidly reaches a maximum, then there is an immediate increase in heart rate which also reaches a maximum, which later limits cardiac output resulting in higher maximum cardiac output. So as the force and rate at which the heart beats are continuously adjusted, the cardiac output is matched to meet the needs of the body. Cardiac output is partly controlled by the cardiovascular centre in the brain.

In evaluation of the practical work the results from the experiment may have been influenced by the main sources of errors. Here are a list of possible sources of errors which may have led to inaccurate results:

Time of day - we carried out this experiment after lunch, this may have had some affect on the results because the majority of everyone had eaten, so the performance levels may have been affected.
There was no control of the subject prior to the experiment, so for example if anyone was exercising or hyperactive before the experiment took place, their heart rate would already be higher than 'normal'.
Not everyone was running at the same pace as everyone else has different levels of fitness, e.g. if someone was running faster than the other person, their heart rate would increase more.
Taking the pulse rate every 6 seconds may have led to the results being 10 beats out when multiplied by 10 once the averages are calculated.
Also the results of the pulse rate being taken for 6 seconds by another person may not have been accurate as the person may have lost the feeling of the pulse or struggled when counting the pulse rate.
Clothing may have restricted movement, so therefore are unable to reach their full potential when sprinting.
The distance at which everyone was supposed to run was not controlled properly as some people were running shorter lengths in order to reach the finish line faster.
Anyone participating in the experiment may have been just recovering or suffering at that time from a fever or another illness e.g. cold, which influences the heartbeat resulting in a faster heart rate than 'normal'.
Between the time the pulse rates were taken at anticipation and the time taken to start the exercise, there would have been a further increase in the heart rate.

The limitations that could adjust the reliability of this experiment is that firstly, I think the experiment should be carried out mid-morning e.g. 10a.m, or mid-afternoon e.g. 2p.m, so that the amount of food eaten in the morning or at lunch will not have much of an affect on the results. To be able to control the subject prior to the experiment, I think that possibly the level of exercise should be restricted or more resting time should have been allowed in order for our heart rates to return to 'normal' incase of any unusually high heart rates. For clothing, I think that it may be best to wear shorts, trainers and t-shirts so that there are no restrictions, it will also lead to a better set of results. I think that even though we were told the distance we had to run, it wasn't properly controlled, to overcome this problem I think that a big improvement would be to carry out the experiment using a running machine. This will enable a set distance and it will also overcome the problem of everyone not running at the same pace, as the running machine will be at a set pace. Also the pulse rate may not have been accurate and reliable, a solution to this could be by using a polar heart rate monitor, where the electrodes can be attached to the chest so that the heart rate is much more accurate and is monitored throughout the experiment.

In this experiment there were no anomalous results which can be shown from the two graphs.