The cardiovascular system serves five important functions (1) during exercise:
- Delivers oxygen to working muscles
- Oxygenates blood by returning it to the lungs
- Transports heat (a by-product of activity) from the core to the skin
- Delivers nutrients and fuel to active tissues
- Transports hormones
Exercise places an increased demand on the cardiovascular system. Oxygen demand by the muscles increases sharply. Metabolic processes speed up and more waste is created. More nutrients are used and body temperature rises. To perform as efficiently as possible the cardiovascular system must regulate these changes and meet the bodys increasing demands (2).
Below we will examine the acute or immediate response to exercise and also the long-term adaptations that take place in the cardiovascular system with repeated exercise. The most important aspects of the cardiovascular system to examine include:
- Heart rate
- Stroke volume
- Cardiac output
- Blood flow
- Blood pressure
Immediate Response of the
Cardiovascular System to Exercise
Resting heart rate averages 60 to 80 beats/min in healthy adults. In sedentary, middle aged individuals it may be as high as 100 beats/min. In elite endurance athletes heart rates as low as 28 to 40 beats/min have been recorded (2).
Before exercise even begins heart rate increases in anticipation. This is known as the anticipatory response. It is mediated through the releases of a neurotransmitters called epinephrine and norepinephrine also known as adrenaline and noradrenaline (1).
After the initial anticipatory response, heart rate increases in direct proportion to exercise intensity until a maximum heart rate is reached. Maximum heart rate is estimated with the formula 220-age. But this is only an estimation, and not particularly accurate. The only direct method for determining maximum heart rate is to exercise at increasing intensities until a plateau in heart rate is found despite the increasing work rate.
Although heart rate increases rapidly with the onset of activity, providing exercise intensity remains constant, heart rate will level off. This is known as steady-state heart rate where the demands of the active tissues can be adequately met by the cardiovascular system. However, there is an exception to this
During prolonged steady-state exercise, particularly in a hot climate, a steady-state heart rate will gradually increase. This phenomenon is known as cardiac drift and is thought to occur due to increasing body temperature (3).
Stroke volume is the amount of blood ejected per beat from left ventricle and measured in ml/beat.
Stroke volume increases proportionally with exercise intensity. In untrained individuals stroke volume at rest it averages 50-70ml/beat increasing up to 110-130ml/beat beat during intense, physical activity. In elite athletes resting stroke volume averages 90-110ml/beat increasing to as much as 150-220ml/beat (2).
Stroke volume may increase only up to 40-60% of maximal capacity after which it plateaus. Beyond this relative exercise intensity, stroke volume remains unchanged right up until the point of exhaustion (4,5). But this is not conclusive and other studies suggest stroke volume continues to rise until the pint of exhaustion (6,7).
Interestingly, swimmers see a smaller increase in stroke volume compared to runners or cyclists for example. It is believed that the supine position prevents blood from pooling in the lower extremities enhancing venous return (2).
Why does stroke volume increase with the onset of exercise? One explanation is that the left ventricle fills more completely, stretching it further, with the elastic recoil producing a more forceful contraction. This is known as the Frank-Starling mechanism. Other contributing factors include increased contractility of the ventricles and reduced peripheral resistance due to greater vasodilation of the blood vessels (1).
Cardiac output is the amount of blood pumped by the heart in 1 minute measured in L/min. It is a product of stroke volume and heart rate (SV x HR). If either heart rate or stroke volume increase, or both, cardiac output increases also.
Cardiac output increases proportionally with exercise intensity – which is predictable from understanding the response of heart rate and stroke volume to activity. At rest the cardiac output is about 5L/min. During intense exercise this can increase to 20-40L/min (1).
The vascular system can redistribute blood to those tissues with the greatest immediate demand and away from areas that have less demand for oxygen.
At rest 15-20% of circulating blood supplies skeletal muscle. During vigorous exercise this increases to 80-85% of cardiac output. Blood is shunted away from major organs such as the kidneys, liver, stomach and intestines. It is then redirected to the skin to promote heat loss (2).
Athletes are often advised not to eat several hours before training or competition. This is advice worth adhering to, as food in the stomach will lead to competition for blood flow between the digestive system and muscles. It has been shown that gastrointestinal blood flow during exercise shortly after a meal is greater compared to exercising on an empty stomach (8).
At rest, a typical systolic blood pressure in a healthy individual ranges from 110-140mmHg and 60-90mmHg for diastolic blood pressure.
During exercise systolic pressure, the pressure during contraction of the heart (known as systole) can increase to over 200mmHg and levels as high as 250mmHg have been reported in highly trained, healthy athletes (2).
Diastolic pressure on the other hand remains relatively unchanged regardless of exercise intensity. In fact an increase of more than 15 mm Hg as exercise intensity increases can indicate coronary heart disease and is used as marker for cessing an exercise tolerance test.
Both systolic and diastolic blood pressure can rise to high, albeit brief, levels during resistance exercise. Values of 480/350mmHg (9) have been reported to coincide with a Valsalva manoeuvre – i.e. trying to exhale against a closed mouth, nose and glottis.
During resting conditions the oxygen content of blood varies from about 20ml of oxygen per 100ml of arterial blood to 14ml of oxygen per 100ml of venous blood (2). The difference in oxygen content of arterial and venous blood is known as a-vO2 difference.
As exercise intensity increase the a-vO2 difference increase also and at maximal exertion the difference between arterial and venous blood oxygen concentration can be three times that at a resting level.
Blood plasma volume decreases with the onset of exercise. The increase in blood pressure and changes in intramuscular osmotic pressures force water from the vascular compartment to the interstitial space. During prolonged exercise, plasma volume can decrease by 10-20% and by 15-20% in 1-minute bouts of exhaustive exercise (10). Resistance training with 40% and 70% one repetition maximum can cause a 7.7% and 13.9% reduction in blood plasma respectively (11).
A reduction in plasma increase the concentration of hemoglobin or hematocrit. Although no extra red blood cells have been produced, the greater concentration of hemoglobin per unit of blood significantly increases the bloods oxygen carrying capacity. This is one of the main adaptations during immediate acclimatization to altitude.
Blood pH can change from a slightly alkaline 7.4 at rest to as low as 6.5 during all-out sprinting activity. This is primarily due to an increased reliance on anaerobic energy systems and the accumulation oh hydrogen ions (1).
Adaptations in the Cardiovascular System
Following training the cardiovascular system and its components go through various adaptations. Here are the most important:
The hearts mass and volume increase and cardiac muscle undergoes hypertrophy.
It is the left ventricle that adapts to the greatest extent. As well as the chamber size increasing as a result of endurance training (12), more recent studies show that the myocardial wall thickness also increases (13).
Resting heart rate can decrease significantly following training in a previously sedentary individual. During a 10-week exercise program, an individual with an initial resting heart rate of 80beats/min can reasonably expect to see a reduction of about 10beats/min in their resting heart rate (2). As mentioned earlier, highly conditioned athletes such as Lance Armstrong can have resting heart rates in the low 30s.
During submaximal exercise, heart rate is lower at any given intensity compared to pre-training. This difference is more marked at higher relative exercise intensities. For example, at low work rates there may only be a marginal difference in heart rate pre and post training. As intensity reaches maximal levels, the difference can be as much as 30beats/min following training (2).
Maximum heart rate tends to remain unchanged by training and seems to be genetically limited. However, there are some reports that maximum heart rate is reduced in elite athletes compared to untrained individuals of the same age.
Following an exercise bout, heart rate remains elevated before slowly recovering to a resting level. After a period of training, the time it takes for heart rate to recover to its resting value is shortened (2). This can be a useful tool for tracking the effects of a training program. However, it is not so useful to compare to other people as various individual factors other than cardiorespiratory fitness play a role in how quickly heart rate returns to a resting level.
Stroke volume increases at rest, during submaximal exercise and maximal exercise following training. Stroke volume at rest averages 50-70 ml/beat in untrained individuals, 70-90ml/beat in trained individuals and 90-110ml/beat in world-class endurance athletes (1).
This all-round increase in stroke volume in attributable to greater end-diastolic filling. This greater filling of the left ventricle is due to a) an increase in blood plasma and so blood volume (see below) and b) reduced heart rate which increases the diastolic filling time (2).
According to the Frank-Starling mechanism, this increased filling on the left ventricle increases its elastic recoil thus producing a more forceful contraction. So not only is the heart filled with more blood to eject, it expels a greater percentage of the end-diastolic volume (referred to as the ejection fraction) compared to before training.
If heart rate decreases at rest and during submaximal exercise and stroke volume increases, what is the net effect on cardiac output?
In actual fact, cardiac output remains relatively unchanged or decreases only slightly following endurance training. During maximal exercise on the other hand, cardiac output increases significantly. This is a result of an increase in maximal stoke volume as maximal heart rate remains unchanged with training. In untrained individuals, maximal cardiac output may be 14-20L/min compared to 25-35L/min in trained subjects. In large, elite athletes, maximal cardiac output can be as high as 40L.min (2).
Skeletal muscle receives a greater blood supply following training. This is due to:
- Increased number of capillaries
- Greater opening of existing capillaries
- More effective blood redistribution
- Increased blood volume
Blood pressure can decrease (both systolic and diastolic pressure) at rest and during submaximal exercise by as much as 10mmHg in people with hypertension. However, at a maximal exercise intensity systolic blood pressure is decreased compared to pre-training (15,16).
It is interesting to note that although resistance exercises can raise systolic and diastolic blood pressure significantly during the activity, it too can lead to a long-term reduction in blood pressure (17).
Endurance training increase blood volume. While plasma volume accounts for the majority of the increase, a greater production of red blood cells can also a contributory factor. Recall that hematocrit is the concentration of hemoglobin per unit of blood. An increase in red blood cells should increase hematocrit but this is not the case. Because blood plasma increases to a greater extent than red blood cells, hematocrit actually reduces following training (2).
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