Chronic psychological stress is commonly said to be deleterious to the heart, and predispose one to cardiovascular disease. Yet the opposite is said about regular aerobic exercise. Physiologically, What is the difference between psychological stress and physical exercise, with respect to their impacts on heart function/health? Both involve an increase in sympathetic cardiac input, and thus more vigorously contracting cardiac muscle; but how is one damaging and the other beneficial?
From the perspective of sympathetic nervous system
- Exercise induced adaptations of heart
A common phenomenon in endurance athletes is the athlete's heart /athletic heart syndrome
You must understand why this type of adaptation is happening in them:
Firstly the athletes train to produce work output at near maximal levels/ sub maximal levels of work done which cannot be physiologically achieved in a day.So the increased heart rate generated by the sympathetic stimulation is sufficient to fulfill oxygen demand of the body only at certain levels of work done by raising the heart rate
As the grade of work done nears submaximal level the oxygen demand increases in the body and the increased sympathetic stimulation (which also restricts the cardiac muscle efficiency by decreasing diastolic time,coronary blood flow) is no longer able to suffice it.Such high oxygen demand is responsible for the useful adaptations of the heart in the form of athlete's heart.Athlete's heart reduces the need for sympathetic stimulation to increase cardiac output .
During intensive prolonged endurance or strength training, the body signals the heart to pump more blood through the body to counteract the oxygen deficit building in the skeletal muscles. Enlargement of the heart is a natural physical adaptation of the body to deal with the high pressures and large amounts of blood that can affect the heart during these periods of time.
- Comparison with chronic psychological stress
During stress the body may or may not have high oxygen demand (the principal stimulus for adaptation) because the sympathetic stimulation is psychological and not in response to physiological work.So the heart instead of adaptating gets afflicted with detrimental pathophysiological effects of prolonged sympathetic stimulation.
stressors’ contributions to diverse pathophysiological changes including sudden death, myocardial infarction, myocardial ischemia, and wall motion abnormalities, as well as to alterations in cardiac regulation as indexed by changes in sympathetic nervous system activity and hemostasis.
From the perspective of cortisol (stress hormone)
- Effect of psychological stress on athletes
When we are talking about the detrimental effects of psychic stress it is important to note that it may override the cardiac adaptations and severely affect performance like,for example high cortisol levels in blood released in response to stress (physical/ psychological).Although cortisol levels in the blood observe a circadian rhythm in normal healthy indivituals, some reports claim that exercise reduces detrimental effects of high cortisol levels on the heart.However,stress may override exercise induced effects in athletes but this is a prospective study, subject to greater research
A prospective study found a clinically significant increase in overnight urinary cortisol:cortisone ratio during a high training load period in triathletes, who subsequently underperformed and reported fatigue
Exercise Physiology And Ergonomics (An Introduction) Asis Goswami Academic Publishers.Pg-39 to 57
Essentials Of Exercise Physiology (4th Ed.) Victor L.Katch,William D.McArdle,Frank I.Katch pg-302-333
I have an answer, but it is not ideal because science's current understanding of the answer to your question is somewhat limited.
The detrimental influence of physiological stress upon the heart is thought to occur because the physiological stress causes the cardiovascular system to prepare to meet anticipated energy needs by 1) increasing pulse and blood pressure, which can cause a heart attack that immediately follows the stress and and 2) increasing the production of glucocorticoids, which are thought to cause atherosclerosis, thereby increasing the likelihood of future cardiac maladies. Why do glucocorticoid levels increase in response to stress? That is somewhat controversial. See this article: https://www.ncbi.nlm.nih.gov/pubmed/10696570.
What about physical exercise? To some degree, whether the exercise is favorable or not is dose-specific. Comparatively moderate exercise (relative to what the body is capable of) will have a favorable effect, while exercise that is too strenuous will not. Physical exercise also increases pulse and blood pressure, which can also cause a heart attack that immediately follows the stress. But if the exercise is of moderate severity, there will be two sources of beneficial effect upon the heart:
1) There is the general phenomenon by which muscle fibers become stronger in response to the imposition of stress upon those muscles, whether those muscles are skeletal or cardiac muscles.
2) There is the phenomenon of hormesis which is characterized by a general improvement process to all cells (including cardiac cells) at the cellular level in response to many different kinds of stress, including physical exercise and caloric restriction, both of which are sources of stress that have a hormetic effect at the right doses. Edward Calabrese has been the most prolific researcher in demonstrating the hormesis effect, but its mechanism is not well-understood.
(A difference between psychological stress and physical exercise is that I do not think even moderate doses of psychological stress offer these two benefits.)
I believe the best answer to your question is that psychological stress increases the levels of a chemical compound called cortisol in the body (a glucocorticoid stress hormone) while physical exercise tends to decrease the levels of this "stress hormone". Of course, physical exercise also positively affects muscles and the heart in other ways, so this is another factor. The benefits of low levels of cortisol vs. the deleterious effects of high levels of it are explained and outlined at length in links below.