Exercise causes activation of sympathetic system & secretion of large amounts of epinephrine & norepinephrin from the medullae of the two adrenal glands, along with release of stress hormones. Theses lead to vasoconstrictor response of the peripheral circulatory system-both arterioles (increase blood pressure) & capacitative areas of circulation including veins (increase the venous return to heart), & also the heart itself is stimulated (heart rate increases, the pumping strength increases etc.).
Due to poor vasoconstrictor innervations the coronary (instead during maximal exercise, coronal blood flow increases by 5 times with 100% coefficient of oxygen utilization) & cerebral systems (no change in cerebral blood flow during any grade of exercise) are spared.
In exercising muscles, however, there is vasodilatation of arterioles due to local chemical changes .Reduction in oxygen is the most important cause- the arterioles walls cannot maintain contraction in absence of oxygen & release of many vasodilators. These are adenosine (most important), potassium ions, ATP, lactic acid & carbon dioxide etc. They cause about 12.5 increases in muscles blood flow during strenuous exercise.
The moderate rise in arterial blood pressure (usually about 30%) & many other factors (like opening up of dormant capillaries which delivers 2-3times more oxygen & nutrients to exercising muscles) occurring during exercise cause another 12.5 increase in muscles blood flow. But during the muscles contraction phase of exercise, blood flow decreases as intramuscular blood vessels are compressed. This explains the rapid muscle fatigue develops during strong tonic muscle contractions due to lack of enough oxygen & nutrients.
Muscle work output, oxygen consumption & cardiac output during exercise are linearly related to one other; i.e; the muscle work output increases oxygen consumption, which in turn dilates the muscle blood vessels, thus increasing the venous return & cardiac output. Physical training can increases the maximal cardiac output in well trained athlete about 1.3-2 times that of an untrained person during exercises, & almost 5.4 times the cardiac output at rest. This is due to the effect of training & exercise on the heart muscles.
In a trained persons heart chambers may enlarge up to 40 %, also the heart mass up to 40% or more. The heart-pumping effectiveness of each heart beat (stroke volume- the amount of blood pumped by each ventricle of the heart per beat) is also 40-50% greater in the highly trained athlete than in a normal person.
Important point to be noted is that heart enlargement & increase in pumping capacity occur entirely in endurance types, not in the sprint types of physical training. Another important point is that though the heart of an athlete is considerable larger than that of a normal person, resting cardiac output (the amount of blood pumped out by each ventricle into the circulation per minute) is almost the same. This is possible only when there is corresponding decrease in the heart rate of trained athlete at rest (as Cardiac output = Stroke volume multiply heart rate).
Stroke volume can be increased twice the normal value during exercise. The stroke volume normally reaches its maximum by the time the cardiac output has increase only halfway to its maximum (Cardiac output can be increased to 5-6 times-approx.25-36 liters/min- during maximum exercise).For further increase in cardiac output, the heart rate must increase correspondingly (heart rate increase linearly with increase in physical effort.
Heart rate during exercise can be used to roughly grade exercise: Grade I/light or mild exercise-below 100/min; Grade II/moderate-100to125/min; Grade III/heavy-125to150/min; Grade IV/very heavy or severe-above 150/min heart rate. Maximum heart rate attainable during exercise is 220 minus age (in years), therefore higher values of maximum cardiac output can be achieved in younger athletes compared to older ones). Hence the increase in heart rate accounts by far for a greater proportion of the increase in cardiac output than does the increase in stroke volume during strenuous exercise.
During maximal exercise, both the heart rate & the stroke volume are increased to about 95% of their maximal levels, this means cardiac output is increased to 90 % of the maximum that a person can attend. On the other hand, as discussed above, pulmonary ventilation is increased to about 65% of the maximum.
This leads to the conclusion that cardiovascular system is the one, rather respiratory system, that limits Vo2max., as oxygen utilization by the body can never be more than the rate at which the cardiovascular system can transport oxygen to the tissues.
Hence athletic performance mainly depends upon the performance capability of his/her heart. And training, as we have discussed above, increases the performance capability of one heart (for example ability to achieve 40% greater cardiac output in trained athlete over the average untrained male) to an optimal level.
Due to generalized vasoconstriction as discussed earlier during exercise many visceral blood flow decreases, specially during severe exercise, which may damage the system. Athletic pseudonephritis is a condition characterizes presence of proteins, cells & other abnormal substances in urine due to prolonged, heavy exercise.
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