Effect of exercise on cardiac output

Endurance training such as running alters the baseline or tonic activity of the sympathetic and parasympathetic systems. In a trained athlete, dominance of the parasympathetic system is even greater than it is in a sedentary individual, resulting in training-induced bradycardia. Although the resting heart rate is 70 to 75 beats/min in a sedentary person, in a trained athlete it may be 50 beats/min or lower. Due to the decrease in heart rate in these individuals, the length of diastole is increased. Assuming a constant rate of venous return, this longer filling period results in a greater end-diastolic volume and an increased stroke volume (40 to 50% greater in the elite athlete compared to the untrained individual). Therefore, at rest when their bodies' metabolic demands are similar, cardiac output in a sedentary person and an athlete is also similar (see Table 14.2).

During exercise, cardiac output increases substantially to meet increased metabolic demand of the working muscles. However, endurance training results in significantly greater increases in cardiac output, which improves oxygen and nutrient delivery to the working muscles (18 to 20 l/min in sedentary individuals; 30 to 35 l/min in trained athletes). As a result, exercise performance is enhanced and fatigue is delayed.

In order to increase cardiac output, heart rate and stroke volume are increased. The maximum heart rate in all individuals is about 195 beats/ min; therefore, the difference in cardiac output in trained vs. untrained people during exercise involves stroke volume. This volume increases approximately 50 to 60% during exercise. Because the athlete has a much larger stroke volume at rest, the increase in stroke volume during exercise is that much greater (see Table 14.2). In this way, even with a similar maximal heart rate, the endurance-trained athlete pumps a significantly greater volume of blood per minute. In order to accommodate these larger stroke volumes, the ventricles of these athletes hypertrophy such that the chambers become larger and increase their diameters.

Table 14.2 Effect of Exercise on Cardiac Output

Rest

Heart rate Stroke volume Cardiac output

Exercise

Heart rate Stroke volume Cardiac output

Sedentary individual Endurance-trained athlete

71 beats/min 70 ml/beat 5000 ml/min

195 beats/min 1000 ml/beat 19,500 ml/min

50 beats/min 100 ml/beat 5000 ml/min

195 beats/min 160 ml/beat 31,200 ml/min

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