O2 Transport

The respiratory system does not show significant changes with training. This is consistent with the observation that VO2 max is not limited by respiratory factors under most conditions (see above); however, important cardiovascular and tissue changes occur with training to support increased O2 transport. Cardiac output increases with training and can achieve levels of up to 35 L/min during maximal exercise with normal right atrial pressures in a well-trained individual. This occurs primarily from an increase in stroke volume, with increased end-diastolic volume (preload) and decreased arterial pressure (afterload). Heart rate is lower after exercise training and does not have to increase as much to achieve the same cardiac output. This may involve downregulation of adrenergic receptors on the heart from high levels of catecholamines during exercise. Heart rate recovers more rapidly after exercise in trained athletes because it is not as high during exercise as in untrained people.

Arterial pressure is lower at a given work rate after training, in part because of increases in the cross-sectional area of the vascular bed. Formation of new capillaries is stimulated by vascular endothelial growth factor (VEGF) with exercise training. Increased vascu-larity increases total blood volume and decreases diffusion distances for O2 in muscle. Myoglobin levels also increase with training which facilitates O2 diffusion within muscles. Together these changes promote tissue-O2 exchange so O2 extraction is increased by muscle, and the arterial-venous O2 difference increases.

The net result of these changes in O2 transport, metabolism and muscle function is, on average, a 15% increase in VO2 with exercise training. People who have relatively low VO2 levels will increase VO2 the most with training; however, genetics explains from 40 to over 60% of the difference in VO2max among individuals. The effects of training depend on duration and intensity, and an effective regimen is exercising large muscle masses (e.g., running) at 50 to 75% of VO2 max for 20 to 60 minutes, 3 to 5 times per week. Training does not change the amount of muscular work done for a given VO2, but it does increase VO2 max; hence, any given workload occurs at a lower percentage of VO2 max after training.

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