Arteriovenous O2 Difference
As mentioned earlier, endurance training induced increases in VO2 are due to increases in both cardiac output (i.e., stroke volume increases) and the a-v On difference. The training-induced increase in the VO2 difference is due to increased O2 extraction from the blood. The increased capacity of the muscle to extra O2 following training is primarily due to the increase in capillary density, with an increase in mitochondrial number being of secondary importance (9, 42), The increase in capillary density in trained muscle accommodates the increase in muscle blood flow during maximal exercise, decreases the diffusion distance to the mitochondria, and slows the rate of blood flow t allow more time for oxygen diffusion from the capillary to the muscle fiber occur. Changes capillary density parallel changes in leg blood flow and VO2 max with training. The increases in mitochondrial number following endurance training increase the muscle fiber's ability to consume oxygen and also contribute to the expanded a VO2 differences. Note, however, the capacity of the mitochondria to use O2 exceeds the capability of the heart to deliver O2, making mitochondrial number not the key factor limiting VO2 max (9, 42). Figure 13.2 provides a summary of the factors responsible for the increase in VO2 max as sociated with an endurance training program
Finally, two scientists who have made major tributions to our understanding of the physiological effects of exercise are Drs. John O. Holloszy and Bengt Saltin. See A Look People in Science for more on these outstanding scientists