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Secondary limitations

Other limitations affecting a marathon runner's VO include pulmonary diffusion, mitochondria enzyme activity, and capillary density. These features of a marathon runner can be enlarged compared to that of an untrained individual but have upper limits determined by the body. Improved mitochondria enzyme activity and increased capillary density likely accommodate more aerobically produced ATP. These increases only occur to a certain point and help to determine peak aerobic capacity. Especially in fit individuals, the pulmonary diffusion of these individuals correlates strongly with VO and can limit these individuals in an inability to efficiently saturate hemoglobin with oxygen due to large cardiac output. The shorter transit time of larger amounts of blood being pumped per unit time can be attributed to this insufficient oxygen saturation often seen in well trained athletes such as marathoners. Not all inspired air and its components make it into the pulmonary system due to the human body's anatomical dead space, which, in terms of exercise, is a source of oxygen wasted.