Measurement of maximal oxygen uptake is a good indicator of overall cardiovascular fitness because

Cardiorespiratory fitness is defined as a component of physiologic fitness that relates to the ability of the circulatory and respiratory systems to supply oxygen during sustained physical activity.

From: The Sports Medicine Resource Manual, 2008

VO2 max depends on oxygen delivery (atmospheric O2, air exchange in the lungs, pumping power of the heart, and arterial blood flow to the muscles) and also oxygen demand by the tissues (mitochondria consume nearly all of the oxygen utilized) [14].

From: The Science of Fitness, 2015

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