VO2 (or oxygen consumption) is a measure of the volume of oxygen that is used by your body to convert the energy from the food you eat into the energy molecules, called adenosine triphosphate (ATP), that your body uses at the cellular level. VO2max (or maximal oxygen consumption) is simply the maximum possible VO2 that a given person can achieve. VO2 and VO2max are important in the context of exercise, because they are a measure of your body’s ability to generate ATP, and ATP is the energy source that allows your muscles to continue working while you are exercising. Therefore, by definition, a VO2max measurement is ultimately a measure of your cardiorespiratory fitness level.
So how does your body “consume” oxygen? Breathing in is only the first step in oxygen consumption and, in fact, not all of the oxygen you breathe in gets consumed. The amount of oxygen in the air that you breathe in is approximately 20.95% by volume. If you consumed all of the oxygen that you breathed in you would expect to find that the amount of oxygen in the air that you breathed out would be 0%, but it is typically in the 15% to 18% range, depending on what you are doing and a number of other factors. To actually consume the oxygen you’ve inhaled your body needs to make use of it in the cellular respiration process that generates ATP. Before your cells can use oxygen to generate ATP, the oxygen must first be delivered to your cells. Delivery of oxygen to your cells requires pulmonary (or lung) ventilation to bring the oxygen into your body, diffusion of the oxygen from the air in your lungs into the blood in the capillaries that surround them, cardiac (or heart) contraction to pump the blood through your body to your muscles, and diffusion of the oxygen from your blood into your muscle cells. Once oxygen has been delivered to your muscle cells it is used in the cellular respiration chemical reaction to produce ATP energy. The byproducts of this cellular respiration chemical reaction are water and carbon dioxide, that’s why we are said to breathe in oxygen and breathe out carbon dioxide.