The oxygen uptake or VO₂ is the amount of oxygen that the body takes in to supply the oxygen which is required for several physiological processes. The oxygen is absorbed by the blood through the lungs and transported to various tissues throughout the body that require oxygen. At rest most oxygen is consumed by the heart, brains and internal organs. When exercise starts, the VO₂ increases (breathing frequency and tidal volume both increase) as exercise intensity increases. The brain still needs the same amount of energt, but the heart and active muscles require the largest part of the oxygen uptake. VO₂ will continue to rise as intensity rises until it reaches its maximum value or VO₂max. VO₂max is often used as a measure of endurance and is seen as the most important determining factor in endurance events such as cycling. The VO₂max is often expressed relative to bodyweight (L*min^-1*kg^-1), but sometimes as an absolute value (L*min^-1).

References:

-Guyton, A.C., Hall, J.E. (2000). The Cell and Its Function. Medical Physiology. Tenth Edition. Philadelphia, USA. W.B. Saunders Company.