What exactly is happening in our body when we train and our VO2 max increases?
Aerobic exercise involves the heart's pumping of oxygen-rich blood to the body, the lungs' consumption of vast amounts of oxygen that diffuses into the bloodstream, and the body's diffusion of oxygen [1,2]. The lungs must be capable of consuming vast amounts of oxygen that diffuses into the bloodstream [1]. A strong set of lungs will make aerobic exercise less demanding and your VO2 max will increase as a result [1]. Factors that range from cardiovascular fitness to personal training to your body's innate ability to diffuse oxygen all play a role in governing the maximum amount of oxygen that one can consume [1,2,3]. The greater your VO2 max, the more oxygen your body can consume, and the more effectively your body can use that oxygen to generate the maximum amount of ATP energy [2]. You can increase your Vo2 max in two ways: by increasing the amount of blood your heart can pump and by increasing how much oxygen [4].
Aerobic fitness is the body's ability to deliver oxygen to muscles, allowing them to do work [1]. When we train, the lungs take in oxygen from the air we breathe, where it gets perfused into the blood stream; the heart and blood vessels deliver it into the working muscles; and the skeletal muscles utilize that oxygen to execute muscular contractions and produce work [1]. A cardiovascular assessment is a good tool to measure the efficiency of the aforementioned physiological functions [1]. 

VO2 is an index of the body's efficiency at producing work; it is expressed in milliliters of oxygen consumed per minute, and adjusted for body weight in kilograms: ml/kg/min [1]. Factors that can influence VO2max include heredity, training, age, gender, and body composition [1]. Generally, VO2max declines with age (about 2% per year after age 30) and males typically have a greater oxygen consumption value than females [1]. Nonetheless, the trend is that a higher VO2max allows one to produce more energy, thereby performing more work [1]. With this in mind, VO2max is the "gold standard" measure of overall fitness [1].

VO2max can also be increased by lowering