There is a common misconception those in the yoga community have. That less carbon dioxide (CO2) and more oxygen (02) is better for us.
To sum it up, this is not true. Above all, its balancing the breath that is most important in yoga.
In fact, high levels of 02 will be toxic to us and C02 plays a vital role in healthy respiration. 
Its importance is traced back to the very beginning of our existence, back to the womb, where C02 levels make up 7-8% of ‘the gaseous mix'. In short, pointing to the ideal human environment’ 
Further, C02 plays an important role in the relaxation of muscle tissue. It acts as a smooth muscle vessel dilator. Smooth muscle surrounds the hollow cavities within the body, such as the bronchial airways and arteries.
On the other hand, decreased amounts of C02 will lead to spasms and constriction. Which will subsequently lead to hyperventilation.
In other words, “Hyperventilation is a paradoxical state. For instance, the body has too much oxygen in the bloodstream, but not enough in its tissues” 
Moreover, carbon dioxide maintains cellular respiration and function in a number of ways.
“It is an end product that is released from body tissues (cells) after cellular respiration is used to release the energy from an ATP molecule.”
To sum it up, it fuels our cells and similarly, ourselves!
Additionally, low levels of C02 will impact on the Bohr effect. Which means, it impacts hemoglobin’s oxygen binding affinity, and therefore 02 will not be released to its optimal level and we suffer a decrease of oxygenation within our tissues and vital organs.
C02 conversion into carbonic acid creates the primary buffer system for our acid-base balance. Keeping the body’s pH levels stable is critical, for death occurs if levels of C02 drop below 3%.
The yogic perspective is that the breath is life. Likewise, there are similarities with Chinese philosophy. Breathing in and the nourishment of the cells with 02 is a yin function.
Similarly, the action of breathing out C02 as a metabolic waste is a yang function.
To clarify, its balancing the breath in yoga, and life, that is most important.
“Prana and Apana are expressions of function.” In other words, life is the result of the balance between these two complementary but opposite forces. The Sanskrit prana is derived from pra- meaning before and an- meaning to breathe and to live. Prana is the act of taking in nutrients, just as the 02 we breathe nourishes us. “prana is the life breath”
In conclusion, Apana means away, off or down. C02 is a by-product of inhalation and needs to be eliminated through the process of exhalation. Apana refers to the ‘waste’ as well as the ‘action of elimination’. In the same vein, this an essential function when maintaining prana, and the balance required for life to exist.