Breath is Spirit.
The act of breathing is Living.
It is a common misconception amongst the yoga community that less CO2 and more 02 is better for us. The idea that ‘C02 is a toxin, a carbon waste product that needs to be replaced with increased levels of 02’  is a harmful ideal that has become like the holy grail of breathing. In fact, high levels of 02 are toxic to our bodies and C02 plays a vital role in healthy respiration. 
C02 is an essential part of maintaining normal cellular biochemical processes. This importance being traced to the womb, where C02 levels make up 7-8% of ‘the gaseous mix in the womb, which is also an interesting indicator of the ideal human environment’ 
C02 plays an important physiological role in relaxation of muscle tissue as it acts as a smooth muscle vessel dilator. Smooth muscle surrounds the hollow cavities within the body, such as the bronchial airways and arteries. Decreasing amounts of C02 lead to spasms and constriction of these smooth muscles and a progressive and continued decrease of C02 levels will lead to hyperventilation. “Hyperventilation is a paradoxical state where there’s too much oxygen in the body’s bloodstream, but not enough in its tissues” 
C02 helps maintain cellular respiration and function in a number of ways. “CO2 serves as an end product that is released from body tissues (cells) after cellular respiration is used to release the energy from an ATP molecule.” In other words, C02 binds with glucose and this leads to ATP production resulting in cellular energy conversion via the mitochondria. Also, low levels of C02 will impact on the Bohr affect, haemoglobin’s oxygen binding affinity, and therefore 02 will not be released to its optimal level and we have decreased oxygenation of the tissues and vital organs.
C02 levels also play a vital role in our overall health. 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%.
From a yogic perspective the breath is a life. Breathing can be seen within the context of yin and yang. Breathing in and the nourishment of the cells with 02 can be seen as a yin function. The opposite action of breathing out C02 as a metabolic waste product derived from
cellular respiration, can be seen as the opposing yang function. “Prana and apana are expressions of function.” Life is the result between the balance of these two complimentary but opposite forces. The Sanskrit prana is derived from pra- meaning before and an- meaning to breath and to live. Prana can also be seen as an action of taking in nutrients, just as the 02 we breathe nourishes us. “prana is the life breath” 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’ making this complimentary opposite essential in maintaining prana and the balance required for life to exist. 
Resources and Bibliography
 D S Salerno, Lecture 1 Australian Yoga Academy
 What Yoga Therapists Should Know about the Anatomy of Breathing, L Kaminoff, , page 5.
 What Yoga Therapists Should Know about the Anatomy of Breathing, L Kaminoff, , page 19
 Yoga Anatomy, L Kaminoff, A Matthews, page 1