Blood carries oxygen to the brain and saturates it with this gas. Therefore, oxygen is the most abundant gas in the brain.The brain requires a lot of energy in order to function properly. Obviously, this energy comes from oxidation of fuel molecules, for which oxygen is of crucial importance. This oxygen is carried by the blood through arteries to the brain capillaries, where it diffuses to the nervous tissue. Since the nervous tissue is most dependent on oxygen (having no oxygen reservoir such as myoglobin and requirering a lot of energy) the organism keeps the brain's oxygen level especially high. On the other hand, carbon dioxide, a catabolism product, diffuses into the blood. However, the carbon dioxide concentration is much lower than the concentration of oxygen, therefore, oxygen is the most abundant gas in the brain.