Breathing is controlled by the autonomic nervous system (ANS), the part of the nervous system that acts automatically or outside of your conscious control. The interesting thing about breathing is that you can also voluntarily affect it by consciously taking a deep breath, coughing and holding your breath. Most ANS controlled body systems do not allow a conscious ‘override’ of function. Along with automatic control, having some conscious effect on breathing shows the importance of maintenance of this system. Air is taken in by the lungs, or inhaled, for the purpose of extracting oxygen from the air. Oxygen passes through tiny vessels in the lungs called alveoli. The alveoli allow oxygen molecules to pass into the bloodstream. The oxygen molecule is then distributed to all cells in the body. When oxygen is metabolized by the body, carbon dioxide is released into the blood. The carbon dioxide waste is carried back to the lungs where it is exhaled back into the air.