Normal breathing involves several different mechanisms.
Shallow breathing is accomplished by the contraction of the diaphragm and the external intercostal muscles for inhalation. During exhalation, the muscles relaxing and the elasticity of the lungs returning to their resting volume expels air out of the lungs.
Deep Breathing is accomplished by an inferior movement of the diaphragm towards the abdomen. The external intercostal muscles along with the sternocleidomastoid and scalene muscles in the neck expand the space between the ribs, increasing the volume of the chest. During deep exhalation, the internal intercostal muscles and abdominal muscles contract to decrease the volume of the thoracic cavity, forcing air out of the lungs. Breathing is controlled by the brain and may be controlled both consciously and unconsciously.
Unconscious control of breathing is maintained by the respiratory center of the brainstem, which monitors the concentration of gases in the blood and adjusts the rate and depth of breathing as needed. During Yoga practice, the respiratory center automatically increases the breathing rate to provide constant levels of oxygen to the blood. During rest, the respiratory center reduces the breathing rate to prevent hyperventilation and maintain healthy oxygen and carbon dioxide levels in the blood.
Conscious control of breathing is maintained by the cerebral cortex of the brain. The cerebral cortex may override the respiratory center during the practice of Pranayama. Unconscious control of breathing resumes as soon as conscious control of breathing ends, preventing the body from suffocating from lack of breathing.