The respiratory system is an amazing process by which the body takes air from the environment into the body to feed the blood and cells with oxygen, and then expels air containing carbon dioxide and toxins back out into the atmosphere. 
This process is called gaseous exchange. This takes place in the lungs. The complete process involves the nasal passages, the pharynx, larynx, trachea, bronchi, lungs and muscles; diaphragm, external and internal intercostals, transverse abdominal, scaleni and sternomastoids.
The physiology:
Inhalation: The diaphragm contracts and flattens; this reduces the pressure in the chest cavity thus increasing the volume of the lungs. Air is drawn in to equalise the pressure. Inhalation comes as a natural result of a full exhalation.
Exhalation: The diaphragm relaxes and rises, the pressure in the chest cavity reduces and the air is forced from the lungs.
In yoga, the breath is the vital link between the mind and the body. Basic breathing does not contain ratios or restrictions, the purpose behind basic breathing is to establish a practice for life; using the breath to manage your response to situations, particularly in challenging or stressful situations, and to help relaxation.
These practices include; simple breath awareness, nostril breathing, linking breath to movement, lengthening the breath, the complete yoga breath.
The Yoga Breath
In yoga a complete breath has 3 physiological phases;
Diaphragmatic (abdominal)
Thoracic (ribcage)
Clavical (upper chest/collar bone)
These phases need to be experienced independently. When this skill is achieved, the breath needs to move in one smooth wave.
Inhale – abdominal – thoracic – clavical
Exhale – clavical – thoracic - abdominal
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