Physiology of Yogic breathing
In process of breathing, one uses diaphragm, intercostals muscles in the chest. The diaphragmatic breathing is called vertical breathing and is considered a more efficient way to inhale air than inhaling while expanding the chest which is called horizontal breathing.
In pranayama, one should utilize the diaphragm efficiently to get more oxygen without making more efforts. The diaphragm is attached to the organs like heart and lings, also the liver, spleen, pancreas and stomach from the bottom side. Efficient movement of the diaphragm makes the functioning of these organs more efficient.