It is through various reflex mechanisms that the respiratory system works involuntarily. The depth and the rate is maintained automatically according to the oxygen requirements of the body from moment to moment. When breathing pattern changes during Pranayama, the whole mechanism of breathing gets altered. We shall now consider the physiological changes occurring due to modifications in the breathing pattern during different phases of Pranayama.  

 
Inhalation of air through one or both nostrils.

Lungs expand considerably and walls
 of the alveoli are stretched to maximum.

Stretch receptors situated in the alveolar walls are stimulated.

No exhalation is possible.

Chest continues to expand under cortical control.

Intra-pulmonary pressure is raised.

Diaphragm does not move freely as abdomen is 
kept slightly inward and controlled.

Alveoli in the upper pulmonary are filled with air.

The filling of lungs is done as per one's limit and is well attended.

A person's inspiratory capacity for prolonged phase of Puraka is achieved.

 
Inhalation is stopped and the inspired air is retained in the 
lungs for proportionate  time.

Alveoli and bronchioles are stretched to their optimum level.

  Stretch receptors, however, cannot bring about the reflex contraction of lungs.

Respiratory muscles cannot relax as they do normally, 
due to strong cortical control.

Metabolic processes continuously produce CO₂ 
which is picked up by blood.

CO₂ level (concentration) increases in blood.

Ratio between volume of inspired air and blood is thus changed.

 Exchange of O₂, and CO₂, across the thin walls of the alveoli and  blood capillaries take place more efficiently as they get more time.

Exchange of gases is not possible after a particular stage when there is a saturation of both gases on both sides i.e. 
in the alveoli and in blood.

Increasing concentration of CO₂ in blood stimulates
the IX ^th  and X ^th cranial nerves and chemo receptors.

Sends impulses to respiratory centre.

A person is trained to hold one's breath i.e. Kumbhaka.

 
Exhalation through one nostril (either right or left) Or through both nostrils

The exhalation is prolonged and the force of air going out is reduced 

Intra-pulmonic pressure is reduced

And the alveoli is gradually deflated

CO2 increases in the blood

The chemoreceptor tries to inhibit the exhalation and to start the inhalation

These reflexes help to breathe out

Acclimatize the receptors to higher CO2 tensions in the blood 

As the expiratory reserve volume of the air is used the air containing CO2 is squeezed out of the lungs

Intra abdominal pressure is maintained for sometime and thus the blood circulation in the lower abdominal region is diverted towards the sacral region thus providing nourishment to the parasympathetic branch of ANS

A person's capacity for prolonged phase of Rechaka is achieved.