Yoga itself is beyond a science or an art. It is a philosophy. Yoga is derived from Yuj - meaning unification or Anusandhaanam. It is defined in the scriptures as Yoga Chittavruttinirodhaha (or making the mind devoid of any action, bringing it to rest) In other words it is the process of controlling the mind, the Ahankaarama(Ego) and the Antarendriyas (internal organs) with the help of Yoga and Pranayama. Eastern philosophy says that just like we strengthen other organs, we can strengthen our mind too. Strengthening of mind is possible through Pranayama, meditation and Yoga, because the mind is also an organ.

Pranayama helps expand one's concentration and to become antarmukham or be able to see your own soul through yourself. Praana is force of life which is the essence of mental, spiritual and cosmic energy. Here, breathing is only a superficial or minute part of the pranayama practice.

Aayama means niyantrana or having control. So, Pranayama means having control over breathing by increasing, decreasing and stopping your breath.

What happens during Pranayama
Steps involved in Pranayama are:

Inhalation - breathing in

Retention - retaining the breath

Exhalation - breathing out

Suspension of breath while exhaling

During Pranayama the chest moves in three different ways/directions in simple harmonic motion- vertical, horizontal and circumferential motion, thereby concentrating the energy in a particular spot. Pranayama is something that is higher than Yoga itself. It must be practiced after three or four hours after food intake, early in the morning, preferably in the Brahma muhurtha (forty-five minutes before sunrise)

To the Body -When respiration is controlled, the heart rate is reduced; this helps to increase supply of oxygen, increase physical rest and cell-rest, increase mental alertness, this again induces respiratory control, this decreases metabolism and the body goes into hibernation, thereby enhancing energy.

To the Mind - When respiratory rate is reduced, alertness of the mind is increased through concentration on just respiratory control. This improves the rhythm of breathing. Such regular breathing develops orderliness. Senses of Ekatvam (onliness - perceiving things as one truth) and Sukshmatvam (subtleness - grasping quickly) are improved. This is how there is a possibility of neuro-physiological rest.

In Pranayama, total filling and emptying the lungs happens - just like we empty a swimming pool or a well. This helps generation of fresh breath. Heart rate is reduced because all resources are used - its efficiency is increased, cardiac muscle strength is improve, micro/capillary circulation is improves, nutrition to cells is improved. All this happens not with physical exertion, but only with control on breathing. Therefore there is saving of physical energy.

The relationship of Pranayama with the various systems of the body:

Respiratory system:
Slow breathing through Pranayama improves oxygenation time. This improves oxygenation itself. Through slow exhalation also, there is absorption of oxygen. This improves capillary circulation and helps nutrition to cells. This is in contrast to rapid breathing (in other exercises) which can cause hyperventilation or dizziness or weakness because of over-exertion. In Yoga and Pranayama, cellular regeneration is increased due to an increase in muscular wear and tear and decrease in cellular wear and tear. This helps in cellular rest, increases blood circulation, increases local immunity, reduces and protects respiratory infections, in today's polluted atmosphere.

Digestive System:
The slow breathing helps stimulation of taste buds and improves secretion. This helps carbohydrates to be better digested - digestion starts in the mouth itself. The diaphragmatic massage in Pranayama helps to massage intestines. This prevents blood to be retained in the veins and so, the entire digestion process is improved, thus reducing and preventing disorders like constipation and indigestion.

Renal System:
Forced blood circulation, effective filtering is improved, elimination of toxins, this helps control renal disorders like kidney malfunctions.

Integumentary system:
The reason we suffer from skin disorders is that there is very less oxygen supply to the skin, as skin is a superficial organ. Supply of oxygen to skin is improved through Pranayama and Yoga. Nutrients are provided to the skin, hydration is improved, elimination of waste takes place and skin disorders are prevented.

Endocrine system:
Forced blood circulation during Yoga and Pranayama increases supply of oxygen, energy and nutrients to the pituitary glands (master glands located in the brain), and this improves and stimulates activities of all other glands.

Nervous system:
Forced blood circulation during Pranayama and Yoga increases cerebral circulation, nerves are soothed and nervous disorders are avoided and controlled.