Shushumna, Ida and Pingala Meridians.

In the same way that the physical body is more than just a collection of organs, the subtle body is more than just a collection of Chakras. The body has a complicated system of nerves, highly developed senses, intricate piecing together of muscles and bones and a vitally important system of hormone regulators. The physical body has pieces that are connected as part of a whole system, and the chakras are also connected together as part of a whole system.

The subtle body has a vital system interconnecting energy channels called meridians and nadis (nad means to flow).

The shushumna, which is the most important of the nadis rises within the base chakra and flows along the spine. There are two other important a channels; Ida (also known as Chandra, the moon) and Pingala (also known as surya, the sun).
Pingala nadi emerges from the right side of the base chakra and travels up the body in a series of of twists and curves crossing over the Shushumna. The Ida nadi emerges from the left side of the base chakra and travels up the body, creating the other half of a symmetrical pattern.
Ida, Pingala and Shushumna meet at the brow center to form a braided knot of energy.

Understanding the Chakras:

By working with the energies of the chakras, we are seeking to understand ourselves wholly.
Yoga offers an integrated system for awakening the energy body, incorporating techniques of pranayama (breath), meditation and asana (postures/poses).
Asana functions on many levels, including the obvious effect on the physical body by releasing muscle tension, strengthening the internal systems and releasing joint stiffness. The asanas also impact and work with the nadis to circulate subtle energy. When combined with meditation and working with the state of the mind, it brings calmness and control.

The breath is a profound tool for creating physical, emotional and intellectual change. The breathing pattern mirrors the way in which you interact with the world and yourself (It is frequently shallow and incomplete).
Controlled breathing is quite different from the often shallow and unconscious rhythms of daily life, and should be practiced in a well ventilated room, on an empty stomach and bladder and the body should be relaxed.

Yogic Breathing:
Yogic breath has three parts as air is brought into the abdomen, the chest and then the nasal passages.
1. Deep inhale
2. Allow the air to fill your belly and feel the expansion within your abdomen.
3. Allow the air to fill your chest and feel your ribcage expand.
4. Allow the air to move into your throat and nasal passages.
5. On the exhale, empty your nasal passages, then your chest, and finally your abdomen.

It is important to move the air smoothly and without a break. There should not be separation between the inhale and the exhale.

source: The elements of the chakras.