Yogis believe that the body is made up of different tiers or layers. There is the physical body, or the actual body; the astral, or emotional, body; the mental body, which receives and sends information and deals with thoughts; and the causal body, which is the spiritual or immortal part of man.
Source: Chakra Therapy for Personal Growth and Healing, Keith Sherwood.
Prana does not run willy nilly through the body. It flows through the nadi, nerve channels. According to ancient yogic texts, there are approximately seventy-five thousand nadi in the body. The three most important nadi for the flow of prana are sushumna, ida, and pingala.
Sushumna is the main channel, and it corresponds with the spinal cord.
Ida is on the left of sushumna and is equated with the moon or a cooler force of energy.
Pingala is opposite ida and on the right. Pingala is equated with the sun or warmer energy.
The ida and pingala are not parallel nadi, but spiral around the sushumna. Both ida and pingala correspond to the sympathetic ganglia of the spinal cord. At the base of the sushumna is the dormant energy called kundalini. Breathing practices help to uncoil the energy at the base of the spine and regulate the flow of prana through the body.