According to the Hatha Pradipika and Goraksha Samhita, we have 72,000 nadis in the human body. These nadis are said to be present in the Pranamaya kosha, a layer that I found in the subtle body but are not seen by the naked eye. Within the nadis, we have three main channels that are the focus of Hatha yoga. These three nadis run from the base of the spine to the head.
The Ida nadi runs from the left side of the pelvic floor and travels up to the left nostril. Ida is associated with lunar energy, is labelled as feminine, and carries more of the apanic energy. Due to it being lunar-based, it is more cool and has similar qualities to tamas (inertia and lethargy).
Pingala is opposing Ida. It travels along the right side of the body mirroring Ida from the right side of the pelvic floor to the right nostril. The qualities of Pingala are warm due it being solar-based, is more pranic, and has attributes of rajas (action, passion and activity) Guna.
The third nadi, and most important of the three, is Susumna nadi. The Susumna nadi is the central energetic channel that runs from the Muladhara chakra to the Sahasrara Chakra. It is said to link the seven chakras together and open the chakras when life force energy, or prana, is locked within the channel.
Hatha yoga practices work to force prana into the central channel by creating a balance between the other two nadis, Ida and Pingala. When Pingala (Ha) and Ida (Tha) join together, we see dualism shift to non-dualism. This shift is the goal of Hatha yoga. We use Hatha yoga practices to help us spiritually transcend into higher levels of yoga with the end goal of Raja Yoga, or the royal yoga. Hatha yoga focuses more on bodily practices, similar to the outer four limbs of Patangali’s Astanga Yoga, to help us purify the body. In the Hatha Pradipika, Svatmarama refers to Hatha as a ladder to raja yoga. We eventually shift from a bodily focus in Hatha yoga to a mental focus in Raja yoga, similar to the inner limbs of Patanjali’s Astanga Yoga with dharana (concentration), dhyana (meditation), and samadhi (absorption).