The outgoing breath [ref]The Essence of the Bhagavad Gita, by Swami Kriyananda. Chapter 4, “Gyana Yoga (Union Through Knowledge of the Divine).”[/ref]; descending energy in the astral (energetic, as opposed to physical) spine. [ref]The Essence of the Bhagavad Gita, by Swami Kriyananda. Glossary.[/ref]
According to Swami Kriyananda, it is the flow of prana (rising energy in the astral spine) and apana (descending energy in the astral spine) that prompts the lungs to breathe. [ref]The Essence of the Bhagavad Gita, bySwami Kriyananda. Chapter 16, “The Supreme Science of Knowing God.”[/ref]
For more explanation of energy and the astral spine, see The Art and Science of Raja Yoga by Swami Kriyananda.