Galen believed the lungs draw pneuma from the air, which the blood communicates throughout the body. This idea is super-ancient:

• In Sumerian mythology, Enki makes people out of clay and then gives them life by breathing upon them.
• In Jewish mythology, “Yahveh formed man from the dust of the ground, and breathed into his nostrils the breath of life; and the man became a living being” (Gen 2:7).
• In Greek mythology, Prometheus shaped man out of mud, and Athena breathed life into his clay figure.
•  In the material monism of Anaximenes, air is the element from which all else originates. “Just as our soul (psyche), being air (aer), holds us together, so do breath (pneuma) and air (aer) encompass the whole world.” (In this early usage, aer and pneuma are synonymous.)
• For Aristotle, semen gives life (specifically, it imparts locomotion and sensation to the baby) because semen contains pneuma.
• For the Stoics, pneuma is the active and generative force that organizes both organisms and the cosmos. (And is similar to Heraclitus’ logos in this regard.) Galen received his notion from here.