From Nephesh ha-Chayim, 1st Gate, Chapter 14:
“Action is the faculty of the Nephesh, as it is written, ‘the soul (nephesh) that will do…’ (Numbers 15:30), ‘the souls (nephashoth) that do…’ (Leviticus 18:29), as well as many similar examples, ‘for the blood is the soul (nephesh)’ (Deuteronomy 12:23), meaning that the nephesh dwells and clothes itself in the blood of man. Therefore, its primary dwelling is in the liver* which is all blood. The flow of blood through all segments of all parts of the limbs, the vessels of action, is what gives them the capacity of movement and arousal, enabling them to act and accomplish that which is within their ability. If the flow of blood to any limb would be blocked, that limb would wither, and would lack ability to move or perform any act; it would be a dead limb.”
*NOTE: No coincidence that the name of the organ that is identified as the primary dwelling of the nephesh (“spirit/life/soul”) is also called in English the liver (emphasis on “live”), i.e. organ of life (nephesh).