Prana is the subtle energy that sustains life in a body. The Prana is like a battery system in a robot. Unless the master switches on the battery system, the robot does not start.
The master is the atman soul that is the real controller of human body. It is this master, controller we called atman soul that manifest human form to work out its karma, remove dross impurities within. In absence of an atman soul, no human body could subsist of its own.
No sooner the master switches on the battery, Prana life energy flows into the body and activates the heart. As the heart takes over the system, it keeps pumping and the body starts functioning. No sooner heart becomes dead (the battery dies); it could not be sustained even by an atman.
In that case the released atman again manifests a new robot to start new cycle of life. The human form from spiritual point of view is actually a robot in control of Prana energy. As long as Prana energy remains in the system in the heart, it keeps pumping.
Prana is the sap of the body that primarily governs the whole system.