lifeblood

Blood regarded as essential for life.

An indispensable or vital part.

The blood necessary to life; vital blood.

That which is essential to the existence or strength of something; that which constitutes or gives strength and energy. Also life's-blood, or, preferably, life's blood.

In pathology, the more or less constant spasmodic quivering of the eyelid or lip: also called life's-blood, live-blood, and cillo.