lifeblood

noun2

1. •
   Blood that is needed for continued life; blood regarded as the seat of life.

   • [Y]ou desire his mana, yet you respect his tabu, for in you and him alike runs the common life-blood. — 1920
2. •
   (figurative) That which is required for continued existence or function.

   • Gasoline is the lifeblood of the modern city.