LIFEBLOOD
lifeblood
(noun) an essential or life-giving force; “water is the lifeblood of India”
lifeblood
(noun) the blood considered as the seat of vitality
Source: WordNet® 3.1
Etymology
Noun
lifeblood (usually uncountable, plural lifebloods)
Blood that is needed for continued life; blood regarded as the seat of life.
Hypernym: blood
(figuratively) That which is required for continued existence or function.
Synonym: essence
Source: Wiktionary
Life"blood`, n.
1. The blood necessary to life; vital blood. Dryden.
2. Fig.: That which gives strength and energy.
Money [is] the lifeblood of the nation. Swift.
Source: Webster’s Unabridged Dictionary 1913 Edition