The Collaborative International Dictionary of English v.0.48:

Lifeblood \Life"blood`\ (l[imac]f"bl[u^]d`), n.
   1. The blood necessary to life; vital blood. --Dryden.
      [1913 Webster]

   2. Fig.: That which gives strength and energy.
      [1913 Webster]

            Money [is] the lifeblood of the nation. --Swift.
      [1913 Webster]

WordNet (r) 3.0 (2006):

lifeblood
    n 1: the blood considered as the seat of vitality
    2: an essential or life-giving force; "water is the lifeblood of
       India"

Moby Thesaurus II by Grady Ward, 1.0:

96 Moby Thesaurus words for "lifeblood":
   Rh factor, Rh-negative, Rh-positive, Rh-type, Rhesus factor, anima,
   animating force, antibody, antigen, arterial blood, atman,
   bathmism, beating heart, biological clock, biorhythm, blood,
   blood bank, blood cell, blood count, blood donor,
   blood donor center, blood group, blood grouping, blood picture,
   blood platelet, blood pressure, blood serum, blood substitute,
   bloodmobile, bloodstream, breath, breath of life, circulation,
   clinical dextran, dextran, divine breath, divine spark, elan vital,
   erythrocyte, essence of life, force of life, globulin, gore,
   growth force, grume, heart, heartbeat, heartblood, hematics,
   hematologist, hematology, hematoscope, hematoscopy, hemocyte,
   hemoglobin, hemometer, humor, ichor, impulse of life,
   inspiriting force, isoantibody, jiva, jivatma, leukocyte,
   life breath, life cycle, life essence, life force, life principle,
   life process, living force, neutrophil, opsonin, phagocyte, plasma,
   plasma substitute, pneuma, prana, red corpuscle, seat of life,
   serum, soul, spark of life, spirit, type O, venous blood,
   vis vitae, vis vitalis, vital energy, vital flame, vital fluid,
   vital force, vital principle, vital spark, vital spirit,
   white corpuscle