Blood

Blood is an essential fluid of many life forms, Gallifreyans and Humans included.

=Significance=

Food
Some species, such as Vampires and Haemovores (DW: The Curse of Fenric) consumed blood in order to survive.

On their home planet, the Ogri subsisted on amino acids, but found that the blood of Earth life forms, including Humans, would do just as well. (DW: The Stones of Blood)

Other species, such as Vortisaurs (BFA: Storm Warning) and Androgums (DW: The Two Doctors) do not subsist off of blood exclusively but will consume it as part of a more varied diet.

Practical use
Mortimus used the blood of five species whose destinies the Time Lords had affected, in order to summon Artemis, the Chronovore. (NA: No Future)

Blood also formed an important part of some Faction Paradox rituals, some of the time biodata is extracted from the blood of a being to use in a Faction Rite or Ritual. (EDA: Alien Bodies)

The Sycorax and other species used a form of biotechnology known as blood control. (DW: The Christmas Invasion)