Blood

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

Food
Some organisms consumed blood in order to survive, namely Vampires, Haemovores and Plasmavores. (DW: State of Decay, The Curse of Fenric, Smith and Jones)

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 and Androgums did not subsist off of blood exclusively but would consume it as part of a more varied diet. (BFA: Storm Warning, DW: The Two Doctors)

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

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

Some species, including the Sycorax, used a form of biotechnology known as blood control. (DW: The Christmas Invasion)

Judoon scanners checked blood to gather information on the units species. (DW: Smith and Jones)

Colour
There are many variatians in blood cloour. Human and Time Lord blood is red but species such as Sontarans have red blood. Many species of insect including Mayflies have yellow blood.