Blood

Blood was an essential fluid of many life forms, Gallifreyans and humans included.

Human blood, which was red, was divided into two types of blood cells, red and white, with red containing iron. (TV: Bad Wolf) Gwen Cooper associated blood with a sour metallic smell. (PROSE: Slow Decay)

Some people, like Chief Steward Weeks, didn't enjoy the sight of blood. (AUDIO: Storm Warning)

Sleep dust contained, among other things, blood cells. (TV: Sleep No More)

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

The Three Who Rule would only drink the blood of the living. However, they killed villagers and used tubing to store their blood in the fuel tanks of the Hydrax. This was then fed to the King Vampire. (TV: State of Decay)

On their home planet, the Ogri subsisted on amino acids, but found that the blood of Earth life forms, including humans, would also suffice. This led to human sacrifices at Stonehedge. (TV: The Stones of Blood)

Other species, such as Vortisaurs (AUDIO: Storm Warning) and Androgums, (TV: The Two Doctors) did not subsist off of blood exclusively, but would consume it as part of a more varied diet. While stranded in four-dimensional space, the Eighth Doctor claimed that Vortisaurs could smell the space-time in his Time Lord blood. (AUDIO: Storm Warning)

As Vastra herself implied, she would drink human blood as a refreshment, calling it "the colour of red wine". (TV: The Snowmen)

Black pudding contained pig's blood; it was consumed by humans as part of a full English breakfast. (PROSE: Slow Decay)

At least one Osiran, Sekhmet, enjoyed feasting on blood. (AUDIO: The Bride of Peladon)

Practical use
The magic potion Lady Peinforte made to travel in time required human blood. (TV: Silver Nemesis)

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

Blood could be transferred to another by the process of blood transfusion. During the Great War, Dr. Sturgis was experimenting with it and threatened the life of the Eighth Doctor by attempting to transfuse blood into his veins. (AUDIO: The Great War) On another occasion, the same incarnation gave blood to his fourth incarnation via this process. (PROSE: The Eight Doctors)

Mortimus used the blood of five species whose destinies the Time Lords had affected in a ritual to summon Artemis, the Chronovore. (PROSE: 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. (PROSE: Alien Bodies)

Judoon scanners checked blood to gather information on the unit's species. (TV: Smith and Jones)

In 2011, Jack Harkness's immortal blood was used by the Three Families to cause Miracle Day, by inserting it into both ends of the Blessing simultaneously. Later that year, the Miracle was stopped when Jack, now mortal because of Miracle Day, inserted his blood into one end of the Blessing at Shanghai, while Rex Matheson, who had a transfusion of Jack's mortal blood, inserted it into the other end in Buenos Aires. (TV: The Blood Line)

As a failsafe, Sekhmet's trisilicate prison was blood locked. The lock required the blood of four royal women to properly release Sekhmet. (AUDIO: The Bride of Peladon)

When trapped in 1941 and trying to send a message to her 21st century Torchwood Three team, Toshiko Sato cut her hand with a sharp edge of a can, and wrote on paper with her blood. (TV: Captain Jack Harkness)

The Twelfth Doctor had a gizmo able to take a sample and examine a specimen of human blood. He administered such a test on Alan-a-Dale, a human from the 12th century, and made a prognosis of several diseases and about six months of life expectancy. (TV: Robot of Sherwood)

Blood types
On at least Earth (TV: The Christmas Invasion) and Gallifrey, (PROSE: A Big Hand for the Doctor) blood was organised into different types.

Colour
There were many variations in blood colour.

Human, (TV: Smith and Jones) Time Lord (TV: The Two Doctors, Heaven Sent, Extremis, The Timeless Children) and Raxacoricofallapatorian (PROSE: The Monsters Inside) blood was red. Time Lord blood was a darker shade of red than human. (TV: The Two Doctors, The Christmas Invasion, Extremis)

The 456 also had red blood. (TV: Children of Earth: Day Five)

The Ood had purple blood, (TV: Planet of the Ood) as did the Venusians. (PROSE: Venusian Lullaby)

One pilot's species had pale blue blood. (PROSE: Joyride)

Foamasi, Kaled/Kaled mutant, Ice Warrior and Meep blood was green. (TV: The Leisure Hive, Revelation of the Daleks, COMIC: Doctor Who and the Star Beast, AUDIO: The Prisoner Of Peladon)

Sontaran blood was mostly green, (TV: The Two Doctors, The Last Sontaran) although according to one account it was red. (HOMEVID: Battlefield)

The Waro had yellow-green blood. (PROSE: The Devil Goblins from Neptune)

Mayflies, Judoon and Krafayis had yellow blood. (TV: Reset, Prisoner of the Judoon, PROSE: Blind Fury)

Nostrovites had black blood. (TV: Something Borrowed)

Silent blood was white. (TV: Day of the Moon)

Other references
Other substances necessary to the existence and activity of living beings were sometimes compared to blood: static electricity, vital to the life of Dalek creatures and to the functioning of their casing, was described by the Second Doctor as "like blood" to the Daleks, (TV: The Power of the Daleks) and Dalek Sec theatrically referred to the genetic fluid flowing into vacated human bodies to create the Dalek-Humans as "the life-blood". (TV: Evolution of the Daleks)