Blood

"Oh. My. God. You're covered in..."

- Janissary James

Blood is used to transport nutrients through the body of a multicellular organism. In the case of humans, it uses a compound called hemoglobin to transport oxygen to the cells. Because of cellular respiration, every single cell in the human body needs oxygen, so blood must deliver oxygen to every cell in the body. This is most likely true all of the other Covenant races, though their blood may be delivering a different compound to the body, resulting in its difference in color to human blood.

Overview
Humans will bleed red blood when shot. Forerunner artifacts, such as the Menachite portal complex, may at times be manipulated by human blood. The Flood will always bleed a pasty, brownish-green color, and the explosion of a carrier form or a Pod infector will leave a brown stain on the ground. In gameplay, however, Pod infectors do not "bleed", but rather explode into bits of flesh.

When shot without the protection of an energy shield, the target will "bleed." In some races, the blood will visibly come out of the target, and leave a stain on the ground. The viscosity and color of the blood varies by race.

Blood and bleeding hold a special significance in Sangheili culture. Having one's blood spilled outside the battlefield is considered dishonorable; for this reason, doctors are not held in a high regard. Blood is also a repeated motif in Sangheili maxims or proverbs. Due to its cultural significance, "blood" is also a Sangheili curse word.

Blood colors

 * Human - Red
 * Sangheili - Dark blue/purple (Halo: Combat Evolved), purple (Halo 2 and Halo 3)
 * Jiralhanae - Dark navy blue/black (Halo 2), purplish red, thick (Halo 3), red (Halo: Reach)
 * Mgalekgolo - Bright orange
 * San'Shyuum - Red
 * Kig-Yar - Dark blue/purple (Halo: Combat Evolved), purple (Halo 2 and Halo 3)
 * Unggoy - Light blue, teal, very thick
 * Yanme'e - White, with slight green tint
 * Huragok - Reddish pink, blue purple
 * Flood - Brownish-green
 * Forerunner - Purple
 * Yonhet - Red
 * Sharquoi - Purple

Blood type
"Hang two units, O-neg, and he needs an airway."

- A UNSC Marine Corps medic during the Battle of Mombasa.

A blood type is a classification of blood based on the presence or absence of inherited antigenic substances on the surface of red blood cells. The blood type is often included on identification patches or on tattoos worn by military personnel, in case they should need an emergency blood transfusion; certain variants of MJOLNIR and ODST armors, for example, are equipped with MEDREF labels displaying the blood type of the wearer. Individuals with known blood types include:
 * Kurt Ambrose: O+
 * Michael Crespo: A-
 * Jorge-052: B+
 * John-117: O+
 * Edward Buck: O-
 * Samantha Wisner: O-
 * Taylor Miles: B+
 * T. Rymann O+
 * Kojo Agu: AB-
 * Jonathan Doherty: O+
 * Talitha Macer: AB+
 * Randall Aiken: AB-
 * Carris-137: A+
 * Caleb Aagard: B-
 * Alo Sebukah: E+

Trivia

 * Sometimes in Halo: Combat Evolved bullet holes with blood and/or blood splatters will be visible at certain angles though nothing was there or nothing was shot or hurt.
 * In Halo: Combat Evolved, when the player repeatedly melee Covenant species (most effectively on Jackals), an unlimited amount of blood will come out.
 * Sometimes in Halo: Combat Evolved on the Xbox, the game will lag if too many blood decals are being rendered.
 * The blood and the amount of blood of Covenant foes changes during the course of the three Halo games. For example, Grunts, Elites, Hunters and Jackals bled profoundly when meleed in Halo: Combat Evolved, and yet, in Halo 2, the player must shoot the corpse in order for it to bleed. In Halo 3, the amount of blood is lessened dramatically, but is given in greater detail. Grunt blood also seems more viscous, as opposed to slightly watery looking in the earlier games.
 * In Halo 3, when a Brute is shot and blood is spilled, there seems to be odd luminous chunks in the blood. This can also be seen in human blood as well.
 * The Brutes are the only race to have significantly different colored blood from one game to another, although in some Halo Legends episodes, other species also have different blood color than the games.
 * As a unique feature among Forerunner machines (such as sentinels or monitors), the Promethean constructs "bleed" an orange fluid when damaged or destroyed.
 * In Halo 5, Grunts bleed more profusely than other alien species or humans. After death, their corpses can also be shot to cause more luminescent blue bleeding.