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 and hormones 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.

In the Halo Series
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.

Covenant Species

 * 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).
 * Mgalekgolo - Bright Orange.
 * San 'Shyuum - Red.
 * Kig-yar - Purple, thick.
 * Unggoy - Light Blue, Teal, Very Thick.
 * Yanme'e - White, with slight green tint.
 * Huragok - Reddish pink.

Others
As in reality, Humans will bleed red blood when shot. Forerunner artifacts, such as the Menachite Forerunner 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 an Infection Form will leave a brown stain on the ground. In gameplay, however, Infection forms do not "bleed", but rather explode into bits of flesh.

Trivia

 * Sometimes on 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(Bullet Hole Glitch).
 * In Halo: The Flood, the word ichor was used instead of the word blood. Ichor was the golden blood of the Greek Gods of Olympus.
 * 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. You can see this in human blood as well.
 * Sometimes in Halo: Combat Evolved on the Xbox, the game will lag if too many blood decals are being rendered.
 * "Blood" is a Sangheili curse word.
 * 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.
 * In Halo: Combat Evolved, when you melee Covenant species (most effectively on Jackals), an unlimited amount of blood will come out.
 * In the Halo: CE level "The Silent Cartographer" when you melee a dead Hunter, he we squirt out Flood blood.
 * In Halo: Reach, blood is less "messy", but much more rich and detailed. It will only appear on the environment when one player is signed in.
 * Unggoy Blood has been described to actually have a stench; close to something like Rotton eggs.