Blood

Biological Use
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.

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 (Halo: CE), Purple (Halo 2 and Halo 3).
 * Jiralhanae - Black (Halo 2), Red (Halo 3).
 * Mgalekgolo - Orange.
 * Kig-yar - Purple, thick.
 * Unggoy - Light Blue, teal.
 * Yanme'e - Whitish-green.

Others
As in reality, Humans will bleed red blood when shot. The Flood will always bleed a pastey, brownish-green color, and the explosion of a Carrier Form or an Infection Form will leave a brown stain on the ground. Regardless of the host organism, Flood combatants will always bleed a brown colored blood.

Trivia
Sometimes on Halo: Combat Evolved bullet holes with blood and/or blood splaters will be visble at certain angles though nothing was here or nothing was shot or hurt.