Blood circulation is a fundamental process that sustains life, delivering essential materials throughout the body. This fluid, which makes up about 7% of a person’s body weight, is recognizable by its characteristic red color. Fresh blood appears a brilliant, bright scarlet red, which directly indicates the successful transport of oxygen from the lungs to the body’s tissues.
The Bright Scarlet of Oxygenated Blood
The bright scarlet color is most commonly associated with a bleeding injury involving an artery or surface capillary. This vivid shade indicates oxygen-rich blood that has been fully saturated with oxygen from the lungs. Arterial blood, which is under higher pressure and travels away from the heart, carries the maximum possible payload of oxygen. This high concentration of oxygen produces the intensely bright red appearance and prevents the blood from darkening before exposure to air.
Hemoglobin: The Pigment and the Mechanism
Blood is red because of hemoglobin, a specialized protein contained within red blood cells. Each hemoglobin molecule is built around four heme groups, with a single iron atom at the center of each group. This iron atom is the site where oxygen temporarily binds for transport. When oxygen binds, the molecular structure of the hemoglobin complex shifts, affecting how light interacts with the protein. This change causes the molecule to absorb light from the blue-green spectrum and reflect the complementary bright scarlet red observed in oxygenated blood.
Why Deoxygenated Blood Looks Darker
As blood circulates, it reaches tissues and releases oxygen to fuel cellular activity. Once the oxygen detaches from the iron atom, the hemoglobin protein undergoes a conformational change. This structural alteration changes the molecule’s light absorption properties. In its deoxygenated state, the blood takes on a much darker, deep crimson or maroon shade instead of bright scarlet. This darker blood flows through the veins, returning to the heart and lungs to be re-oxygenated.
When Blood Color Indicates an Issue
While blood is always a shade of red, colors outside the normal bright scarlet or deep crimson range can signal a medical issue. For instance, carbon monoxide poisoning causes the blood to appear an abnormally bright, cherry-red color. This happens because carbon monoxide binds to hemoglobin with a higher affinity than oxygen, forming carboxyhemoglobin, which retains a saturated red hue but cannot deliver oxygen. Blood that appears brownish or blue-tinged can indicate methemoglobinemia, a condition where the iron in the heme group is oxidized and cannot bind oxygen effectively. Blood exposed to air, such as in dried wounds, oxidizes and turns a dark brown or black color, while pink or very pale blood can indicate dilution or severe anemia.