Why Does Blood Turn Black?
Blood doesn't inherently turn black; its color change depends on several factors, including oxidation, the presence of specific compounds like sulfur, and the location of bleeding.
When exposed to air, blood undergoes oxidation. This chemical reaction causes the iron in hemoglobin to change, resulting in a darker color. Initially, this might appear as dark red or brown before potentially darkening further. This is the reason dried blood often appears black or dark brown. Quora explains this process, highlighting the reaction of oxygen with iron in hemoglobin.
Additionally, blood low in oxygen, such as blood in the veins or in certain medical conditions, appears dark red or even bluish. This deoxygenated state is significantly darker than oxygen-rich arterial blood. MedlinePlus notes that deoxygenated blood is a dark bluish-red, contrasting with the brighter red of oxygenated blood.
Sulfhemoglobinemia
A less common reason for dark or black-appearing blood is the presence of sulfhemoglobin. This abnormal hemoglobin variant contains sulfur bound to the iron atom, preventing oxygen binding. Without oxygenated iron, the blood loses its characteristic red hue and can appear dark blue, green, or even black. As noted by McGill University, the sulfur atom's interference with oxygen binding leads to the color change.
Gastrointestinal Bleeding
Blood in the digestive tract can also appear black, especially in the stool. This isn't because the blood itself has turned black, but due to the digestive process. As the blood travels through the digestive system, it is chemically altered and mixed with other digestive substances, causing a darkening effect. According to Cleveland Clinic, this process transforms blood into a black, tarry substance (melena). It takes a significant amount of blood (50ml or more) to make the stool distinctly black. NCBI confirms this. Iron supplements can also darken stool to an almost black color. UpToDate mentions this effect.
Gangrene
In cases of gangrene, where blood flow to tissue is cut off, the affected area turns black due to tissue death and the lack of oxygenated blood. Johns Hopkins Medicine highlights this discoloration as a characteristic symptom of dry gangrene.
Menstrual Blood
Menstrual blood can also vary in color, appearing darker and sometimes almost black. This is typically due to older blood that has been exposed to air and has oxidized over time before exiting the uterus. Medical News Today describes this oxidation process, indicating that older menstrual blood darkens before ultimately exiting the body.
