Blood is always red inside the human body, a color resulting directly from the transport of oxygen. The appearance of blood can range from a brilliant scarlet to a deep maroon, but it never naturally appears blue or green. Understanding the chemistry behind this hue explains why these color variations occur and when a change in color might signal a health concern.
The Chemical Reason Blood Is Red
The color of blood originates from the millions of tiny red blood cells that circulate throughout the body. Within these cells is the protein known as hemoglobin, which is responsible for binding and transporting oxygen. Each hemoglobin molecule is built around four ring-shaped chemical structures called heme groups.
At the center of each heme group sits a single iron atom. The interaction between this iron atom and oxygen gives blood its distinctive red coloration. When oxygen binds to the iron, it alters the molecule’s light absorption and reflection properties, resulting in a vibrant red.
Why Blood Has Different Shades of Red
The shade of red observed in blood is determined by its oxygen saturation level. Arterial blood, which has just been oxygenated in the lungs and is traveling to the body’s tissues, is highly saturated. This high saturation results in the bright, vivid scarlet color often seen in fresh wounds.
Conversely, venous blood has already delivered its oxygen load to the tissues and is returning to the heart and lungs. The hemoglobin in this blood is deoxygenated, causing it to appear a darker, deeper red, sometimes described as a dark maroon or brick red. This difference in color is purely a function of oxygen content. The common misconception that deoxygenated blood is blue stems from the visual illusion created by light passing through the layers of skin and tissue above the veins.
When Blood Color Indicates a Health Issue
Any color appearing outside of the bright red and dark red spectrum can indicate a serious medical problem caused by toxins or an abnormal form of hemoglobin. A notable example is the presence of carboxyhemoglobin, which occurs when carbon monoxide binds to hemoglobin instead of oxygen. This compound is a bright, cherry-red color, and its presence signals carbon monoxide poisoning, a life-threatening condition.
Another pathological color change results in blood that appears a chocolate-brown color, a symptom of methemoglobinemia. This condition occurs when the iron atom in the hemoglobin molecule is oxidized to the ferric (Fe3+) state, which renders it unable to bind to oxygen effectively. When methemoglobin levels are elevated, the blood takes on this characteristic brown shade.
In extremely rare cases, a greenish or bluish tinge to the blood can be observed, which is a hallmark of sulfhemoglobinemia. This condition is typically drug-induced and happens when a sulfur atom incorporates itself into the hemoglobin structure, creating a greenish derivative called sulfhemoglobin. Unlike normal hemoglobin, sulfhemoglobin is permanent for the life of that red blood cell and is completely incapable of transporting oxygen.