Different blood types are not different colors. All human blood, regardless of its type, shares the same fundamental red color, determined by a specific protein found within the red blood cells. The color of this circulating tissue is entirely dependent on the presence and state of the oxygen-carrying molecule within those red cells. The microscopic markers that define an individual’s blood type have no bearing on this physical color.
The Chemistry That Colors All Human Blood Red
The universal red color of human blood is due to the protein hemoglobin (Hgb), which is contained within the red blood cells. Hemoglobin is a complex molecule designed to transport oxygen from the lungs to the body’s tissues. The structure of this protein includes four subunits, each containing a heme group with a single iron atom at its core.
It is this iron atom that reversibly binds to oxygen, and the interaction between iron and oxygen determines the blood’s precise shade of red. When oxygen is fully bound to the hemoglobin, the resulting compound, called oxyhemoglobin, appears bright, vivid red. This is the color of blood circulating through the arteries.
When the hemoglobin releases its oxygen to the body’s tissues, it transforms into deoxyhemoglobin. This deoxygenated state causes a change in the protein’s shape and its light absorption properties. Consequently, the blood becomes a darker, deeper red, often described as maroon. This chemical mechanism is consistent across every human, meaning blood color is identical regardless of blood type under the same oxygenation conditions.
The Invisible Markers That Define Blood Type
The differences between blood types are not visible to the naked eye but are defined by microscopic structures on the surface of the red blood cells. These structures are called antigens, which are essentially protein or sugar markers extending outward from the cell membrane. The ABO blood group system is defined by the presence or absence of two specific antigens, A and B.
The ABO System
A person with Type A blood has the A antigen marker on their red cells, while a person with Type B blood has the B antigen marker. Individuals with Type AB blood possess both the A and B markers, and those with Type O blood have neither of these specific markers. These antigens are complex carbohydrate chains, often attached to proteins or lipids on the cell surface.
The plasma, the liquid component of blood, contains corresponding antibodies that will react against any foreign antigens. Type A blood plasma contains anti-B antibodies, and Type B plasma contains anti-A antibodies. Type O blood has both anti-A and anti-B antibodies, and Type AB blood has neither. This antigen-antibody relationship dictates compatibility for blood transfusions.
The Rh Factor
In addition to the ABO system, the Rh factor determines whether a blood type is positive or negative. The Rh factor is a separate protein, the D antigen, which is either present (Rh-positive) or absent (Rh-negative) on the red blood cell surface.
The differences between the eight major blood types (A+, O-, AB+, etc.) are purely immunological and biochemical. They involve tiny surface molecules, not pigments that would alter the overall red color.
Why Blood Color Appears to Change (But Isn’t Related to Type)
Although blood type does not affect color, blood color does appear to change based on its location in the body, which can cause confusion. The most common variation is the difference between arterial blood and venous blood. Arterial blood, which is pumped away from the heart and lungs, is high in oxygen saturation and is the bright red color associated with oxyhemoglobin.
Venous blood, which is returning to the heart after delivering oxygen to the tissues, carries lower oxygen levels. This deoxygenated blood is darker, appearing as a deep maroon or dark red due to the structure of deoxyhemoglobin. This is why blood drawn during a standard venipuncture procedure often appears much darker than blood from a superficial cut.
A common misconception is that deoxygenated blood is blue because veins often look blue through the skin. This blue appearance is an optical illusion caused by how light interacts with skin and tissue. When light penetrates the skin, the red wavelengths are absorbed by the blood, but the blue wavelengths are scattered back to the eye, making the deeper veins appear bluish or greenish. The blood inside those veins, regardless of whether the person is A-positive or B-negative, remains a shade of red.