Deoxygenated blood is blue
Human blood is always red, never blue, regardless of its oxygen level. Deoxygenated blood in veins is a darker red or maroon color, not blue. Veins appear bluish or greenish through the skin because of how different wavelengths of light penetrate and reflect off subcutaneous tissue, not because the blood inside them is actually blue.
What we know
The belief that blood is blue while inside the body, only turning red upon exposure to oxygen in the air, is an extremely common misconception, often reinforced visually by medical diagrams, textbook illustrations, and anatomical charts that conventionally color veins blue and arteries red purely as a visual coding convention to help distinguish the two types of vessels, not as an attempt to represent the actual color of blood inside them.
Human blood owes its red color to hemoglobin, an iron-containing protein within red blood cells responsible for binding and transporting oxygen throughout the body. The iron atom at the center of each hemoglobin molecule's heme group changes its light-absorbing properties depending on whether it is bound to oxygen or not, but critically, this change never produces a blue color at any point in the cycle. Oxygenated blood, found in arteries and freshly oxygenated tissue, appears bright red because oxygen-bound hemoglobin (oxyhemoglobin) strongly reflects red wavelengths of light. Deoxygenated blood, found in veins on its way back to the lungs, appears a darker red or maroon color, sometimes described as brownish-red, because deoxygenated hemoglobin absorbs light somewhat differently, but this darker shade remains firmly within the red-brown part of the visible spectrum and never crosses into blue at any point in normal human physiology.
The visual perception that veins appear blue or greenish through the skin is a distinct optical phenomenon involving how light interacts with skin and underlying tissue, not a reflection of the blood's actual color. When light strikes skin overlying a vein, different wavelengths penetrate to different depths and are absorbed and scattered differently by skin and fatty tissue. Blue and green light, which have shorter wavelengths, tend to penetrate less deeply into tissue and scatter back toward the observer's eye more readily than red light, which penetrates deeper and is more completely absorbed by the tissue and blood, before it can scatter back out. This differential light penetration and scattering effect, well documented and quantitatively modeled in biomedical optics research, is what produces the visual impression of a bluish or greenish vein beneath the skin, even though the blood inside that vein, if drawn out and directly viewed, would appear dark red, never blue.
This can be directly demonstrated through simple, repeatedly performed clinical and educational observation: blood drawn from a vein during a routine blood draw is visibly dark red immediately upon leaving the body, before any exposure to atmospheric oxygen has had time to occur, directly contradicting the idea that blood is blue while inside the body and only becomes red upon air exposure. Some marine and invertebrate animals do have blood or blood-like fluid that is genuinely blue, including horseshoe crabs and octopuses, whose respiratory pigment is hemocyanin, a copper-based molecule rather than the iron-based hemoglobin used by humans and other vertebrates, and this genuine, well-documented instance of blue blood in other parts of the animal kingdom may contribute to why the human-blue-blood misconception feels intuitively plausible to some people, even though the underlying biochemistry involved is entirely different from human blood chemistry.
Biology and anatomy educators, along with phlebotomy and medical training programs, specifically address this misconception directly in coursework, given how frequently it recurs among students and the general public, often using the direct observation of freshly drawn venous blood as the simplest and most immediately convincing demonstration that human blood is never blue at any point in its oxygenation cycle.
Common claims
- Blood is blue inside the body and turns red on contact with air.Not supported
- Deoxygenated blood is dark red or maroon, not blue.Accurate
- Veins appear blue due to light scattering through skin, not blood color.Accurate
- Some invertebrate animals have genuinely blue blood.Accurate