Human blood is not actually blue. The appearance of blue veins is an optical illusion. The color we perceive is influenced by how our eyes process light and color, not the inherent color of the blood itself. Deoxygenated blood is darker red, and the way light penetrates the skin creates the blueish hue we often see in veins.
The Reality of Blood Color
- Oxygenated Blood: Blood pumped directly from the heart is rich in oxygen and bright red.
- Deoxygenated Blood: Blood returning to the heart has less oxygen and is a darker, more purplish-red. It is never blue.
- Hemoglobin: The red color comes from hemoglobin, a protein containing heme, crucial for oxygen transport.
The misconception of blue blood likely stems from a combination of factors, including:
- Optical Illusion: The way light interacts with the skin and veins can make the darker, deoxygenated blood appear blue.
- Cultural Associations: The term "blue blood," meaning aristocratic lineage, developed separately, possibly due to the pale skin of the upper classes, reflecting less red blood. The historical origins, traced to the early 19th century and even earlier notions, are not directly related to the actual color of blood. (https://www.historyextra.com/period/georgian/why-people-noble-called-blue-blooded/, https://www.quora.com/How-did-the-term-Blue-Blood-come-about)
Some organisms do have blue blood. This is because they use hemocyanin, a different oxygen-carrying protein, which is blue. Horseshoe crabs are a prime example. (https://biobeat.nigms.nih.gov/2019/02/roses-are-red-and-so-is-blood/)
However, this is not the case for humans. Our blood is always shades of red, varying in brightness depending on the oxygen level. (http://theconversation.com/blood-in-your-veins-is-not-blue-heres-why-its-always-red-97064)
