Kids believed that blood inside your body is actually blue, and it only turns red when it's exposed to air or comes out as a wound.
Blood is always red inside the body. Oxygenated blood is bright red, and deoxygenated blood is darker red, but never blue. The blue appearance of veins under the skin is an optical illusion caused by how light scatters through tissue layers. Venous blood drawn in medical labs is dark red, not blue, confirming blood is red regardless of oxygen levels.
Reception
Sources
- Wikipedia: Blood PRIMARY
- Snopes: Blue Blood Myth REFERENCE
- Mayo Clinic: Blood Color and Oxygen REFERENCE