Human Blood: What Color is Our Blood? Is It Actually Blue?

The blood in a healthy human’s veins is red.

Our erythrocytes, also known as red blood cells, are called “red” for a reason: heme molecules. These molecules are structurally red. Therefore, a blood molecule that doesn’t carry oxygen will appear in red.

However, sometimes the blood in our veins, especially when it is not clean and do not carry oxygen, appear blue, purple, or even green, leading people to think that the original color of blood is blue, not red. This is completely wrong. Even venous blood is never blue, it only looks that way through the skin. Blood’s color varies between bright red (clean blood) and dark red (venous blood).

Hemoglobin molecules, which are normally oxygen carriers, are called "oxyhemoglobin" and these molecules, which make up 96% of red blood cells, are red. Nevertheless, hemoglobin can also carry 10% carbon dioxide and when it carries such, a “carboxyhemoglobin” molecule that looks blue through the skin is formed.

Due to some diseases, blood may appear to be blue, this is called “cyanosis”. For example, when hemoglobin is permanently oxidized (turned into a Fe⁺² ion instead of Fe⁺³), it turns into a brown molecule called "methemoglobin" and can no longer carry oxygen. It looks blueish through skin. This makes up 2-3% of red blood cells.

Sometimes hemoglobin partially binds oxygen and this is called "sulphemoglobinemia". In this very rare disease, the veins appear blue. In some invertebrates (especially shrimp or crabs) the blood is indeed blue. However, in vertebrates, blood is always red in color.