The color of blood is a direct result of the molecule responsible for carrying oxygen. While blood is universal across many life forms, the specific chemical composition of this oxygen-carrying molecule determines its hue. This variation is seen across the animal kingdom and can also manifest as abnormal color shifts within human circulation. Understanding the chemistry behind this coloration involves metallic elements and evolutionary adaptation.
The Iron Foundation Why Human Blood Appears Red
The characteristic red color of human blood is due to the protein hemoglobin, packed inside red blood cells. This complex molecule contains iron atoms at its core, specifically in the ferrous (Fe2+) state, which are responsible for binding oxygen. When oxygen attaches to the iron, the resulting compound, called oxyhemoglobin, strongly absorbs blue-green light, making the blood appear bright scarlet.
As blood circulates and delivers oxygen to tissues, the iron releases the oxygen molecule. This deoxygenated state, known as deoxyhemoglobin, changes the light absorption properties, causing the blood to become a darker, deeper red. Human blood is never blue inside the body; the blue appearance of veins under the skin is an optical illusion caused by how light penetrates and reflects off the skin and tissue layers.
Abnormal Color Shifts in Human Circulation
Changes in hemoglobin’s chemical structure can lead to shifts in human blood color. One condition is methemoglobinemia, where the iron atom is oxidized from the normal ferrous (Fe2+) state to the ferric (Fe3+) state. This altered molecule, methemoglobin, cannot bind oxygen, leading to functional anemia. When methemoglobin levels are elevated, the blood takes on a chocolate-brown color, causing the skin to appear bluish or purple (cyanosis). This condition is usually acquired through exposure to certain oxidizing agents, such as some medications or industrial chemicals, though it can be inherited.
A rarer condition is sulfhemoglobinemia, which occurs when a sulfur atom binds to hemoglobin, creating sulfhemoglobin. This derivative renders the blood permanently dark green to blackish-green. Sulfhemoglobin cannot effectively carry oxygen. Since this is an irreversible change in the red blood cell, the condition only resolves once the body replaces the damaged cells, which takes up to three months.
The liquid component of blood, the plasma, can also change color due to disease or diet. High levels of fats (lipemia) can make the plasma appear cloudy and milky white. Similarly, the buildup of the bile pigment bilirubin due to liver dysfunction causes jaundice, which turns the plasma and tissues a yellowish hue.
Diverse Hues in the Animal Kingdom
The animal kingdom features diverse blood colors determined by different oxygen-carrying proteins and their metallic core. The most common alternative is blue blood, found in many mollusks (like octopuses and snails) and arthropods (including horseshoe crabs and spiders). These creatures utilize hemocyanin, a protein that employs copper atoms (Cu2+) instead of iron to bind oxygen. When deoxygenated, hemocyanin is colorless, but oxygen exposure causes the copper to reflect blue light. This copper-based pigment is thought to be more efficient at transporting oxygen in cold, low-oxygen environments, suitable for many marine invertebrates.
Another color is green, resulting from chlorocruorin, an iron-containing pigment found dissolved in the blood of certain marine segmented worms, such as lugworms. Chlorocruorin is chemically similar to hemoglobin but contains a different chemical group that makes its solution appear light green when oxygenated, though it can look light red when highly concentrated.
Some marine invertebrates, such as peanut worms and brachiopods, have violet or pink blood due to the protein hemerythrin. Hemerythrin uses iron to bind oxygen but lacks the characteristic heme group. It turns violet-pink when oxygenated, becoming colorless when deoxygenated.