Human blood is universally recognized by its distinct red color. This consistent hue often leads to questions about its underlying biological cause. The answer lies within its microscopic components and the chemistry that governs their function.
What Blood Is Made Of
Blood is a complex fluid composed of several main elements, each playing a distinct role. It includes plasma, a yellowish liquid that constitutes over half of the total blood volume and carries various substances like proteins, salts, and nutrients. Suspended within this plasma are different types of blood cells: white blood cells for immune defenses, and platelets for blood clotting. The most numerous components are red blood cells, also known as erythrocytes, which account for approximately 40% to 45% of blood’s volume. These red blood cells are the primary reason for blood’s characteristic color.
Hemoglobin and Iron
The red color of human blood originates from hemoglobin, a protein found within red blood cells. Hemoglobin’s main function is to transport oxygen from the lungs to the body’s tissues and return carbon dioxide. Each hemoglobin molecule contains four subunits, each with a ring-shaped heme group. At the center of each heme group sits a single iron atom. This iron atom directly interacts with oxygen, giving blood its red appearance. When light interacts with hemoglobin, the iron’s unique chemical structure absorbs all colors of light except red. The red wavelengths are then reflected, making the blood appear red. The presence of iron is fundamental to blood’s crimson hue.
How Oxygen Changes Blood Color
Blood’s shade can vary depending on its oxygen content. Oxygenated blood, which has picked up oxygen in the lungs and is pumped through arteries, appears a bright, scarlet red. As this blood delivers oxygen to the body’s tissues and becomes deoxygenated, its color shifts to a darker, maroon red. This change in hue occurs because the binding and release of oxygen molecules alter the shape of the hemoglobin protein, which in turn subtly changes how light is absorbed and reflected.
A common misconception is that deoxygenated blood is blue. This is inaccurate; human blood is never blue. The bluish appearance of veins seen through the skin is an optical illusion caused by how light penetrates and reflects off the skin and blood vessels. The skin scatters blue light more effectively than red light, making the underlying dark red deoxygenated blood appear blue or sometimes green.
Blood Colors Beyond Red
While red blood is characteristic of humans and most vertebrates, the animal kingdom exhibits diverse blood colors. Some invertebrates, such as octopuses, squids, and horseshoe crabs, possess blue blood. Their blood contains a copper-based protein called hemocyanin, which binds oxygen and gives their blood a blue tint when oxygenated. This copper functions similarly to iron in human hemoglobin.
Other creatures display different blood colors. Certain segmented worms and leeches have green blood due to a respiratory pigment called chlorocruorin, an iron-containing protein similar to hemoglobin. Additionally, some skink species, a type of lizard, have vivid green blood because of high concentrations of a green bile pigment called biliverdin. These examples highlight that the color of blood is determined by the specific oxygen-carrying molecules present in different species.