Blood color varies widely across the animal kingdom, far beyond the familiar red seen in humans. This diversity is directly linked to the specific oxygen-carrying molecules in an animal’s circulatory system. Different chemical compounds used for oxygen transport result in varied blood hues, reflecting diverse evolutionary adaptations for circulating oxygen.
What Makes Blood Red
The red color of blood in humans and many vertebrates is primarily due to hemoglobin, a protein found within red blood cells. Hemoglobin binds and transports oxygen throughout the body. Each hemoglobin molecule contains a heme component, crucial for this oxygen-carrying function.
At the center of each heme group, an iron atom binds to oxygen, causing blood to appear bright red. As oxygen is released to tissues, blood darkens to a more purplish red. This iron-oxygen interaction is the primary reason for the characteristic red color of vertebrate blood.
The Chemical Palette of Animal Blood
Beyond red, animal blood comes in a surprising array of colors, each determined by a different oxygen-binding protein. These pigments showcase diverse biochemical adaptations, with the observed color typically appearing when the pigment is oxygenated.
Many mollusks (e.g., octopuses, squids, snails) and arthropods (e.g., crabs, lobsters, horseshoe crabs) have blue blood. Their blood contains hemocyanin, a copper-based protein that turns blue when it binds to oxygen, giving it a distinct azure tint.
Certain marine worms, including annelids like leeches and lugworms, exhibit green blood due to chlorocruorin. This iron-based pigment is structurally similar to hemoglobin but imparts a green hue when oxygenated. Interestingly, chlorocruorin can appear green when deoxygenated but turns red when oxygenated, depending on its concentration. New Guinean skinks also have green blood, where high concentrations of the green bile pigment biliverdin, a byproduct of hemoglobin breakdown, overwhelm their hemoglobin’s red color.
Purple blood is found in some marine invertebrates, such as peanut worms (sipunculids) and brachiopods (lamp shells). Their pigment, hemerythrin, is iron-based like hemoglobin but lacks the heme group. When oxygenated, hemerythrin gives their blood a violet-pink or purple color; it is colorless when deoxygenated.
The Antarctic icefish possesses virtually colorless or clear blood, lacking hemoglobin and red blood cells entirely. They rely on oxygen dissolved directly in their blood plasma for transport. Many insects also have hemolymph, a fluid analogous to blood that appears clear or yellowish because it does not carry oxygen and lacks respiratory pigments.
Why Blood Colors Differ
Blood colors differ due to the various respiratory pigments animals have evolved to transport oxygen. These pigments, despite varied metallic components and structures, serve the same purpose. The choice of pigment often adapts to an animal’s specific environmental conditions.
Hemocyanin, found in blue-blooded animals, is effective at transporting oxygen in cold, low-oxygen environments. This efficiency benefits marine invertebrates in deep-sea or cold-water habitats. The metals (e.g., iron, copper) in these pigments determine their color when oxygenated. This color is a byproduct of the chemical structure and light interaction, not a direct adaptive purpose.
Essential Functions of Animal Blood
Regardless of color or oxygen-transport pigment, blood performs several functions across the animal kingdom. It acts as a transport system, moving essential substances throughout the body.
Blood’s primary function is delivering oxygen from respiratory organs to tissues and removing carbon dioxide. It also transports nutrients from digestion to cells, carries waste products to excretory organs, and distributes hormones. Additionally, blood plays a role in immune defense by circulating immune cells and regulates body temperature and pH balance.