The popular phrase “blue blood” does not reflect the actual color of human blood. While veins can appear blue through the skin, this is a visual trick rather than an indication of blood’s true hue. The biological reality of blood color is far more nuanced and, in some cases, surprisingly diverse across the animal kingdom.
The Actual Color of Human Blood
Human blood is never truly blue; its color is always some shade of red. Oxygenated blood, found in arteries and arterioles, appears bright red, often described as scarlet or cherry red. This vibrant color is due to the hemoglobin molecules within red blood cells carrying a full load of oxygen.
As blood circulates through the body and delivers oxygen to tissues, it becomes deoxygenated. This deoxygenated blood, found in veins and venules, is a darker red. The difference in shade is subtle, and both oxygenated and deoxygenated human blood remain distinctly red. When blood leaves the body and is exposed to air, it quickly re-oxygenates, turning bright red almost immediately.
Why Veins Appear Blue
The perception of veins as blue or bluish-green under the skin is a widespread optical illusion, not a reflection of the blood’s color. This phenomenon is primarily due to how light interacts with skin, tissues, and the blood vessels themselves.
White light, which contains all colors of the spectrum, penetrates the skin. Red light, with its longer wavelength, penetrates deeper into the skin and is absorbed by the hemoglobin in the blood. Blue light, having a shorter wavelength, does not penetrate as deeply and is largely reflected back to the observer’s eyes. The depth of the veins beneath the skin also plays a role; veins are typically closer to the surface than arteries, making this light interaction more noticeable. Our eyes and brain then interpret the scattered blue light that returns from the superficial veins, leading to the perception of a bluish hue.
The Science of Blood Color
The color of human blood is intrinsically linked to hemoglobin, a complex protein found within red blood cells. Hemoglobin’s primary function is to transport oxygen from the lungs to the body’s tissues. At the core of each hemoglobin molecule are four heme groups, and each heme group contains a single iron atom. It is this iron atom that directly binds to oxygen, and the interaction between iron and oxygen dictates the blood’s color.
When oxygen binds to the iron in hemoglobin, it forms oxyhemoglobin, which absorbs blue-green light and reflects red-orange light, resulting in the bright red color of oxygenated blood. Conversely, when oxygen is released, hemoglobin becomes deoxyhemoglobin, which reflects a darker shade of red. This chemical interaction, rather than the presence of oxygen itself, is what gives blood its characteristic red appearance.
Blue Blood in Other Organisms
While human blood is always red, some animals genuinely possess blue blood. This striking difference in blood color is due to a different type of oxygen-carrying protein. Instead of iron-based hemoglobin, these creatures utilize a copper-based protein called hemocyanin for oxygen transport.
When hemocyanin binds with oxygen, the copper within its structure turns blue, giving the blood its distinctive color. When deoxygenated, hemocyanin-based blood typically becomes colorless or a dull gray. Notable examples of animals with blue blood include horseshoe crabs, octopuses, squid, and some spiders. Hemocyanin is particularly efficient at transporting oxygen in cold and low-oxygen environments, making it well-suited for marine invertebrates.