Is Blood Blue?

Medically reviewed by Amelia MacIntyre, DO

If you're under the impression that blood inside the body is blue, you're not alone. There is a longstanding myth that blood inside of our bodies is blue. While some animals do have blue blood, human blood is red. Blood both inside and outside of our bodies is red due to the presence of pigment found in hemoglobin.

This article will discuss why blood is red.

Why Do Some People Think Blood Is Blue?

There are a few reasons people may think blood inside the body is blue. A longstanding misconception about blue blood stems from the way that veins look through the skin.

Visible veins can appear blue, green, or purple in skin, depending on the skin tone and depth of the veins within the skin. How veins appear is affected by how light is filtered by the skin. The color of bruises and hematomas (collections of blood) are also dependent on light filtering through the skin.

Medical images also often portray blood in veins as blue to differentiate deoxygenated blood (blood with low oxygen levels) in veins from the oxygenated (oxygen-rich) blood in arteries. The path of blood in the circulatory system involves blood going from the lungs where it receives oxygen to the left side of the heart.

From there, it's pumped to arteries (often portrayed in diagrams as red) to supply the body with blood. The tissues in the body remove much of the oxygen from the blood, where it gets deoxygenated, or made oxygen-poor. Then, travels through veins to the right side of the heart and is pumped to the lungs to receive more oxygen.

What Color Is Blood?

Blood in humans is red. It may be a dark red or a bright red, depending on the amount of oxygen present, but human blood is not blue. Blood with lots of oxygen appears a bright red color, whereas venous blood with less oxygen appears a darker, deeper red color.

The protein hemoglobin makes blood red. This important protein is found in red blood cells and carries oxygen to tissues throughout the body. Hemoglobin is made from protein subunits that hold the heme molecule. This structure is also called a porphyrin ring, which gives blood its color. The reason for blood being red is due to the wavelengths of lights absorbed and reflected by the porphyrin ring.

Why Do Veins Appear Blue Through the Skin?

If you take a look at your wrist, you may be able to see some superficial veins. They might appear blue, purple, and/or green depending on your skin tone. The reason these veins don't appear red is due to the way that the skin tissue absorbs and reflects light.

Visible light is composed of a range or spectrum of wavelengths that travel through space. We sense visible light as various colors, depending on the wavelengths reflected to our eyes. For example, we see the color red when red light, which has a specific wavelength, is sensed by the retina in our eyes.

When blood is very near the skin (such as can be seen in a fair-skinned person who is blushing or flushed), it appears red. However, the veins deeper in the skin appear blue, purple, or green because the red wavelength of light is more easily absorbed in the tissue, while the blue, purple, and green wavelengths are reflected and sensed by our eyes.

Blue Skin: A Warning Sign

If you notice skin on the lips or nail beds turning blue, it can be a sign of a serious problem called cyanosis. Cyanosis can be caused by different medical conditions, and in some cases can be a sign of a serious problem. It might appear at the tips of your fingers in cold weather due to the narrowing of blood vessels (vasoconstriction) in an attempt to keep your body warm. However, it can also occur with low oxygen levels from heart and lung problems or when a blood clot in an artery blocks blood flow, such as to a leg. People with darker-toned skin may notice cyanosis more prominently in the gums, lips, and around the eyes. If you have cyanosis unrelated to a cold environment, seek medical attention immediately.

Summary

Despite misconception that human blood is blue, blood is, in fact, red. Depending on the amount of oxygen present, it may be dark, deep red or bright red color. Hemoglobin is a protein in the blood of humans that is responsible for the red color. Certain other animals do have blue blood due to the presence of a different pigment in their blood.

Veins appear blue, green or purple because of the way that skin tissue absorbs and reflects light. Blue skin, or cyanosis, can signal low oxygen levels in blood, and may be due to a medical emergency.

Read the original article on Verywell Health.