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This is a great question that needs an elaborate answer. Let's explore together what is blood as well as its function. Blood is the major component of the body's circulatory system. It is constantly pumped through our bodies and organs to distribute oxygen and nutrients to all cells in the body. Oxygen is needed to produce energy that fuels most the body's metabolic processes. Upon oxygen release, the blood picks-up carbon dioxide generated by the cells as a waste product and brings it to the lungs for exhalation. It is extremely important to get rid of carbon dioxide as it increases blood acidity. In order to meet the increased demand of the body for oxygen, blood cells are equipped with a metalloprotein (a protein with a metal core) known as hemoglobin. The metal core in hemoglobin, which is iron, binds to oxygen and helps with its transport across the body.

To get back to your question, human blood is always red and never blue. However, the color of the blood ranges from bright red, when it is oxygenated, to dark red when it is not. When blood leaves the heart to be transported to organs and tissues, it carries oxygen in its hemoglobin. When oxygen is bound to iron, the blood looks bright red. Once oxygen is released, and carbon dioxide binds, the deoxygenated blood looks darker in color.

The mistaken belief that deoxygenated blood is blue is due to the fact that darker blood absorbs less light. Light is an electromagnetic radiation of different wavelengths. Visible light consists of different colors (the colors of the rainbow) when combined, they look white. According to some researchers, tissue optics (how light is absorbed and scattered) is responsible for the blue blood phenomenon. According to this theory, the depths of the veins as well as the darkness of the deoxygenated blood reflect the blue color of the light spectrum. Since the blue color is not absorbed, it is "bounced back" and the eye sees the veins as blue.

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