The primary reason human blood appears red is the presence of a protein called hemoglobin. Hemoglobin is found in red blood cells and plays a crucial role in transporting oxygen from the lungs to various tissues and organs.
Hemoglobin contains iron, which binds to oxygen molecules in the lungs. When it binds to oxygen, it gives blood its red color.
Iron Content
When hemoglobin is oxygenated (carrying oxygen), it forms a bright red compound called oxyhemoglobin. This is what makes arterial blood, which carries oxygen, appear bright red.
Oxygen-Bound Hemoglobin
Deoxygenated Hemoglobin
After delivering oxygen to body tissues, hemoglobin returns to the lungs with less oxygen, and it forms a darker red compound called deoxyhemoglobin.
Veins vs. Arteries
Arteries carry oxygenated blood away from the heart to the body's tissues, and they appear bright red due to the oxygen-bound hemoglobin. Veins carry deoxygenated blood back to the heart and appear darker red.
Color Changes
Blood can appear various shades of red, from bright scarlet in arteries to dark maroon in veins, depending on the oxygen content.
Reflection and Scattering
The way that light interacts with the hemoglobin in the blood can also affect the perceived color. Light scattering and reflection contribute to the red appearance.
Wavelength Absorption
Hemoglobin absorbs light most strongly in the blue and green portions of the electromagnetic spectrum, allowing the red wavelengths to be reflected back to our eyes.
Blood pH
Blood pH can affect the color slightly. More acidic or alkaline blood may have a slightly different hue.
Health and Oxygen Levels
Blood color can also be influenced by an individual's health and oxygen levels. When blood oxygen levels drop significantly, it can appear bluish, a condition called cyanosis.