Blood is a specialized fluid that delivers oxygen and nutrients throughout the body while removing waste products like carbon dioxide. Human blood is characterized by a distinctive red hue, a color that dynamically indicates its current state and chemical composition.
The Chemistry Behind the Red
The color of blood is traceable to the red blood cells circulating within the fluid. These cells contain hemoglobin, a large protein responsible for transporting oxygen. Hemoglobin’s structure includes four subunits, each containing a heme group with a single iron atom at its core.
The iron atom dictates the blood’s red appearance. When iron binds with oxygen molecules, this chemical interaction changes how the molecule absorbs and reflects light. The resulting iron-oxygen complex specifically reflects the red wavelengths of the visible light spectrum. The sheer concentration of hemoglobin within the red blood cells results in the intense red color we observe.
The Difference Between Bright and Dark Red
The shade of red blood displays is determined by its oxygen saturation level. When blood leaves the lungs via the arteries, its hemoglobin is fully loaded with oxygen, known as oxyhemoglobin. This high concentration of oxygen bound to the iron causes the blood to appear a vibrant, bright cherry red color.
As blood reaches the body’s tissues, oxygen is released to fuel cellular processes. The hemoglobin that has released its oxygen is referred to as deoxyhemoglobin. This change in oxygen status alters the molecule’s three-dimensional structure slightly, causing it to absorb more red light. This results in the darker, more maroon or purplish-red appearance observed in venous blood. The bluish appearance of veins under the skin is an optical illusion caused by how light is scattered by the skin and tissue layers, not because deoxygenated blood is blue.
What Causes Blood to Change Color Over Time
Blood that leaves the circulation and pools under the skin, such as in a bruise, undergoes a predictable sequence of color changes as it breaks down. Initially, the bruise appears dark red or purplish as the pooled blood releases its remaining oxygen. Over the next few days, the body begins metabolizing the trapped hemoglobin.
This metabolic process converts hemoglobin into a series of other colored compounds. The first breakdown product is biliverdin, a pigment that gives the bruise a greenish tint around day five to seven. Biliverdin is then converted into bilirubin, which produces a yellowish color. This progressive shift from red to purple, then to green and yellow, is the body’s visible method of recycling blood components before the bruise fades.
When Blood Color Signals a Problem
While bright and dark red are normal variations, rare conditions can cause blood to deviate significantly from the red spectrum, signaling a serious medical issue. One condition is methemoglobinemia, where the iron atom in hemoglobin is oxidized from its normal state. This chemical change prevents oxygen from binding effectively, causing the blood to take on a brownish or chocolate-brown color.
Another condition is sulfhemoglobinemia, which occurs when a sulfur atom binds irreversibly to the hemoglobin molecule. This dysfunctional hemoglobin cannot transport oxygen and gives the blood a distinctly greenish hue. Both conditions are typically induced by exposure to certain drugs or toxins. The presence of these non-red colors indicates a loss of oxygen-carrying capacity and requires immediate medical intervention.