Blood is a vital bodily fluid that transports oxygen and nutrients throughout the body. The color of blood is often associated with the color red due to its bright hue when oxygenated. However, the color of blood before it comes into contact with oxygen is quite different.
The Role of Hemoglobin in Blood Color
Hemoglobin is the protein molecule contained within red blood cells that binds to oxygen. It contains iron atoms that can form bonds with oxygen molecules. When hemoglobin is oxygenated, it turns bright red, which leads to the association between arterial blood and the color red.
However, hemoglobin takes on a different color when not bound to oxygen. Unoxygenated blood has a dark red, purple, or even blue hue depending on how deoxygenated the blood is. This darker color is more apparent in veins, where blood is returning to the heart and lungs devoid of oxygen.
Venous and Arterial Blood Colors
The difference between vibrant red oxygenated blood and darker deoxygenated blood leads to the characterization of two main types of blood:
- Arterial blood: Blood pumped from the heart to the lungs and body through the arteries. It is bright red in color due to its high oxygen saturation (95-100%).
- Venous blood: Blood that returns to the heart through the veins. It is dark red or purple as it has lost oxygen while circulating through the tissues. Veins carry blood with an oxygen saturation of only 75%.
The stark difference between arterial and venous blood is apparent under a microscope or when blood is drawn during a blood test. The tube collecting the crimson oxygenated arterial sample contrasts with the deeper purple hue of the venous sample.
Deoxygenated Blood Color Variance
While venous blood is relatively deoxygenated compared to arterial blood, the exact hue can vary based on just how oxygen deprived it is. The spectrum of color includes:
- Dark red – Venous blood returning from the extremities may retain some oxygen, resulting in a dark yet vivid red color.
- Purple – Blood draining from organs tends to be very dark purple, with less than 50% oxygen saturation.
- Blue – Some veins like those draining from the kidneys have oxygen saturations as low as 25%, resulting in a deep bluish-purple color.
So in summary, while the color red is associated with blood, the true color of blood depends on its oxygenation status. Blood returning to the lungs through the veins can range from dark red to deep purple to even blue hues.
Cyanosis Causes Skin Discoloration
While veins carry deoxygenated blood, this blood should re-oxygenate once it reaches the lungs. Some medical conditions can prevent proper oxygenation, leading to lower systemic oxygen levels. This condition, known as cyanosis, can sometimes be detected by looking at a patient’s skin color.
Cyanosis leads to a blue or purple discoloration of the skin and mucous membranes. Lips or fingernails may take on a bluer than normal hue. This happens because hemoglobin overwhelmed with deoxygenated blood shows through surface capillaries. Some causes of cyanosis include:
- Pneumonia or chronic respiratory conditions
- Heart defects
- Lung diseases like COPD
- Low red blood cell count
In most people, venous blood remains hidden from view. But in cyanosis, the dark color of deoxygenated blood becomes more evident through discolored skin. Prompt medical treatment is needed in these cases to restore proper oxygenation.
Fetal Circulation Shunts Blood Away from Lungs
Human blood takes on a different path of circulation before birth compared to after. This is because the fetus does not use its lungs to gather oxygen from the outside air.
Instead, the baby gets oxygen from the mother’s blood supply through the placenta and umbilical cord. Two special blood vessels regulate circulation:
- Ductus arteriosus: Shunts blood from the pulmonary artery to the aorta, bypassing the non-functioning fetal lungs
- Foramen ovale: Connects the right and left atrium to shunt blood away from the pulmonary circuit
These vessels allow most fetal blood to skip the lungs and prevent excess fluid building up in those organs. As a result, fetal blood remains oxygen-rich and vivid red in color as it circulates from the placenta back to the body.
Blood Appears Blue in Veins Under Skin
Many people associate veins with the color blue as they appear through the skin. This phenomenon results from how light interacts with blood vessels at this very surface level.
Veins located just below the skin surface absorb low frequency red wavelengths. The remaining blue hues bounce back, giving veins their bluish tint. But this is just a trick of the light. The blood contained in subsurface veins retains its dark red-purple color.
The exact color that veins take on depends on skin pigmentation and the thickness of the vessels. But while veins may fool they eye into seeing blue, deoxygenated blood never turns blue inside the body.
Hemoglobin Color Changes in Disease States
The color of blood can indicate certain disease states relating to how hemoglobin functions. Some examples include:
- Methaemoglobinemia – Too much methaemoglobin in the blood turns it a chocolate brown color
- Sulfhemoglobinemia – Caused by medicines or chemicals, blood turns greenish-black
- Carboxyhemoglobinemia – Carbon monoxide poisoning makes blood cherry-red
In these cases, changes to the hemoglobin molecule prevent it from carrying oxygen normally. The altered blood color can clue doctors into the diagnosis.
To summarize, here are some key points about blood color:
- Arterial blood is bright red due to high oxygen levels
- Venous blood is a darker red/purple as it returns to the heart
- The bluish color in veins under the skin is an optical illusion
- Fetal circulation allows blood to skip the lungs and remain oxygenated
- Some diseases alter hemoglobin function, drastically changing blood color
So while the idea of blue blood is a misconception, there are many interesting facets to the color of blood as it flows through the body! Understanding these subtleties helps illustrate the fascinating physiology that allows our circulatory system to function.
|Blood Type||Color||Oxygen Saturation|
|Cyanosis||Blue/purple tinted skin||Low|
|Fetal||Bright red||High (from placenta)|
This table summarizes the differences in color and oxygen saturation between types of blood flow in the body.
The Vibrant History of Bloodletting
For thousands of years, bloodletting was a common medical practice used to treat illness. This involved releasing blood from the body, based on ancient beliefs that it could “re-balance” the humors.
Bloodletting was done in a variety of ways, including:
- Cutting open a vein with a lancet or sharpened instrument
- Using suction cups to draw blood to the skin’s surface
- Applying leeches to open incisions to allow blood to escape
Interestingly, the red color of oxygenated blood was rarely seen during bloodletting. Veins or capillaries were one source of blood, producing the darker venous hue. Arteries were avoided as they posed greater risk for the patient.
The gallons of blood spilled during bloodletting resulted in many deaths. George Washington arguably died after nearly half his blood was drawn to treat an infection. But bloodletting persisted for centuries before being phased out by modern medicine.
Common Conditions Bloodletting was Used to Treat
- Psychiatric disorders
- Childhood illnesses like whooping cough
Ironically, bloodletting likely weakened patients already afflicted by sickness or injury. But its practice endured as physicians clung to the faulty humoral theory of illness.
Hemoglobin Believed to Influence Skin Color
For centuries, scientists presumed that hemoglobin was responsible for skin pigmentation. This idea tied into outdated beliefs about race and biology.
In the 1600s, scholar Robert Boyle hypothesized that black skin contained more hemoglobin than white skin. This greater “proportion of blood” was believed to darken skin color.
This view aligned with the now discredited one-drop rule, where visible African ancestry conferred black racial identity. Enslaved Africans were deemed predisposed to hard outdoor labor based on skin color.
In truth, hemoglobin levels do not affect skin pigmentation. The amount of eumelanin produced by melanocytes determines skin color instead. But the myth linking blood and race persisted for centuries.
Modern genetics disproved supposed racial differences in blood. All humans share a common origin in Africa. And while hemoglobin transports oxygen, it does not impact the color of the skin.
Key Events Challenging Blood Theories of Race
- 18th Century – European scholars classify race based partly on blood composition
- 1901 – Karl Landsteiner discovers human blood groups (ABO system)
- Early 1900s – Sickle cell anemia found prevalent in African Americans
- 1970s – Population geneticists demonstrate biological unity of human races
Research now underscores that race is an artificial social construct, not a biological attribute. And while blood continues to be represented as red in cultural depictions, its true color varies greatly based on its vital circulatory path.