Blood appears red to our eyes, but it is actually different shades of red depending on whether it is oxygenated or deoxygenated. The color can range from bright red when oxygenated to a darker maroon/red when deoxygenated. The varying color is related to how light is absorbed and reflected by hemoglobin, the protein that carries oxygen in red blood cells.
What Makes Blood Red?
The red color of blood comes from the hemoglobin inside red blood cells. Hemoglobin contains iron and proteins that allow it to bind and transport oxygen from the lungs to tissues and organs throughout the body. The iron in hemoglobin makes it red, as iron oxides are usually red or brown in color.
When hemoglobin is loaded up with oxygen it takes on a bright cherry red hue. This is the color we normally associate with arterial blood, which flows from the heart to deliver oxygenated blood to the body. As tissues extract oxygen from the blood, the hemoglobin changes to a darker maroon/red color. This deoxygenated blood returns to the heart and lungs through the veins to be replenished with oxygen again.
The Spectrum of Blood Color
Though we generally think of blood as being red, its exact shade depends on the oxygenation level. Here are the colors blood can range between:
- Bright red – When blood is highly oxygenated, containing over 95% oxygen saturation, it appears a vivid cherry red.
- Red – Normal oxygenated blood traveling through arteries is a bright red color, with over 90% oxygen saturation.
- Dark red – As blood starts losing oxygen, it transitions to a darker scarlet red shade. Venous blood returns to the heart at about 75% oxygen saturation.
- Maroon – Deoxygenated blood heading back to the lungs has less than 75% oxygen and appears maroon or burgundy colored.
- Dark maroon – Blood that has lost most of its oxygen appears dark maroon or brownish-maroon.
So in summary, the color spectrum goes from bright cherry red for arterial blood down to a very dark maroon-brown for venous blood. While the darkest maroon shades could be mistaken for brown or purple, all shades of blood contain some tint of red.
What Affects Blood’s Red Color?
There are a few key factors that can affect the red coloring of blood:
Oxygenation Level – As described, the oxygen content of blood greatly impacts its shade. Brighter red indicates higher oxygen, while darker maroon reflects lower oxygenation.
Hemoglobin Levels – Hemoglobin gives blood cells their red color, so low hemoglobin levels can make blood appear less vibrantly red. Conditions like anemia, bleeding, malnutrition, or bone marrow disorders can lower hemoglobin concentrations.
Plasma Volume – Plasma is the liquid component of blood and is pale yellow. Higher plasma volume dilutes blood, giving it a slightly less saturated red hue. Dehydration concentrates blood cells and makes the color appear deeper red.
Blood Cell Disorders – Certain blood diseases like polycythemia (high RBC count) or thalassemias cause abnormal hemoglobin production and alter blood’s red color.
Venous vs. Arterial Blood – As mentioned, venous and arterial blood differ in oxygenation and therefore shade. Venous blood pools appear darker due to their maroon deoxygenated color.
Typical Shades of Arterial vs. Venous Blood
Arterial and venous blood have distinct appearances:
Arterial Blood:
- Color: Bright red to red
- Oxygen saturation: 95% and above
- Source: Pulmonary arteries carrying blood from lungs to body
Venous Blood:
- Color: Dark red to maroon
- Oxygen saturation: 75% or less
- Source: Veins carrying blood back to heart and lungs
The higher oxygen content makes arterial blood a vivid red, while venous blood is nearly maroon from oxygen depletion by body tissues. But there is overlap where darkly oxygenated arterial blood appears maroon and highly oxygenated venous blood seems redder.
Comparing Maroon and Red Blood
Maroon and red are the two main color varieties of blood:
| Attribute | Maroon Blood | Red Blood |
|---|---|---|
| Oxygen saturation | Less than 75% | 90% or higher |
| Hemoglobin | Deoxygenated | Oxygenated |
| Blood type | Venous | Arterial |
| Source | Veins | Arteries |
| Direction | Flowing to heart | Flowing from heart |
| Purpose | Returns waste CO2 | Transports oxygen |
While arterial blood is always red and venous is maroon, there can be exceptions. But in general, darker maroon shades reflect deoxygenated venous blood, while brighter reds are well-oxygenated arterial blood.
What’s Considered Normal Blood Color?
Normal arterial blood is a bright red color. It flows from the heart’s left ventricle through the body’s arteries. Any signs of darkening from its normal bright red hue could indicate an issue.
Normal venous blood has a darker maroon, purple, or blue tint. This is the blood returning from the veins to the heart. Veins contain valves and blood pools, so venous blood often has a darker maroon appearance.
Here are key indicators of normal arterial and venous blood color:
Arterial blood:
- Vivid cherry red
- 95% or higher oxygenated
- No signs of darkening
Venous blood:
- Maroon, purple, or blue tint
- 75% or lower oxygenated
- Darker than arterial blood
As long as arterial blood remains bright red and venous blood seems maroon or purple, the coloring is normal. Any deviations could reflect an underlying condition.
When to See a Doctor
Consult a doctor if blood exhibits an abnormal color, including:
- Dark red arterial blood – May indicate low oxygen levels
- Bright red venous blood – Could signal an arteriovenous malformation
- Brown/green color – Suggests bile pigments due to liver disease
- Pink foamy blood – Can be a sign of pulmonary edema
- Black or gray blood – Indicates old blood from gastrointestinal bleeding
Blood color outside the normal range of bright red to deep maroon could be caused by:
- Respiratory disorders
- Heart defects
- Carbon monoxide poisoning
- Hemoglobin abnormalities
- Bleeding disorders
- Liver dysfunction
- Blood clotting issues
See a doctor promptly for evaluation of any abnormal blood color, as it may reflect a significant medical problem.
Takeaway Points
The key points about blood’s color include:
- Blood appears different shades of red due to oxygenation levels
- Bright cherry red reflects well-oxygenated arterial blood
- Dark maroon is deoxygenated venous blood returning to the lungs
- Changes from normal red arterial blood or maroon venous blood can indicate health conditions
- Consult a doctor if blood seems an abnormal color like brown, green, gray, or pink
Though often described as red, blood’s exact shade shifts between bright red and maroon depending on its oxygen content. While some red blood varieties are normal, abnormal colors can be a sign of underlying medical issues.
