As a data analyst fascinated by scientific patterns, I‘ve long wondered about the different shades blood can turn. My own bruises morph from red to purple to green. Monthly, my menstrual flow changes from dark to bright and back again. After a hospitalization, I was startled by how vampiric my drawn blood appeared.
Blood‘s chameleonic nature is mostly healthy, simply indicating age or oxygen levels. Yet at times, troubling colors can signal underlying illness. This comprehensive guide will arm you with knowledge on blood‘s hues – from the normal spectrum to warning signs requiring medical attention.
A deep dive into what gives blood its red color
Before examining variations, let‘s first understand the pigments that typically give blood its rich, red appearance:
Hemoglobin
Hemoglobin is an iron-containing protein in red blood cells that binds to oxygen in the lungs. This hemoglobin-oxygen compound, called oxyhemoglobin, lends blood its bright scarlet color. Hemoglobin accounts for over 90% of blood‘s pigmentation.
Bilirubin
A yellowish waste pigment from old red blood cell breakdown also contributes to blood‘s red-orange tone. Higher bilirubin levels lead to a darker red. Jaundice results from excess bilirubin.
Red blood cells
Comprising up to 45% of blood volume, RBCs contain hemoglobin proteins that change color depending on oxygen levels. More red cells mean darker blood.
| Blood Component | Function | Impact on Color |
|---|---|---|
| Hemoglobin | Oxygen transport | Bright red when oxygenated |
| Bilirubin | Waste pigment | Darker red in higher levels |
| Red blood cells | Contain hemoglobin | More RBCs = darker red |
Now let‘s explore the factors that can turn blood darker red or even brown, black, or green.
Oxygen – the key to bright red blood
Oxygen plays the biggest role in blood‘s color. Oxygenated arterial blood blazes a fiery red, while deoxygenated venous blood appears almost purple.
Arterial blood
Arterial blood gushes from the heart to body tissues, freshly oxygenated bright red. This oxygenated hemoglobin makes arteries appear scarlet.
- Oxygen saturation 95%+
- Bright cherry red hue
- Flows from heart to tissues
Venous blood
After delivering oxygen, darker blood returns to the lungs through veins. With less oxygen, hemoglobin changes shape, altering blood‘s color.
- Oxygen saturation 50-80%
- Dark maroon/purple red
- Flows from tissues back to heart
| Blood Type | Oxygen Saturation | Color | Role |
|---|---|---|---|
| Arterial | 95%+ | Bright red | Carries blood from heart to body |
| Venous | 50-80% | Dark red | Returns blood to heart from tissues |
Some key takeaways:
- Deoxygenation causes blood darkening, not deoxygenated blood itself.
- Darker venous blood is healthy and expected.
- Unusually dark arterial blood can signal respiratory conditions or heart defects.
When dark blood indicates illness
Certain diseases darken blood through decreased oxygen supply:
Respiratory disorders
Pneumonia, COPD, and other lung issues can prevent adequate oxygenation. This manifests as darker systemic blood, particularly visible in nail beds and lips.
Heart defects
Defects like septal holes or valve abnormalities can cause oxygen-rich and oxygen-poor blood to mix, diluting blood‘s bright redness.
Shock
In shock, blood pressure drops severely, impairing oxygen delivery and darkening blood. Septic shock involves both low BP and respiratory failure.
Carbon monoxide poisoning
CO binds to hemoglobin, forming carboxyhemoglobin instead of oxyhemoglobin. This impairs oxygen transport, significantly darkening blood.
Of 330,000 CO poisonings annually in the U.S., around 50,000 visit the ER, per CDC data.
Blood thickening disorders
Blood thickening, either from too many cells or internal clotting, also darkens color.
Polycythemia vera
A rare blood cancer, polycythemia vera proliferates excessive red blood cells. More red cells leads to more hemoglobin pigment, darkening blood.
Median age of diagnosis is 60 years old. The incidence rate is 0.8-2.8 per 100,000 people.
Thrombocytosis
With thrombocytosis, the body overproduces platelets, thickening blood. This hyperviscosity also darkens color.
Gastrointestinal bleeding
GI bleeds display a wide spectrum of blood colors depending on the source.
Bright red blood
Arterial bleeding in the lower GI tract produces vivid red blood. Sources include:
- Colorectal cancers
- Hemorrhoids
- Diverticulitis
- Angiodysplasia
Dark red blood
Slow venous oozing in the upper GI tract manifests as dark maroon or black blood. Causes include:
- Esophageal varices
- Peptic ulcer
- Gastritis
Dark brown stool can indicate old blood from past upper GI bleeding.
Menstrual blood color changes
The uterine lining cyclically builds up and sheds, causing period color variation:
Dark brown/black blood
Common at the very beginning and end of periods. This darker blood is older and slower flowing.
- Beginning: Residual old blood from previous cycle
- End: Older blood cleared out at finish
Bright red blood
Most prominent mid-cycle during highest flow days. This fresh blood has not had time to oxidize.
Abnormal dark blood
While some color variation is normal during menses, very dark brown or grey blood can indicate:
- Fibroids
- Endometriosis
- Polyps
- Cancer
Infection can also cause abnormal discharge. See your OB-GYN if dark bleeding persists across cycles.
When blood turns green, black, purple, and blue
Less common pigments can also color blood:
Green blood
Bile pigment biliverdin tints blood green. Causes:
- Hemolytic anemia
- Cholecystitis
- Liver cancer
- Strep infections
1 in 50,000 people have green blood, per a 2008 case report.
Black blood
Very deoxygenated blood appears black or coffee-colored. Causes include:
- Blood thickening disorders
- Gangrene
- Internal wounds
Purple blood
Rare hemoglobin disorders like sulfhemoglobinemia bond sulfur to hemoglobin. This turns blood purple.
Blue blood
Blood only turns blue when oxygenated as seen in veins. Deoxygenated blood remains dark red, never blue.
When to seek care for dark blood
While some blood color variation is normal, consult a doctor if you notice:
- Black, green, purple, or blue blood
- Blood thick like oil
- Foul smelling blood
- Grey, white, or yellow discharge
- Severely darkened lips/nails
- Unexplained blood clots
- Dizziness, weakness, or chest pain
Improving unhealthy dark blood
If a medical condition causes problematic blood darkening, treating the underlying cause is key.
For low oxygenation:
- Oxygen therapy, intubation, ventilation
- Treating pneumonia, heart failure, anemia
For infections:
- Antibiotics, antivirals
- Draining abscesses
For blood thickening:
- Phlebotomy to remove excess red blood cells
- Blood thinners like aspirin
- Managing clotting disorders
For bleeding:
- Volumen replacement
- Cauterization, embolization
- Medication to slow bleeding
In some cases, blood transfusions or surgical intervention may be required.
The takeaway: Don‘t fear the darker depths of blood
Hopefully this deep dive demystifies blood‘s myriad shades of red and beyond. While our circulatory rivers run scarlet most days, know that darker blood can often be entirely normal. Yet be vigilant for symptoms suggesting underlying illness. With a healthy respect for blood‘s chameleonic nature, we can monitor its colors for vital insight into the body‘s inner workings.
