Blood is a vital bodily fluid that transports oxygen and nutrients throughout the body. Under normal circumstances, blood appears red due to the abundance of red blood cells that contain hemoglobin. Hemoglobin is a protein that contains iron, which gives blood its red color when oxygenated. However, in some cases, blood can take on a darker color ranging from brown to black.
Quick Answers
Here are quick answers to some common questions about black blood:
- Blood can turn black due to the presence of deoxygenated blood, dried blood, or blood clots.
- Deoxygenated blood is blood that lacks oxygen. It appears dark red or brownish-red.
- Dried blood turns black as the water content evaporates.
- Blood clots contain fibrin, platelets, and red blood cells. Large clots may appear black or dark brown.
- Certain medical conditions like methemoglobinemia can also cause blood to turn black.
- Most causes of black blood are not immediately life-threatening, but some can indicate an underlying health issue.
- Black stool due to blood in the gastrointestinal tract is called melena and requires medical evaluation.
What Makes Blood Turn Black?
There are a few key reasons why blood may turn black or an unusually dark color:
Deoxygenated Blood
Under normal conditions, blood leaving the heart to go to the lungs is deoxygenated. Deoxygenated blood has a dark reddish-brown color due to the iron in hemoglobin not being bound to oxygen molecules. This blood is pumped from the heart to the lungs where it picks up oxygen and returns to the heart as bright red oxygenated blood ready to circulate to the body.
However, some medical conditions can cause larger quantities of deoxygenated blood to circulate through the body without being oxygenated. This deoxygenated blood has a darker color and can make veins look blue or almost black through the skin.
Dried Blood
As fresh blood sits exposed to air, the liquid components evaporate leaving behind just the cells and proteins. These coagulated proteins have a black/brown appearance. Old blood stains and scabs often appear black due to the dried blood. This is sometimes referred to as eschar.
Blood Clots
Blood clots form when proteins like fibrin and platelets mesh together to stop bleeding after an injury. Small clots may maintain a reddish color, but larger clots often appear black or dark brown. These clots are thicker and contain more solid material than liquid blood.
Methemoglobinemia
Methemoglobinemia is a condition where the iron in hemoglobin is oxidized from ferrous (Fe2+) to ferric (Fe3+) form. This ferric iron cannot bind oxygen, causing the blood to turn chocolate brown and appear black through the skin. Methemoglobinemia can be congenital or caused by certain medications and toxins.
Medical Conditions That Can Cause Black Blood
There are several medical conditions that may result in black or unusually dark blood:
- Sepsis – Sepsis occurs when the body has an extreme immune response to infection. Septic shock reduces oxygen delivery throughout the body leading to deoxygenated black-ish blood.
- Pulmonary embolism – Clots in the pulmonary arteries block blood flow to the lungs. Deoxygenated blood returns to the heart and circulation.
- Hemolytic anemia – Red blood cells rupture prematurely. The iron released from hemoglobin may appear black in urine or stool.
- Intestinal bleeding – Bleeding in the digestive tract causes black, tarry stool called melena. The blood is exposed to digestive enzymes and appears black.
- Kidney disease – In advanced kidney disease, damaged blood leaks into the urine and can appear black or brown.
- Methemoglobinemia – As described earlier, this blood disorder interferes with oxygen transport and turns blood chocolate brown.
Other less common causes include exposure to toxins like benzene, severe burns, hypothermia, and reactions to some antimicrobial and antimalarial drugs.
When to See a Doctor
In most cases, black or dark blood is not an emergency. However, it can indicate an underlying problem, so it is important to see a doctor in the following situations:
- Blood in urine or stool appears black/brown
- Unexplained bruising under the skin looks black
- Skin, lips, or nail beds appear blackish due to lack of oxygen
- Chest pain or trouble breathing occurs along with blackish blood
- Exposure to toxins or drugs known to impact red blood cells
- Blood changes color after surgery, injury, or illness
It is always best to see a doctor if blood changes color persistently or if other unexplained symptoms develop. Oftentimes, blood tests can help determine the cause of abnormal blood color.
Treatments for Black Blood
Treatment focuses on identifying and addressing the underlying cause of the black blood:
- Oxygen therapy – Additional oxygen can be given to increase oxygenated hemoglobin.
- Medications – Antibiotics for infections, medications to treat blood disorders, iron supplements.
- Blood transfusion – May be necessary in cases of severe anemia or oxygen deficiency.
- Surgery – Procedures to stop internal bleeding or remove clots.
- Dialysis – Used to filter the blood if the kidneys are not functioning properly.
The prognosis depends on the underlying condition. With appropriate treatment, many of the causes are reversible.
Prevention
It is not always possible to prevent black discoloration of blood, but the following precautions may help in some cases:
- Avoiding toxins and drugs known to affect red blood cells
- Promptly treating infections before they progress to sepsis
- Managing chronic conditions like heart failure and lung disease
- Not ignoring symptoms of internal bleeding or blockages
- Following safety precautions to avoid injury and excess bruising
Conclusion
While blood is normally red, it can sometimes turn black due to deoxygenation, drying, clotting, or medical disorders. Black stool, urine, or bruising may be the first signs something is wrong. Most causes are treatable with appropriate medical care, so it is important to see a doctor for an evaluation. With prompt assessment and treatment, black blood is often reversible.