The question of why blood appears black is rooted in the transport of oxygen throughout the body. While human blood is always red, its precise shade is a dynamic indicator of its oxygen saturation. The appearance of “black” blood is, in reality, a perception of an extremely dark red, deep maroon, or purplish color. This dramatic darkening occurs when the blood has released its oxygen content to the body’s tissues.
The Role of Hemoglobin and Oxygen in Blood Color
The color of blood is determined by hemoglobin, a protein packed inside red blood cells. Hemoglobin contains four subunits, each centering around a heme group that holds a single iron atom. This iron atom is the physical site where oxygen molecules bind, and the state of this binding dictates the color seen.
When oxygen binds to the iron atom, the hemoglobin molecule undergoes a slight conformational shift, creating a substance known as oxyhemoglobin. This oxygen-rich state causes the blood to absorb blue-green light wavelengths while reflecting red light, resulting in the characteristic bright, scarlet red color seen in arteries. This is the hue that signifies freshly oxygenated blood moving away from the lungs.
Conversely, when the oxygen is released to feed the body’s cells, the molecule shifts again, forming deoxyhemoglobin. This deoxygenated state changes the light absorption properties of the blood, causing it to reflect a different spectrum. The resulting color is a much darker, purplish-red or deep maroon, which can easily be mistaken for black, particularly in lower light or when viewed in bulk.
Common External Scenarios Where Blood Appears Dark
Dark blood observed outside the body is often simply deoxygenated venous blood. When a slow-flowing cut or scrape occurs, the blood emerging is typically venous blood that has completed its circuit of oxygen delivery and is returning to the heart. This blood is already the deep, dark red of deoxyhemoglobin, which is why it appears darker than the bright red arterial blood.
A secondary process known as oxidation also contributes to the perceived blackness in external scenarios like scabs or menstrual flow. Menstrual blood, for example, is characteristically dark brown or black because the flow is relatively slow, especially at the start and end of a cycle. This slower transit time allows the blood to linger in the uterus, giving it more time to react with oxygen before it exits the body.
This lingering and exposure to air breaks down the hemoglobin into other compounds, most notably methemoglobin, which is a rust-colored derivative. Similarly, a scab or dried blood on a surface is not simply deoxygenated; it has oxidized and dried, resulting in the familiar dark brown or black rust-like color. The longer blood is outside the circulatory system and exposed to the environment, the further this chemical breakdown progresses, leading to the darkest shades observed.
When Dark Blood Indicates a Serious Medical Concern
While dark blood is often a normal observation, a truly black or tarry appearance in certain bodily waste products can signal a serious medical concern, specifically internal bleeding. The most recognized example is melena, which describes black, foul-smelling, and tarry stools. Melena is typically caused by bleeding that originates in the upper gastrointestinal (GI) tract, such as the esophagus, stomach, or upper small intestine.
The blood turns black because it has been partially digested by stomach acid and digestive enzymes as it passes through the intestinal tract. This chemical interaction drastically alters the hemoglobin, resulting in the distinct dark color and consistency. Another related sign is vomit that resembles coffee grounds, known as coffee-ground emesis. This appearance indicates that blood has been sitting in the stomach, where it has mixed with and been oxidized by gastric acid, creating a grainy, dark substance.
Bleeding that results in melena or coffee-ground vomit is a sign of a gastrointestinal hemorrhage and requires immediate medical attention. Individuals should seek emergency care if the dark blood is accompanied by other severe symptoms, including dizziness, lightheadedness, weakness, a rapid pulse, or a large volume of persistent bleeding.