When blood is drawn, it often appears dark, sometimes described as “purple.” While this might seem concerning, blood color naturally varies due to several factors. Understanding the science behind blood color helps clarify why the shade observed during a blood draw is usually normal. This article explores the biological reasons for blood’s appearance and other influences on its perceived color.
The True Colors of Blood
Blood’s color is primarily determined by hemoglobin, a protein found within red blood cells. Hemoglobin contains iron, which binds to oxygen and gives blood its characteristic red hue. When hemoglobin is fully saturated with oxygen, such as in arterial blood leaving the lungs, it appears bright, vibrant red. This is often described as a cherry red color.
As blood circulates through the body, delivering oxygen to tissues and organs, the hemoglobin releases its oxygen. This deoxygenated hemoglobin changes its structure slightly, causing the blood to appear a darker shade of red, often described as maroon or even a blackish-red. Blood is always red, though its specific shade depends on its oxygen content.
Why Venous Blood Looks Darker
Blood drawn during a routine medical procedure, known as venipuncture, is almost always collected from a vein. Venous blood has already traveled throughout the body, delivering oxygen to various tissues and collecting carbon dioxide as a waste product. Consequently, the hemoglobin in this blood is largely deoxygenated.
This deoxygenated state gives venous blood its characteristically dark red or maroon appearance. The darker shade is simply a reflection of its lower oxygen saturation compared to arterial blood. Therefore, when blood appears dark or “purple-ish” after being drawn, it typically indicates deoxygenated venous blood.
Other Influences on Blood Color Perception
While the oxygenation level is the main physiological determinant of blood color, external factors can significantly influence how we perceive its shade. The type and intensity of lighting in the room can alter the appearance of the blood, making a dark red seem more purple. Different light sources can emphasize certain color tones, leading to varying perceptions.
The material and color of the collection tube itself can also subtly affect the perceived color of the blood inside. For instance, the tint of the plastic or glass, or even the presence of certain anticoagulants, might slightly modify the visual appearance. Beyond these environmental factors, individual differences in color perception mean that people might describe the same shade of dark red blood in slightly different ways.
In extremely rare medical conditions, blood can genuinely take on unusual colors. Methemoglobinemia, for example, can cause blood to appear chocolate-brown due to altered hemoglobin that cannot effectively carry oxygen. Another rare condition, sulfhemoglobinemia, results from sulfur binding irreversibly to hemoglobin, leading to a greenish or bluish tint in the blood.
When to Be Concerned
Observing dark, “purple-ish” blood when drawn is typically a normal and expected physiological occurrence. There is usually no need for concern if this is the only observation.
However, you should seek medical advice if the blood color is truly unusual, such as a bright blue, black, or a distinctly green shade, especially if it is accompanied by other concerning symptoms. Symptoms like dizziness, shortness of breath, severe pain, or unusual bruising, combined with an abnormal blood color, warrant immediate medical attention. A healthcare professional can accurately assess the situation and determine if any underlying medical conditions require investigation.