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What color is human blood without oxygen?

Human blood is a complex fluid that transports oxygen, nutrients, hormones, and waste throughout the body. The color of blood is largely determined by the presence of hemoglobin, the protein in red blood cells that carries oxygen from the lungs to tissues and organs.

Hemoglobin and Oxygen

When hemoglobin is loaded with oxygen, it imparts a bright red color to blood. This oxygenated blood is pumped from the heart to the body through arteries and capillaries. After delivering oxygen to tissues, the blood returns to the heart through veins. At this point, the hemoglobin is depleted of oxygen, making the blood appear dark red.

Blood without oxygen is known as deoxygenated blood. The dark color comes from hemoglobin in its deoxygenated state. Without oxygen bound to it, hemoglobin takes on a more purple-red hue. This color difference allows us to easily distinguish between oxygenated and deoxygenated blood.

What Makes Blood Red

In addition to hemoglobin, there are several other pigments and components in blood that contribute to its red color:

  • Heme – This iron-containing molecule gives hemoglobin its red color. Heme is found in myoglobin, cytochromes, catalases, and peroxidases as well.
  • Red blood cells – Also known as erythrocytes, these abundant cells get their color from hemoglobin. A normal blood sample is 40-45% red blood cells.
  • Plasma – This straw-colored liquid makes up about 55% of blood volume. Plasma contains water, proteins, nutrients, and waste products.
  • Platelets – These tiny cell fragments are clear individually but give a solution of platelets a cloudy appearance.
  • White blood cells – Also known as leukocytes, these colorless cells make up less than 1% of blood.

While all the non-red components dilute the color somewhat, the high concentration of hemoglobin makes blood appear red overall.

Oxygenated Blood vs Deoxygenated Blood

Here’s a comparison of the two types of blood:

Characteristic Oxygenated Blood Deoxygenated Blood
Color Bright red Dark red or purple-red
Oxygen Saturation 95-100% 75% or less
Hemoglobin Oxyhemoglobin (hemoglobin bound to oxygen) Deoxyhemoglobin (hemoglobin without oxygen)
Location Arteries and capillaries Veins

As shown, the two types of blood can be distinguished based on color, oxygen content, type of hemoglobin, and location in the circulatory system.

Deoxygenated Blood Color Variation

While deoxygenated blood is generally darker red or purple, the exact shade can vary somewhat depending on viewing conditions and anatomical site. Contributing factors include:

  • Vessel size – Deoxygenated blood is darkest in the large veins near the heart and becomes lighter red in the smaller veins.
  • Volume – A larger amount of deoxygenated blood appears darker than a thinner layer.
  • Flow rate – Fast-flowing deoxygenated blood looks lighter than slow-moving blood.
  • Skin color – The color of underlying skin and tissue can affect perceived blood color, especially with small volumes.
  • Lighting – Bright light makes deoxygenated blood appear brighter red.

For these reasons, deoxygenated blood can range from dark purple-red in large vessels to lighter brick-red in smaller vessels or thinner layers. However, it is always darker than bright oxygenated arterial blood.

Causes of Deoxygenated Blood

There are several medical conditions that can cause the presence of deoxygenated blood:

  • Respiratory disease – Conditions like COPD, pneumonia, pulmonary edema can prevent proper oxygenation of blood in the lungs.
  • Heart disease – Weakened heart pumps may not circulate blood effectively to tissues.
  • Severed artery – Deoxygenated venous blood may come from a severed artery.
  • Carbon monoxide poisoning – Prevents hemoglobin from carrying oxygen.
  • Hemoglobin disorders – Such as sickle cell disease disrupt oxygen transport.
  • Cyanosis – Causes blue discoloration of skin indicating poorly oxygenated blood.

Seeing deoxygenated blood in an artery or from a fresh wound is always abnormal and a cause for concern. Prompt medical treatment is important.


In summary, human blood without oxygen becomes a dark red or purple color due to deoxygenated hemoglobin in red blood cells. The dark coloration helps distinguish deoxygenated venous blood from bright oxygenated arterial blood. While the exact shade can vary based on several factors, deoxygenated blood is always darker than normal. Understanding the difference in coloration and causes can assist with prompt diagnosis and treatment of many medical conditions.