Human blood is never black. The color of blood flowing through arteries and veins is red. However, there are some misconceptions about why blood may appear darker or even black in certain situations.
Why does blood look red?
Human blood owes its red color to the hundreds of millions of red blood cells that float in plasma. Red blood cells contain a protein called hemoglobin, which binds to oxygen molecules and transports them throughout the body. Hemoglobin contains iron, which gives blood its red hue.
Arterial blood — blood flowing from the heart to the rest of the body through arteries — is bright red. This is because arterial blood is full of oxygen-rich hemoglobin traveling to organs and tissues. Venous blood — blood returning to the heart through veins — is a darker red, as much of the oxygen has already been delivered to the body.
When does blood look darker or black?
There are a few situations when blood may wrongly appear to be a dark or black color:
- Dried blood – As blood dries, the water in it evaporates. This concentrates the red blood cells and makes the blood appear darker.
- Blood under skin – Blood trapped or accumulated under the skin, like in a bruise or hematoma, can look blue-black through the skin.
- Veins through skin – Veins carrying venous blood may appear blue or green through light skin because of how light scatters at the skin’s surface.
- Low oxygen – Blood low in oxygen, like blood returning from limbs, can be a darker red.
- Coagulated blood – Clotted or coagulated blood is very dark, almost black.
What makes blood red?
As mentioned earlier, the red color of blood comes from the hemoglobin inside red blood cells. Hemoglobin gets its red hue primarily from the heme component. Heme contains an iron atom that can bind to oxygen molecules. It is the interaction between the iron atom and oxygen that gives heme its distinctive red color.
The red blood cells that transport hemoglobin start out life in the bone marrow. Here, they lack heme and are synthesized without iron, making them paler. As they mature, they are released into the bloodstream where they bind with iron-containing heme to attain their signature red color.
Hemoglobin
Hemoglobin is a protein made up of four subunits – two alpha subunits and two beta subunits. Each of these subunits contains a heme group with an iron atom that can bind to one oxygen molecule. A single hemoglobin protein can bind four oxygen molecules.
In blood plasma, a single red blood cell contains about 280 million hemoglobin proteins. This allows each cell to transport up to 1.1 billion oxygen molecules around the body!
Heme
Heme is a complex organic molecule that consists of an iron ion bound to a large heterocyclic ring called a porphyrin. The iron gives heme its red color. Heme is a vital component of hemoglobin that allows it to bind to oxygen.
Heme is highly hydrophobic, meaning it repels water. This allows hemoglobin to maintain its structure as red blood cells travel through blood plasma.
How does blood transport oxygen?
The main role of red blood cells is to collect oxygen from the lungs and deliver it to tissues and organs around the body. This is made possible by hemoglobin:
- In the lungs, each heme group in hemoglobin binds to an oxygen molecule forming oxyhemoglobin. This gives arterial blood its bright red color.
- The oxygen-rich blood travels through arteries to capillaries surrounding tissues.
- Here, the higher oxygen concentration causes the oxyhemoglobin to release its oxygen into the tissues.
- This oxygen is used by cells for respiration. The oxygen-depleted hemoglobin now travels back through veins.
- Back in the lungs, the hemoglobin binds to more oxygen, starting the cycle again.
This efficient system allows red blood cells to transport oxygen to tissues at all times. In fact, hemoglobin binds and releases oxygen so readily that a red blood cell takes only about 20 seconds to complete a full circuit around the body!
How much oxygen does blood carry?
On average, each liter of blood can transport around 200 ml of oxygen. Of this:
- 100 ml is transported by plasma, dissolved directly
- 100 ml is carried bound to hemoglobin
However, the hemoglobin provides the vast majority of oxygen delivered to tissues. The levels of oxygen saturation in arterial and venous blood can be approximated as:
| Blood Type | Oxygen Saturation |
|---|---|
| Arterial blood | 97–99% |
| Venous blood | 75% |
So while venous blood returning to the heart still carries some oxygen, arterial blood transports full oxygen capacity to the body’s organs and tissues.
What if blood couldn’t carry oxygen?
Without hemoglobin to bind and transport oxygen, human life would not be possible. Some medical conditions that affect the ability of blood to carry oxygen include:
Anemia
Anemia occurs when there are too few red blood cells or not enough hemoglobin per cell. This reduces the oxygen-carrying capacity of blood. Symptoms of anemia include fatigue, weakness, and shortness of breath.
Carbon monoxide poisoning
Carbon monoxide binds to hemoglobin much more readily than oxygen does. This means carbon monoxide poisoning prevents hemoglobin from transporting oxygen around the body. This can lead to tissue damage, organ failure, and death.
Sickle cell disease
Sickle cell disease causes red blood cells to become misshapen and inflexible. This prevents them moving freely through blood vessels. The resulting lack of oxygen causes periodic pain and organ damage.
Without hemoglobin shuttling oxygen in the bloodstream, cells would be unable to access enough oxygen to create energy via respiration. Humans depend on this oxygen transport system in order to live.
Conclusion
While blood may wrongly appear darker or even black in certain situations, healthy human blood is always red. This red color comes from millions of red blood cells filled with the oxygen-transporting protein hemoglobin.
Hemoglobin’s unique structure allows it to efficiently bind and release oxygen molecules. This allows oxygen absorbed in the lungs to be delivered to tissues all around the body. Without this vital function, we simply could not live.
So next time you get a cut or scrape, remember that the red liquid that oozes out is keeping you alive thanks to some clever chemistry. The blood flowing through your veins may look darker at times, but rest assured it’s red as can be!