Blood appears red due to the presence of hemoglobin, a protein that contains iron. Hemoglobin binds to oxygen in the lungs and transports it through the bloodstream to tissues and organs. The iron in hemoglobin makes blood appear red. There are several factors that influence the exact shade of red in blood, including the concentration of hemoglobin and how saturated it is with oxygen.
Hemoglobin Structure
Hemoglobin is comprised of four protein chains – two alpha chains and two beta chains – that surround an iron-containing group called heme. Heme contains iron in the Fe2+ (ferrous) state when no oxygen is bound to it. This gives it a purplish-red color. When heme binds to oxygen, the iron atom oxidizes to the Fe3+ (ferric) state. This bright red oxyhemoglobin is responsible for the scarlet color we associate with arterial blood.
Oxygen Saturation
As blood circulates through the body and delivers oxygen to tissues and cells, the saturation of hemoglobin decreases. Deoxygenated hemoglobin appears darker, deeper red due to the iron in heme being in the Fe2+ state. The more oxygenated the blood is, the brighter red it appears. Arterial blood straight from the lungs is highly saturated with oxygen, typically 95-100%. Venous blood returning to the heart and lungs is about 75% saturated on average.
| Blood Type | Oxygen Saturation | Color |
|---|---|---|
| Arterial Blood | 95-100% | Scarlet Red |
| Venous Blood | ~75% | Dark Red |
This table summarizes the differences in color between oxygenated arterial blood and deoxygenated venous blood.
Hemoglobin Concentration
The concentration of hemoglobin also affects the appearance of blood. Hemoglobin levels are measured in grams per deciliter (g/dL) of blood. Normal levels are generally 13.5-17.5 g/dL in men and 12.0-15.5 g/dL in women. Higher hemoglobin concentration makes blood appear brighter red, while lower levels make it seem darker.
Conditions like anemia that reduce hemoglobin levels make blood paler in color. Erythrocytosis disorders that increase hemoglobin concentration deepen the red hue. Extreme high hemoglobin levels can make blood almost purple-black.
Blood Plasma and Platelets
While hemoglobin is the main contributor to blood’s red color, plasma and platelets also play a role. Plasma makes up about 55% of blood volume. It is mostly water along with proteins, nutrients, and waste products. The yellowish fluid color of plasma dilutes the deep red of hemoglobin slightly.
Platelets are colorless cell fragments involved in clotting. But when activated, platelets release chemicals that alter the pH balance of plasma. This pH change can intensify the red hue of hemoglobin in the same sample of blood.
Diseases and Disorders
Certain diseases and health conditions can modify blood’s red color in different ways. Red blood cells can rupture due to trauma, infections, autoimmune disorders, and mechanical heart valves. This releases hemoglobin directly into plasma, giving it a pinkish-red tint rather than just diluting the blood.
Bile pigments are also released into plasma when red blood cells are destroyed, staining blood an orange-yellow color known as jaundice. This is often seen in newborns but can also indicate liver disease in adults.
Blood can appear bluish-purple if there are high levels of deoxygenated hemoglobin. This is seen in conditions like cyanosis that impair lung function. Exposure to certain toxins like carbon monoxide prevents hemoglobin from binding normally with oxygen, causing a blue discoloration as well.
Diet and Nutrition
Diet and nutrition status influence blood’s red hue to some extent. Deficiencies in iron, vitamin B12, folate, and protein can all potentially lead to various degrees of anemia and paler red blood over time.
Excessive intake of colorful plant pigments like lycopene and anthocyanins can also result in discoloration. A diet high in carotenoids may give plasma a yellow-orange tint. Beets in particular are known to impart a reddish color to both plasma and urine after eating.
However, these dietary factors have relatively minor effects on blood color compared to the major impact of hemoglobin concentration and oxygen saturation.
Conclusion
In summary, hemoglobin is the key component that makes blood appear red. The oxidation state of iron in heme groups, along with hemoglobin concentration and oxygen saturation levels, primarily determine the exact shade from bright scarlet to deep crimson. Plasma content, diet, and certain diseases can also alter the red color to some degree through various mechanisms.