Reasons why venous blood turns black

Veins are very important blood vessels in the human body, and their main function is to transport nutrients and blood. When injecting or infusing fluids, intravenous injection is generally chosen because of the function of veins in transporting nutrients. When doing a blood test, the doctor also draws blood from the patient's veins, because venous blood can effectively reflect the health of a person's blood. But why is venous blood black after being drawn?

In fact, the color of blood drawn from most people is dark red. For people with anemia, the color of the blood drawn becomes lighter, appearing light pink, because the red blood cells in the blood are too low.

The reason why the donated blood is "black" is because we collect venous blood!

Venous blood & arterial blood

Venous blood usually refers to the blood flowing in the veins of the body's circulation. That is, the blood flowing in the veins in the large circulation and the blood in the pulmonary artery from the right atrium in the pulmonary circulation (small circulation).

Arterial blood is the blood that flows in the arteries of the systemic circulation (macrocirculation) and the blood in the pulmonary veins that returns from the lungs to the left atrium in the pulmonary circulation (macrocirculation).

There is a difference in color between venous blood and arterial blood: the red color of blood comes from the hemoglobin in red blood cells. Since arterial blood transports nutrients and oxygen, its hemoglobin contains more oxygen, so its color is bright red. Venous blood mainly transports carbon dioxide, and its hemoglobin contains less oxygen, so it is dark red.

Therefore, don’t worry if the blood you collected turns black when you donate blood, because venous blood is originally this color!

Why don't we collect arterial blood when donating blood?

Generally speaking, blood donation is done by drawing blood from the median vein of the elbow, and most general clinical tests also involve drawing venous blood. Because veins are generally located on the surface of the human body, blood flow is slower, making it easier to draw blood, and it is safer and more convenient.

Why don't we draw blood from the artery when donating blood?

First of all, arterial blood is rich in oxygen. Drawing arterial blood may cause local hypoxia in the body. Venous blood contains almost no oxygen and there is no such danger.

Secondly, the diameter of arteries in the same area is smaller than that of veins, making it difficult to insert the needle.

Secondly, arteries are generally below the dermis, while veins are mainly in the upper layer of the skin. In comparison, arteries are more difficult to locate and extract than veins.

Finally, the blood flow in the artery is fast, and it is not easy to stop bleeding after extraction.

Arteries are generally located deeper and have thicker blood vessel walls, making them difficult to extract. Furthermore, once an artery is punctured, it is difficult to stop bleeding and the operation is difficult, so arterial blood is not collected for blood donation.