Dark red blood is venous blood. The color of blood to the naked eye is related to the composition of the blood, and venous blood is dark red because it contains large amounts of tissue metabolic waste and CO₂. As the oxygen content gradually decreases, the color of the blood becomes darker and darker. Blood is divided into arterial and venous blood, and most of the blood tests are done with venous blood. Arterial blood is bright red because of its high oxygen content and oxygenated hemoglobin; venous blood is dark red and occasionally black because of its low oxygen content and reduced hemoglobin. The color of venous blood varies slightly between individuals and is related to the oxygen content of hemoglobin. If the blood is found to be other colors, such as white, cherry red, pink, yellow-brown, etc., it may be due to some kind of disease, and the cause should be found out as soon as possible by completing the relevant examinations under the guidance of the physician.