In normal people, venous blood is usually dark red and arterial blood usually appears bright red. The color of blood is different in normal human body circulation due to the different oxygen content in the blood. The color of blood mainly depends on how much oxygen is contained in the hemoglobin in the red blood cells. Hemoglobin in arteries has higher oxygen content, so it appears bright red; while venous blood contains low oxygen content and higher carbon dioxide, so the blood appears dark red. However, the color of venous blood is different in some patients due to their individual constitution and diseases. For example, in hyperlipidemic patients, or after transfusion of fat emulsion, the color of blood may be milky red as the fat content in blood is high. Therefore, normal human venous blood is usually dark red, which is of discriminatory significance for venipuncture.