For the cause of the darkening of blood vessels, it is due to the high level of reduced hemoglobin in the venous blood that has lower blood oxygen in the blood vessels, therefore, the blood in the blood vessels has a dark red color, which leads to the darkening of the blood vessels. The common clinical darkening of blood vessels comes from superficial veins, which, after gas exchange with the tissues, have to bring together all the blood from the tissue cells and then converge to the deep veins and finally to the inferior vena cava through the body circulation. This venous blood has a lower concentration of oxygen and lower concentration of oxygen and hemoglobin, so this venous blood is dark red and less bright compared to the common arterial blood. This darkening of blood vessels is a relatively normal clinical phenomenon and does not require special treatment or excessive worry.