Normal human venous blood color is usually dark red, darker, the main reason is because the venous blood is metabolized and reduced by the body, which contains a lot of metabolic waste, so its color is dark. Then again the main reason is that the color of venous blood is not as bright red as arterial blood because the oxygen content and oxygen saturation of venous blood is very low. If the patient has no other symptoms, it does not need to be treated, but some people may have obvious cyanosis, which should be considered some lack of oxygen or the patient has poisoning. This is because in the case of oxygen deprivation, especially in the case of congenital heart disease, the patient’s venous blood will be significantly darker and there will be corresponding cyanosis of the extremities, cyanosis of the lips, especially a decrease in activity tolerance and other corresponding symptoms. In addition, there are some poisonings that can also lead to significant darkening of venous blood, for example, methemoglobinemia caused by nitrite poisoning can also occur, so for patients with serious abnormalities and symptoms, it is necessary to conduct the corresponding examination, except for some organic diseases.