The color of venous blood is dark, the common causes are as follows: 1. Normal: In clinical practice, it is generally a relatively normal phenomenon. Blood contains a large amount of hemoglobin, if hemoglobin and oxygen combined to form oxyhemoglobin, and oxygen separation to form reduced hemoglobin. The blood with more oxygenated hemoglobin will be bright red, and the blood with more reduced hemoglobin will be dark red. 2, secondary factors: there are some secondary factors that will aggravate the content of reduced hemoglobin in venous blood, and the oxygen saturation will drop, and some obstructive lung diseases, such as Pulmonary edema, COPD, severe pneumonia, etc., and venous diseases that cause slow venous return will also lead to further darkening and darkening of venous blood, such as venous thrombosis, venous obstructive diseases, venous return disorders, etc.