Venous blood usually refers to the blood flowing in the veins of the body circulation, and the blood flowing in the arteries is arterial blood, that is, the blood flowing in the veins of the general circulation and the blood in the pulmonary arteries of the pulmonary circulation. Venous blood can check: 1, blood routine, liver function, kidney function, electrolytes, and in the blood can understand the content of carbon dioxide, oxygen content, partial pressure of oxygen, partial pressure of carbon dioxide, as well as the ph value of the blood, that is, we often say that the blood gas analysis; 2, indicators of infections: such as hepatitis B and C, etc.; 3, tumor markers, a variety of tumor markers can be found with the venous blood; 4, the patient has a situation of drug abuse, such as whether the patient has abused Drugs, such as whether the patient has drug abuse, or other abusive drugs, through the blood test can also be clearly known.