In the circulatory system, venous blood is blood returning to the heart. With one exception (the pulmonary vein) this blood is deoxygenated and high in carbon dioxide, having released oxygen and absorbed CO2 in the tissues.
Venous blood can be obtained by venesection or phlebotomy, or in small quantities by fingerprick. Most medical laboratory tests are conducted on venous blood, with the exception of arterial blood gasses.
