Venous blood
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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. It is also typically warmer than arterial blood, has a lower pH, has lower concentrations of glucose and other nutrients, and has higher concentrations of urea and other waste products.
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 gases.
Venous blood is often depicted as blue in color in medical diagrams, and veins sometimes look blue when seen through the skin. However, venous blood is actually a dark red color, while arterial blood is bright red. This is due to the difference in color between deoxyhemoglobin and oxyhemoglobin; the red color ultimately originates from the iron atom in heme. If blood is drawn for a medical test, the dark red color can be seen; however, if it is exposed to oxygen in the air, it will turn bright red like arterial blood.