Venous blood is deoxygenated blood in the circulatory system. It runs in the systemic veins from the organs to the heart. Deoxygenated blood is then pumped by the heart to lungs via the pulmonary arteries, one of the few arteries in the body that carries deoxygenated blood (the pulmonary veins likewise return the newly oxygenated blood to the heart).
Venous blood is typically warmer than arterial blood, and has a lower oxygen content and pH. It also has lower concentrations of glucose and other nutrients, and has higher concentrations of urea and other waste products. The difference in the oxygen content of the blood between the arterial blood and the venous blood is known as the arteriovenous oxygen difference.[1]
Most medical laboratory tests are conducted on venous blood, with the exception of arterial blood gas tests. It is obtained for lab work by venipuncture (also called phlebotomy), or by fingerprick for small quantities.
Venous blood is not blue as it is often depicted in many medical diagrams.
Veins often look blue when seen through the skin, but this is due to Rayleigh scattering – venous blood itself is actually a dark red color (but looks purple through the opaque skin).[2]
The appearance of veins as dark blue is a wavelength phenomenon, having to do with the reflection of blue light away from the outside of venous tissue if the vein is at 0.02 inches deep or more. Deoxygenated blood is darker than oxygenated blood due to the difference in color between deoxyhemoglobin and oxyhemoglobin.
Venous blood is used during stem cell donation.