<P> Venous blood is typically colder 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 arterial blood and venous blood is known as the arteriovenous oxygen difference . </P> <P> Most medical laboratory tests are conducted on venous blood, with the exception of arterial blood gas tests . Venous blood is obtained for lab work by venipuncture (also called phlebotomy), or by finger prick for small quantities . </P> <P> Human blood is red in color, ranging from bright red when oxygenated to a very dark, almost blackish - red when deoxygenated . It owes its color to hemoglobin, to which oxygen binds . Deoxygenated blood is darker due to the difference in color between deoxyhaemoglobin and oxyhaemoglobin . </P> <P> The blue appearance of surface veins is caused mostly by the scattering of blue light away from the outside of venous tissue if the vein is at 0.5 mm deep or more . Veins and arteries appear similar when skin is removed and are seen directly . </P>

Blue coloured blood is present in which of the following